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ΓòÉΓòÉΓòÉ 1. -Preface- ΓòÉΓòÉΓòÉ
This file documents the use of the GNU compiler. Published by the Free Software
Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA
Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998 Free
Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this manual
provided the copyright notice and this permission notice are preserved on all
copies.
Permission is granted to copy and distribute modified versions of this manual
under the conditions for verbatim copying, provided also that the sections
entitled ``GNU General Public License,'' ``Funding for Free Software,'' and
``Protect Your Freedom---Fight `Look And Feel''' are included exactly as in the
original, and provided that the entire resulting derived work is distributed
under the terms of a permission notice identical to this one.
Permission is granted to copy and distribute translations of this manual into
another language, under the above conditions for modified versions, except that
the sections entitled ``GNU General Public License,'' ``Funding for Free
Software,'' and ``Protect Your Freedom---Fight `Look And Feel''', and this
permission notice, may be included in translations approved by the Free
Software Foundation instead of in the original English.
ΓòÉΓòÉΓòÉ 2. Introduction ΓòÉΓòÉΓòÉ
This manual documents how to run and install the GNU compiler, as well as its
new features and incompatibilities, and how to report bugs. It corresponds to
GNU CC version 2.8.1.
G++ and GCC You can compile C or C++ programs.
Invoking GCC Command options supported by 'gcc'.
Installation How to configure, compile and install GNU CC.
C Extensions GNU extensions to the C language family.
C++ Extensions GNU extensions to the C++ language.
Gcov gcov: a GNU CC test coverage program.
Trouble If you have trouble installing GNU CC.
Bugs How, why and where to report bugs.
Service How to find suppliers of support for GNU CC.
Contributing How to contribute to testing and developing GNU
CC.
VMS Using GNU CC on VMS.
Index Index of concepts and symbol names.
ΓòÉΓòÉΓòÉ 3. Compile C, C++, or Objective C ΓòÉΓòÉΓòÉ
The C, C++, and Objective C versions of the compiler are integrated; the GNU C
compiler can compile programs written in C, C++, or Objective C.
``GCC'' is a common shorthand term for the GNU C compiler. This is both the
most general name for the compiler, and the name used when the emphasis is on
compiling C programs.
When referring to C++ compilation, it is usual to call the compiler ``G++''.
Since there is only one compiler, it is also accurate to call it ``GCC'' no
matter what the language context; however, the term ``G++'' is more useful
when the emphasis is on compiling C++ programs.
We use the name ``GNU CC'' to refer to the compilation system as a whole, and
more specifically to the language-independent part of the compiler. For
example, we refer to the optimization options as affecting the behavior of
``GNU CC'' or sometimes just ``the compiler''.
Front ends for other languages, such as Ada 9X, Fortran, Modula-3, and Pascal,
are under development. These front-ends, like that for C++, are built in
subdirectories of GNU CC and link to it. The result is an integrated compiler
that can compile programs written in C, C++, Objective C, or any of the
languages for which you have installed front ends.
In this manual, we only discuss the options for the C, Objective-C, and C++
compilers and those of the GNU CC core. Consult the documentation of the
other front ends for the options to use when compiling programs written in
other languages.
G++ is a compiler, not merely a preprocessor. G++ builds object code directly
from your C++ program source. There is no intermediate C version of the
program. (By contrast, for example, some other implementations use a program
that generates a C program from your C++ source.) Avoiding an intermediate C
representation of the program means that you get better object code, and
better debugging information. The GNU debugger, GDB, works with this
information in the object code to give you comprehensive C++ source-level
editing capabilities (see Section C and C++ of Debugging with GDB).
ΓòÉΓòÉΓòÉ 4. GNU CC Command Options ΓòÉΓòÉΓòÉ
When you invoke GNU CC, it normally does preprocessing, compilation, assembly
and linking. The ``overall options'' allow you to stop this process at an
intermediate stage. For example, the '-c' option says not to run the linker.
Then the output consists of object files output by the assembler.
Other options are passed on to one stage of processing. Some options control
the preprocessor and others the compiler itself. Yet other options control
the assembler and linker; most of these are not documented here, since you
rarely need to use any of them.
Most of the command line options that you can use with GNU CC are useful for C
programs; when an option is only useful with another language (usually C++),
the explanation says so explicitly. If the description for a particular
option does not mention a source language, you can use that option with all
supported languages.
See Compiling C++ Programs, for a summary of special options for compiling C++
programs.
The gcc program accepts options and file names as operands. Many options have
multiletter names; therefore multiple single-letter options may not be
grouped: '-dr' is very different from '-d -r'.
You can mix options and other arguments. For the most part, the order you use
doesn't matter. Order does matter when you use several options of the same
kind; for example, if you specify '-L' more than once, the directories are
searched in the order specified.
Many options have long names starting with '-f' or with '-W'---for example,
'-fforce-mem', '-fstrength-reduce', '-Wformat' and so on. Most of these have
both positive and negative forms; the negative form of '-ffoo' would be
'-fno-foo'. This manual documents only one of these two forms, whichever one
is not the default.
Option Summary Brief list of all options, without explanations.
Overall Options Controlling the kind of output: an executable,
object files, assembler files, or preprocessed
source.
Invoking G++ Compiling C++ programs.
C Dialect Options Controlling the variant of C language compiled.
C++ Dialect Options Variations on C++.
Warning Options How picky should the compiler be?
Debugging Options Symbol tables, measurements, and debugging
dumps.
Optimize Options How much optimization?
Preprocessor Options Controlling header files and macro definitions.
Also, getting dependency information for Make.
Assembler Options Passing options to the assembler.
Link Options Specifying libraries and so on.
Directory Options Where to find header files and libraries. Where
to find the compiler executable files.
Target Options Running a cross-compiler, or an old version of
GNU CC.
Submodel Options Specifying minor hardware or convention
variations, such as 68010 vs 68020.
Code Gen Options Specifying conventions for function calls, data
layout and register usage.
Environment Variables Env vars that affect GNU CC.
Running Protoize Automatically adding or removing function
prototypes.
ΓòÉΓòÉΓòÉ 4.1. Option Summary ΓòÉΓòÉΓòÉ
Here is a summary of all the options, grouped by type. Explanations are in
the following sections.
Overall Options
See Options Controlling the Kind of Output.
-c -S -E -o file -pipe -pass-exit-codes -v -x language
C Language Options
See Options Controlling C Dialect.
-ansi -fallow-single-precision -fcond-mismatch -fno-asm
-fno-builtin -ffreestanding -fhosted -fsigned-bitfields -fsigned-char
-funsigned-bitfields -funsigned-char -fwritable-strings
-traditional -traditional-cpp -trigraphs
C++ Language Options
See Options Controlling C++ Dialect.
-fall-virtual -fdollars-in-identifiers -felide-constructors
-fenum-int-equiv -fexternal-templates -ffor-scope -fno-for-scope
-fhandle-signatures -fmemoize-lookups -fname-mangling-version-n
-fno-default-inline -fno-gnu-keywords -fnonnull-objects -fguiding-decls
-foperator-names -fstrict-prototype -fthis-is-variable
-ftemplate-depth-n -nostdinc++ -traditional +en
Warning Options
See Options to Request or Suppress Warnings.
-fsyntax-only -pedantic -pedantic-errors
-w -W -Wall -Waggregate-return -Wbad-function-cast
-Wcast-align -Wcast-qual -Wchar-subscript -Wcomment
-Wconversion -Werror -Wformat
-Wid-clash-len -Wimplicit -Wimplicit-int
-Wimplicit-function-declarations -Wimport -Winline
-Wlarger-than-len -Wmain -Wmissing-declarations
-Wmissing-prototypes -Wnested-externs
-Wno-import -Wold-style-cast -Woverloaded-virtual -Wparentheses
-Wpointer-arith -Wredundant-decls -Wreorder -Wreturn-type -Wshadow
-Wsign-compare -Wstrict-prototypes -Wswitch -Wsynth
-Wtemplate-debugging -Wtraditional -Wtrigraphs
-Wundef -Wuninitialized -Wunused -Wwrite-strings
Debugging Options
See Options for Debugging Your Program or GCC.
-a -ax -dletters -fpretend-float
-fprofile-arcs -ftest-coverage
-g -glevel -gcoff -gdwarf -gdwarf-1 -gdwarf-1+ -gdwarf-2
-ggdb -gstabs -gstabs+ -gxcoff -gxcoff+
-p -pg -print-file-name=library -print-libgcc-file-name
-print-prog-name=program -print-search-dirs -save-temps
Optimization Options
See Options that Control Optimization.
-fbranch-probabilities
-fcaller-saves -fcse-follow-jumps -fcse-skip-blocks
-fdelayed-branch -fexpensive-optimizations
-ffast-math -ffloat-store -fforce-addr -fforce-mem
-ffunction-sections -finline-functions
-fkeep-inline-functions -fno-default-inline
-fno-defer-pop -fno-function-cse
-fno-inline -fno-peephole -fomit-frame-pointer
-frerun-cse-after-loop -fschedule-insns
-fschedule-insns2 -fstrength-reduce -fthread-jumps
- funroll-all-loops -funroll-loops
-O -O0 -O1 -O2 -O3
Preprocessor Options
See Options Controlling the Preprocessor.
-Aquestion(answer) -C -dD -dM -dN
-Dmacro[=defn] -E -H
-idirafter dir
-include file -imacros file
-iprefix file -iwithprefix dir
-iwithprefixbefore dir -isystem dir
-M -MD -MM -MMD -MG -nostdinc -P -trigraphs
-undef -Umacro -Wp,option
Assembler Option
See Passing Options to the Assembler.
-Wa,option
Linker Options
See Options for Linking.
object-file-name -llibrary
-nostartfiles -nodefaultlibs -nostdlib
-s -static -shared -symbolic
-Wl,option -Xlinker option
-u symbol
Directory Options
See Options for Directory Search.
-Bprefix -Idir -I- -Ldir -specs=file
Target Options
See Target Options.
-b machine -V version
Machine Dependent Options
See Hardware Models and Configurations.
M680x0 Options
-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040
-m68060 -m5200 -m68881 -mbitfield -mc68000 -mc68020 -mfpa
-mnobitfield -mrtd -mshort -msoft-float -malign-int
VAX Options
-mg -mgnu -munix
SPARC Options
-mcpu=cpu type
-mtune=cpu type
-mcmodel=code model
-malign-jumps=num -malign-loops=num
-malign-functions=num
-m32 -m64
-mapp-regs -mbroken-saverestore -mcypress -mepilogue
-mflat -mfpu -mhard-float -mhard-quad-float
-mimpure-text -mlive-g0 -mno-app-regs -mno-epilogue
-mno-flat -mno-fpu -mno-impure-text
-mno-stack-bias -mno-unaligned-doubles
-msoft-float -msoft-quad-float -msparclite -mstack-bias
-msupersparc -munaligned-doubles -mv8
Convex Options
-mc1 -mc2 -mc32 -mc34 -mc38
-margcount -mnoargcount
-mlong32 -mlong64
-mvolatile-cache -mvolatile-nocache
AMD29K Options
-m29000 -m29050 -mbw -mnbw -mdw -mndw
-mlarge -mnormal -msmall
-mkernel-registers -mno-reuse-arg-regs
-mno-stack-check -mno-storem-bug
-mreuse-arg-regs -msoft-float -mstack-check
-mstorem-bug -muser-registers
ARM Options
-mapcs-frame -mapcs-26 -mapcs-32
-mlittle-endian -mbig-endian -mwords-little-endian
-mshort-load-bytes -mno-short-load-bytes
-msoft-float -mhard-float
-mbsd -mxopen -mno-symrename
MN10300 Options
-mmult-bug
-mno-mult-bug
M32R/D Options
-mcode-model=model type -msdata=sdata type
-G num
M88K Options
-m88000 -m88100 -m88110 -mbig-pic
-mcheck-zero-division -mhandle-large-shift
-midentify-revision -mno-check-zero-division
-mno-ocs-debug-info -mno-ocs-frame-position
-mno-optimize-arg-area -mno-serialize-volatile
-mno-underscores -mocs-debug-info
-mocs-frame-position -moptimize-arg-area
-mserialize-volatile -mshort-data-num -msvr3
-msvr4 -mtrap-large-shift -muse-div-instruction
-mversion-03.00 -mwarn-passed-structs
RS/6000 and PowerPC Options
-mcpu=cpu type
-mtune=cpu type
-mpower -mno-power -mpower2 -mno-power2
-mpowerpc -mno-powerpc
-mpowerpc-gpopt -mno-powerpc-gpopt
-mpowerpc-gfxopt -mno-powerpc-gfxopt
-mnew-mnemonics -mno-new-mnemonics
-mfull-toc -mminimal-toc -mno-fop-in-toc -mno-sum-in-toc
-mxl-call -mno-xl-call -mthreads -mpe
-msoft-float -mhard-float -mmultiple -mno-multiple
-mstring -mno-string -mupdate -mno-update
-mfused-madd -mno-fused-madd -mbit-align -mno-bit-align
-mstrict-align -mno-strict-align -mrelocatable
-mno-relocatable -mrelocatable-lib -mno-relocatable-lib
-mtoc -mno-toc -mtraceback -mno-traceback
-mlittle -mlittle-endian -mbig -mbig-endian
-mcall-aix -mcall-sysv -mprototype -mno-prototype
-msim -mmvme -mads -myellowknife -memb
-msdata -msdata=opt -G num
-mlongcall
RT Options
-mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs
-mfull-fp-blocks -mhc-struct-return -min-line-mul
-mminimum-fp-blocks -mnohc-struct-return
MIPS Options
-mabicalls -mcpu=cpu type -membedded-data
-membedded-pic -mfp32 -mfp64 -mgas -mgp32 -mgp64
-mgpopt -mhalf-pic -mhard-float -mint64 -mips1
-mips2 -mips3 -mlong64 -mlong-calls -mmemcpy
-mmips-as -mmips-tfile -mno-abicalls
-mno-embedded-data -mno-embedded-pic
-mno-gpopt -mno-long-calls
-mno-memcpy -mno-mips-tfile -mno-rnames -mno-stats
-mrnames -msoft-float
-m4650 -msingle-float -mmad
-mstats -EL -EB -G num -nocpp
i386 Options
-mcpu=cpu type
-march=cpu type
-mieee-fp -mno-fancy-math-387
-mno-fp-ret-in-387 -msoft-float -msvr3-shlib
-mno-wide-multiply -mrtd -malign-double
-mreg-alloc=list -mregparm=num
-malign-jumps=num -malign-loops=num
-malign-functions=num
HPPA Options
-mbig-switch -mdisable-fpregs -mdisable-indexing -mfast-indirect-calls
-mgas -mjump-in-delay -mlong-load-store -mno-big-switch -mno-disable-fpregs
-mno-disable-indexing -mno-fast-indirect-calls -mno-gas
-mno-jump-in-delay
-mno-long-load-store
-mno-portable-runtime -mno-soft-float -mno-space -mno-space-regs
-msoft-float
-mpa-risc-1-0 -mpa-risc-1-1 -mportable-runtime
-mschedule=list -mspace -mspace-regs
Intel 960 Options
-mcpu type -masm-compat -mclean-linkage
-mcode-align -mcomplex-addr -mleaf-procedures
-mic-compat -mic2.0-compat -mic3.0-compat
-mintel-asm -mno-clean-linkage -mno-code-align
-mno-complex-addr -mno-leaf-procedures
-mno-old-align -mno-strict-align -mno-tail-call
-mnumerics -mold-align -msoft-float -mstrict-align
-mtail-call
DEC Alpha Options
-mfp-regs -mno-fp-regs -mno-soft-float -msoft-float
-malpha-as -mgas
-mieee -mieee-with-inexact -mieee-conformant
-mfp-trap-mode=mode -mfp-rounding-mode=mode
-mtrap-precision=mode -mbuild-constants
-mcpu=cpu type
-mbwx -mno-bwx -mcix -mno-cix -mmax -mno-max
Clipper Options
-mc300 -mc400
H8/300 Options
-mrelax -mh -ms -mint32 -malign-300
SH Options
- m1 -m2 -m3 -m3e -mb -ml -mrelax
System V Options
-Qy -Qn -YP,paths -Ym,dir
V850 Options
-mlong-calls -mno-long-calls -mep -mno-ep
-mprolog-function -mno-prolog-function -mspace
-mtda=n -msda=n -mzda=n
-mv850 -mbig-switch
Code Generation Options
See Options for Code Generation Conventions.
-fcall-saved-reg -fcall-used-reg
-ffixed-reg -finhibit-size-directive
-fcheck-memory-usage -fprefix-function-name
-fno-common -fno-ident -fno-gnu-linker
-fpcc-struct-return -freg-struct-return
-fshared-data -fpic -fPIC -fexceptions
-funwind-tables -fshort-enums -fshort-double
-fvolatile -fvolatile-global -fvolatile-static
-fverbose-asm -fpack-struct -fstack-check +e0 +e1
Overall Options Controlling the kind of output: an executable,
object files, assembler files, or preprocessed
source.
C Dialect Options Controlling the variant of C language compiled.
C++ Dialect Options Variations on C++.
Warning Options How picky should the compiler be?
Debugging Options Symbol tables, measurements, and debugging
dumps.
Optimize Options How much optimization?
Preprocessor Options Controlling header files and macro definitions.
Also, getting dependency information for Make.
Assembler Options Passing options to the assembler.
Link Options Specifying libraries and so on.
Directory Options Where to find header files and libraries. Where
to find the compiler executable files.
Target Options Running a cross-compiler, or an old version of
GNU CC.
ΓòÉΓòÉΓòÉ 4.2. Options Controlling the Kind of Output ΓòÉΓòÉΓòÉ
Compilation can involve up to four stages: preprocessing, compilation proper,
assembly and linking, always in that order. The first three stages apply to
an individual source file, and end by producing an object file; linking
combines all the object files (those newly compiled, and those specified as
input) into an executable file.
For any given input file, the file name suffix determines what kind of
compilation is done:
file.c
C source code which must be preprocessed.
file.i
C source code which should not be preprocessed.
file.ii
C++ source code which should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the library
'libobjc.a' to make an Objective-C program work.
file.h
C header file (not to be compiled or linked).
file.cc
file.cxx
file.cpp
file.C
C++ source code which must be preprocessed. Note that in '.cxx',
the last two letters must both be literally 'x'. Likewise, '.C'
refers to a literal capital C.
file.s
Assembler code.
file.S
Assembler code which must be preprocessed.
other
An object file to be fed straight into linking. Any file name with
no recognized suffix is treated this way.
You can specify the input language explicitly with the '-x' option:
-x language
Specify explicitly the language for the following input files
(rather than letting the compiler choose a default based on the file
name suffix). This option applies to all following input files
until the next '-x' option. Possible values for language are:
c objective-c c++
c-header cpp-output c++-cpp-output
assembler assembler-with-cpp
-x none
Turn off any specification of a language, so that subsequent files
are handled according to their file name suffixes (as they are if
'-x' has not been used at all).
If you only want some of the stages of compilation, you can use '-x' (or
filename suffixes) to tell gcc where to start, and one of the options '-c',
'-S', or '-E' to say where gcc is to stop. Note that some combinations (for
example, '-x cpp-output -E' instruct gcc to do nothing at all.
-c
Compile or assemble the source files, but do not link. The linking
stage simply is not done. The ultimate output is in the form of an
object file for each source file.
By default, the object file name for a source file is made by
replacing the suffix '.c', '.i', '.s', etc., with '.o'.
Unrecognized input files, not requiring compilation or assembly, are
ignored.
-S
Stop after the stage of compilation proper; do not assemble. The
output is in the form of an assembler code file for each
non-assembler input file specified.
By default, the assembler file name for a source file is made by
replacing the suffix '.c', '.i', etc., with '.s'.
Input files that don't require compilation are ignored.
-E
Stop after the preprocessing stage; do not run the compiler proper.
The output is in the form of preprocessed source code, which is sent
to the standard output.
Input files which don't require preprocessing are ignored.
-o file
Place output in file file. This applies regardless to whatever sort
of output is being produced, whether it be an executable file, an
object file, an assembler file or preprocessed C code.
Since only one output file can be specified, it does not make sense
to use '-o' when compiling more than one input file, unless you are
producing an executable file as output.
If '-o' is not specified, the default is to put an executable file
in 'a.out', the object file for 'source.suffix' in 'source.o', its
assembler file in 'source.s', and all preprocessed C source on
standard output.
-v
Print (on standard error output) the commands executed to run the
stages of compilation. Also print the version number of the
compiler driver program and of the preprocessor and the compiler
proper.
-pipe
Use pipes rather than temporary files for communication between the
various stages of compilation. This fails to work on some systems
where the assembler is unable to read from a pipe; but the GNU
assembler has no trouble.
- pass-exit-codes
Normally the gcc program will exit with the code of 1 if any phase
of the compiler returns a non-success return code. If you specify
'-pass-exit-codes', the gcc program will instead return with
numerically highest error produced by any phase that returned an
error indication.
ΓòÉΓòÉΓòÉ 4.3. Compiling C++ Programs ΓòÉΓòÉΓòÉ
C++ source files conventionally use one of the suffixes '.C', '.cc', 'cpp', or
'.cxx'; preprocessed C++ files use the suffix '.ii'. GNU CC recognizes files
with these names and compiles them as C++ programs even if you call the
compiler the same way as for compiling C programs (usually with the name gcc).
However, C++ programs often require class libraries as well as a compiler that
understands the C++ language---and under some circumstances, you might want to
compile programs from standard input, or otherwise without a suffix that flags
them as C++ programs. g++ is a program that calls GNU CC with the default
language set to C++, and automatically specifies linking against the C++
library. xx (1) On many systems, the script g++ is also installed with the
name c++.
When you compile C++ programs, you may specify many of the same command-line
options that you use for compiling programs in any language; or command-line
options meaningful for C and related languages; or options that are meaningful
only for C++ programs. See Options Controlling C Dialect, for explanations of
options for languages related to C. See Options Controlling C++ Dialect, for
explanations of options that are meaningful only for C++ programs.
ΓòÉΓòÉΓòÉ 4.4. Options Controlling C Dialect ΓòÉΓòÉΓòÉ
The following options control the dialect of C (or languages derived from C,
such as C++ and Objective C) that the compiler accepts:
-ansi
Support all ANSI standard C programs.
This turns off certain features of GNU C that are incompatible with
ANSI C, such as the asm, inline and typeof keywords, and predefined
macros such as unix and vax that identify the type of system you are
using. It also enables the undesirable and rarely used ANSI
trigraph feature, and it disables recognition of C++ style '//'
comments.
The alternate keywords __asm__, __extension__, __inline__ and
__typeof__ continue to work despite '-ansi'. You would not want to
use them in an ANSI C program, of course, but it is useful to put
them in header files that might be included in compilations done
with '-ansi'. Alternate predefined macros such as __unix__ and
__vax__ are also available, with or without '-ansi'.
The '-ansi' option does not cause non-ANSI programs to be rejected
gratuitously. For that, '-pedantic' is required in addition to
'-ansi'. See Warning Options.
The macro __STRICT_ANSI__ is predefined when the '-ansi' option is
used. Some header files may notice this macro and refrain from
declaring certain functions or defining certain macros that the ANSI
standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.
The functions alloca, abort, exit, and _exit are not builtin
functions when '-ansi' is used.
-fno-asm
Do not recognize asm, inline or typeof as a keyword, so that code
can use these words as identifiers. You can use the keywords
__asm__, __inline__ and __typeof__ instead. '-ansi' implies
'-fno-asm'.
In C++, this switch only affects the typeof keyword, since asm and
inline are standard keywords. You may want to use the
'-fno-gnu-keywords' flag instead, as it also disables the other,
C++-specific, extension keywords such as headof.
-fno-builtin
Don't recognize builtin functions that do not begin with two leading
underscores. Currently, the functions affected include abort, abs,
alloca, cos, exit, fabs, ffs, labs, memcmp, memcpy, sin, sqrt,
strcmp, strcpy, and strlen.
GCC normally generates special code to handle certain builtin
functions more efficiently; for instance, calls to alloca may become
single instructions that adjust the stack directly, and calls to
memcpy may become inline copy loops. The resulting code is often
both smaller and faster, but since the function calls no longer
appear as such, you cannot set a breakpoint on those calls, nor can
you change the behavior of the functions by linking with a different
library.
The '-ansi' option prevents alloca and ffs from being builtin
functions, since these functions do not have an ANSI standard
meaning.
-fhosted
Assert that compilation takes place in a hosted environment. This
implies '-fbuiltin'. A hosted environment is one in which the
entire standard library is available, and in which main has a return
type of int. Examples are nearly everything except a kernel. This
is equivalent to '-fno-freestanding'.
-ffreestanding
Assert that compilation takes place in a freestanding environment.
This implies '-fno-builtin'. A freestanding environment is one in
which the standard library may not exist, and program startup may
not necessarily be at main. The most obvious example is an OS
kernel. This is equivalent to '-fno-hosted'.
-trigraphs
Support ANSI C trigraphs. You don't want to know about this
brain-damage. The '-ansi' option implies '-trigraphs'.
-traditional
Attempt to support some aspects of traditional C compilers.
Specifically:
1. All extern declarations take effect globally even if they are
written inside of a function definition. This includes
implicit declarations of functions.
2. The newer keywords typeof, inline, signed, const and volatile
are not recognized. (You can still use the alternative
keywords such as __typeof__, __inline__, and so on.)
3. Comparisons between pointers and integers are always allowed.
4. Integer types unsigned short and unsigned char promote to
unsigned int.
5. Out-of-range floating point literals are not an error.
6. Certain constructs which ANSI regards as a single invalid
preprocessing number, such as '0xe-0xd', are treated as
expressions instead.
7. String ``constants'' are not necessarily constant; they are
stored in writable space, and identical looking constants are
allocated separately. (This is the same as the effect of
'-fwritable-strings'.)
8. All automatic variables not declared register are preserved
by longjmp. Ordinarily, GNU C follows ANSI C: automatic
variables not declared volatile may be clobbered.
9. The character escape sequences '\x' and '\a' evaluate as the
literal characters 'x' and 'a' respectively. Without
'-traditional', '\x' is a prefix for the hexadecimal
representation of a character, and '\a' produces a bell.
10.In C++ programs, assignment to this is permitted with
'-traditional'. (The option '-fthis-is-variable' also has
this effect.)
You may wish to use '-fno-builtin' as well as '-traditional' if
your program uses names that are normally GNU C builtin functions
for other purposes of its own.
You cannot use '-traditional' if you include any header files that
rely on ANSI C features. Some vendors are starting to ship
systems with ANSI C header files and you cannot use '-traditional'
on such systems to compile files that include any system headers.
The '-traditional' option also enables the '-traditional-cpp'
option, which is described next.
-traditional-cpp
Attempt to support some aspects of traditional C preprocessors.
Specifically:
1. Comments convert to nothing at all, rather than to a space.
This allows traditional token concatenation.
2. In a preprocessing directive, the '#' symbol must appear as
the first character of a line.
3. Macro arguments are recognized within string constants in a
macro definition (and their values are stringified, though
without additional quote marks, when they appear in such a
context). The preprocessor always considers a string
constant to end at a newline.
4. The predefined macro __STDC__ is not defined when you use
'-traditional', but __GNUC__ is (since the GNU extensions
which __GNUC__ indicates are not affected by '-traditional').
If you need to write header files that work differently
depending on whether '-traditional' is in use, by testing
both of these predefined macros you can distinguish four
situations: GNU C, traditional GNU C, other ANSI C compilers,
and other old C compilers. The predefined macro
__STDC_VERSION__ is also not defined when you use
'-traditional'. See Section Standard Predefined Macros of
The C Preprocessor, for more discussion of these and other
predefined macros.
5. The preprocessor considers a string constant to end at a
newline (unless the newline is escaped with '\'). (Without
'-traditional', string constants can contain the newline
character as typed.)
-fcond-mismatch
Allow conditional expressions with mismatched types in the second
and third arguments. The value of such an expression is void.
-funsigned-char
Let the type char be unsigned, like unsigned char.
Each kind of machine has a default for what char should be. It is
either like unsigned char by default or like signed char by default.
Ideally, a portable program should always use signed char or
unsigned char when it depends on the signedness of an object. But
many programs have been written to use plain char and expect it to
be signed, or expect it to be unsigned, depending on the machines
they were written for. This option, and its inverse, let you make
such a program work with the opposite default.
The type char is always a distinct type from each of signed char or
unsigned char, even though its behavior is always just like one of
those two.
-fsigned-char
Let the type char be signed, like signed char.
Note that this is equivalent to '-fno-unsigned-char', which is the
negative form of '-funsigned-char'. Likewise, the option
'-fno-signed-char' is equivalent to '-funsigned-char'.
You may wish to use '-fno-builtin' as well as '-traditional' if your
program uses names that are normally GNU C builtin functions for
other purposes of its own.
You cannot use '-traditional' if you include any header files that
rely on ANSI C features. Some vendors are starting to ship systems
with ANSI C header files and you cannot use '-traditional' on such
systems to compile files that include any system headers.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bitfield is signed or unsigned, when
the declaration does not use either signed or unsigned. By default,
such a bitfield is signed, because this is consistent: the basic
integer types such as int are signed types.
However, when '-traditional' is used, bitfields are all unsigned no
matter what.
-fwritable-strings
Store string constants in the writable data segment and don't
uniquize them. This is for compatibility with old programs which
assume they can write into string constants. The option
'-traditional' also has this effect.
Writing into string constants is a very bad idea; ``constants''
should be constant.
-fallow-single-precision
Do not promote single precision math operations to double precision,
even when compiling with '-traditional'.
Traditional K&R C promotes all floating point operations to double
precision, regardless of the sizes of the operands. On the
architecture for which you are compiling, single precision may be
faster than double precision. If you must use '-traditional', but
want to use single precision operations when the operands are single
precision, use this option. This option has no effect when
compiling with ANSI or GNU C conventions (the default).
ΓòÉΓòÉΓòÉ 4.5. Options Controlling C++ Dialect ΓòÉΓòÉΓòÉ
This section describes the command-line options that are only meaningful for
C++ programs; but you can also use most of the GNU compiler options regardless
of what language your program is in. For example, you might compile a file
firstClass.C like this:
g++ -g -felide-constructors -O -c firstClass.C
In this example, only '-felide-constructors' is an option meant only for C++
programs; you can use the other options with any language supported by GNU CC.
Here is a list of options that are only for compiling C++ programs:
-fno-access-control
Turn off all access checking. This switch is mainly useful for
working around bugs in the access control code.
-fall-virtual
Treat all possible member functions as virtual, implicitly. All
member functions (except for constructor functions and new or delete
member operators) are treated as virtual functions of the class
where they appear.
This does not mean that all calls to these member functions will be
made through the internal table of virtual functions. Under some
circumstances, the compiler can determine that a call to a given
virtual function can be made directly; in these cases the calls are
direct in any case.
-fcheck-new
Check that the pointer returned by operator new is non-null before
attempting to modify the storage allocated. The current Working
Paper requires that operator new never return a null pointer, so
this check is normally unnecessary.
-fconserve-space
Put uninitialized or runtime-initialized global variables into the
common segment, as C does. This saves space in the executable at
the cost of not diagnosing duplicate definitions. If you compile
with this flag and your program mysteriously crashes after main()
has completed, you may have an object that is being destroyed twice
because two definitions were merged.
-fdollars-in-identifiers
Accept '$' in identifiers. You can also explicitly prohibit use of
'$' with the option '-fno-dollars-in-identifiers'. (GNU C allows
'$' by default on most target systems, but there are a few
exceptions.) Traditional C allowed the character '$' to form part of
identifiers. However, ANSI C and C++ forbid '$' in identifiers.
-fenum-int-equiv
Anachronistically permit implicit conversion of int to enumeration
types. Current C++ allows conversion of enum to int, but not the
other way around.
-fexternal-templates
Cause template instantiations to obey '#pragma interface' and
'implementation'; template instances are emitted or not according to
the location of the template definition. See Template
Instantiation, for more information.
This option is deprecated.
-falt-external-templates
Similar to -fexternal-templates, but template instances are emitted
or not according to the place where they are first instantiated. See
Template Instantiation, for more information.
This option is deprecated.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a
for-init-statement is limited to the 'for' loop itself, as specified
by the draft C++ standard. If -fno-for-scope is specified, the scope
of variables declared in a for-init-statement extends to the end of
the enclosing scope, as was the case in old versions of gcc, and
other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but to
allow and give a warning for old-style code that would otherwise be
invalid, or have different behavior.
-fno-gnu-keywords
Do not recognize classof, headof, signature, sigof or typeof as a
keyword, so that code can use these words as identifiers. You can
use the keywords __classof__, __headof__, __signature__, __sigof__,
and __typeof__ instead. '-ansi' implies '-fno-gnu-keywords'.
-fguiding-decls
Treat a function declaration with the same type as a potential
function template instantiation as though it declares that
instantiation, not a normal function. If a definition is given for
the function later in the translation unit (or another translation
unit if the target supports weak symbols), that definition will be
used; otherwise the template will be instantiated. This behavior
reflects the C++ language prior to September 1996, when guiding
declarations were removed.
This option implies '-fname-mangling-version-0', and will not work
with other name mangling versions.
-fno-implicit-templates
Never emit code for templates which are instantiated implicitly
(i.e. by use); only emit code for explicit instantiations. See
Template Instantiation, for more information.
-fhandle-signatures
Recognize the signature and sigof keywords for specifying abstract
types. The default ('-fno-handle-signatures') is not to recognize
them. See C++ Signatures: Type Abstraction using Signatures.
-fhuge-objects
Support virtual function calls for objects that exceed the size
representable by a 'short int'. Users should not use this flag by
default; if you need to use it, the compiler will tell you so. If
you compile any of your code with this flag, you must compile all of
your code with this flag (including the C++ library, if you use it).
This flag is not useful when compiling with -fvtable-thunks.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions
controlled by '#pragma implementation'. This will cause linker
errors if these functions are not inlined everywhere they are
called.
-fmemoize-lookups
-fsave-memoized
Use heuristics to compile faster. These heuristics are not enabled
by default, since they are only effective for certain input files.
Other input files compile more slowly.
The first time the compiler must build a call to a member function
(or reference to a data member), it must (1) determine whether the
class implements member functions of that name; (2) resolve which
member function to call (which involves figuring out what sorts of
type conversions need to be made); and (3) check the visibility of
the member function to the caller. All of this adds up to slower
compilation. Normally, the second time a call is made to that member
function (or reference to that data member), it must go through the
same lengthy process again. This means that code like this:
cout << "This " << p << " has " << n << " legs.\n";
makes six passes through all three steps. By using a software cache, a
``hit'' significantly reduces this cost. Unfortunately, using the cache
introduces another layer of mechanisms which must be implemented, and so
incurs its own overhead. '-fmemoize-lookups' enables the software cache.
Because access privileges (visibility) to members and member functions may
differ from one function context to the next, G++ may need to flush the cache.
With the '-fmemoize-lookups' flag, the cache is flushed after every function
that is compiled. The '-fsave-memoized' flag enables the same software cache,
but when the compiler determines that the context of the last function
compiled would yield the same access privileges of the next function to
compile, it preserves the cache. This is most helpful when defining many
member functions for the same class: with the exception of member functions
which are friends of other classes, each member function has exactly the same
access privileges as every other, and the cache need not be flushed.
The code that implements these flags has rotted; you should probably avoid
using them.
-fstrict-prototype
Within an 'extern "C"' linkage specification, treat a function
declaration with no arguments, such as 'int foo ();', as declaring
the function to take no arguments. Normally, such a declaration
means that the function foo can take any combination of arguments,
as in C. '-pedantic' implies '-fstrict-prototype' unless overridden
with '-fno-strict-prototype'.
This flag no longer affects declarations with C++ linkage.
-fname-mangling-version-n
Control the way in which names are mangled. Version 0 is compatible
with versions of g++ before 2.8. Version 1 is the default. Version
1 will allow correct mangling of function templates. For example,
version 0 mangling does not mangle foo<int, double> and foo<int,
char> given this declaration:
template <class T, class U> void foo(T t);
-fno-nonnull-objects
Don't assume that a reference is initialized to refer to a valid
object. Although the current C++ Working Paper prohibits null
references, some old code may rely on them, and you can use
'-fno-nonnull-objects' to turn on checking.
At the moment, the compiler only does this checking for conversions
to virtual base classes.
-foperator-names
Recognize the operator name keywords and, bitand, bitor, compl, not,
or and xor as synonyms for the symbols they refer to. '-ansi'
implies '-foperator-names'.
-fthis-is-variable
Permit assignment to this. The incorporation of user-defined free
store management into C++ has made assignment to 'this' an
anachronism. Therefore, by default it is invalid to assign to this
within a class member function; that is, GNU C++ treats 'this' in a
member function of class X as a non-lvalue of type 'X *'. However,
for backwards compatibility, you can make it valid with
'-fthis-is-variable'.
-fvtable-thunks
Use 'thunks' to implement the virtual function dispatch table
('vtable'). The traditional (cfront-style) approach to implementing
vtables was to store a pointer to the function and two offsets for
adjusting the 'this' pointer at the call site. Newer
implementations store a single pointer to a 'thunk' function which
does any necessary adjustment and then calls the target function.
This option also enables a heuristic for controlling emission of
vtables; if a class has any non-inline virtual functions, the vtable
will be emitted in the translation unit containing the first one of
those.
-ftemplate-depth-n
Set the maximum instantiation depth for template classes to n. A
limit on the template instantiation depth is needed to detect
endless recursions during template class instantiation. ANSI/ISO C++
conforming programs must not rely on a maximum depth greater than
17.
-nostdinc++
Do not search for header files in the standard directories specific
to C++, but do still search the other standard directories. (This
option is used when building the C++ library.)
-traditional
For C++ programs (in addition to the effects that apply to both C
and C++), this has the same effect as '-fthis-is-variable'. See
Options Controlling C Dialect.
In addition, these optimization, warning, and code generation options have
meanings only for C++ programs:
-fno-default-inline
Do not assume 'inline' for functions defined inside a class scope.
See Options That Control Optimization.
-Wold-style-cast
-Woverloaded-virtual
-Wtemplate-debugging
Warnings that apply only to C++ programs. See Options to Request or
Suppress Warnings.
-Weffc++
Warn about violation of some style rules from Effective C++ by Scott
Myers.
+en
Control how virtual function definitions are used, in a fashion
compatible with cfront 1.x. See Options for Code Generation
Conventions.
ΓòÉΓòÉΓòÉ 4.6. Options to Request or Suppress Warnings ΓòÉΓòÉΓòÉ
Warnings are diagnostic messages that report constructions which are not
inherently erroneous but which are risky or suggest there may have been an
error.
You can request many specific warnings with options beginning '-W', for
example '-Wimplicit' to request warnings on implicit declarations. Each of
these specific warning options also has a negative form beginning '-Wno-' to
turn off warnings; for example, '-Wno-implicit'. This manual lists only one
of the two forms, whichever is not the default.
These options control the amount and kinds of warnings produced by GNU CC:
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.
-pedantic
Issue all the warnings demanded by strict ANSI standard C; reject
all programs that use forbidden extensions.
Valid ANSI standard C programs should compile properly with or
without this option (though a rare few will require '-ansi').
However, without this option, certain GNU extensions and traditional
C features are supported as well. With this option, they are
rejected.
'-pedantic' does not cause warning messages for use of the alternate
keywords whose names begin and end with '__'. Pedantic warnings are
also disabled in the expression that follows __extension__.
However, only system header files should use these escape routes;
application programs should avoid them. See Alternate Keywords.
This option is not intended to be useful; it exists only to satisfy
pedants who would otherwise claim that GNU CC fails to support the
ANSI standard.
Some users try to use '-pedantic' to check programs for strict ANSI
C conformance. They soon find that it does not do quite what they
want: it finds some non-ANSI practices, but not all---only those for
which ANSI C requires a diagnostic.
A feature to report any failure to conform to ANSI C might be useful
in some instances, but would require considerable additional work
and would be quite different from '-pedantic'. We recommend,
rather, that users take advantage of the extensions of GNU C and
disregard the limitations of other compilers. Aside from certain
supercomputers and obsolete small machines, there is less and less
reason ever to use any other C compiler other than for bootstrapping
GNU CC.
-pedantic-errors
Like '-pedantic', except that errors are produced rather than
warnings.
-w
Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of '#import'.
-Wchar-subscripts
Warn if an array subscript has type char. This is a common cause of
error, as programmers often forget that this type is signed on some
machines.
-Wcomment
Warn whenever a comment-start sequence '/*' appears in a '/*'
comment, or whenever a Backslash-Newline appears in a '//' comment.
-Wformat
Check calls to printf and scanf, etc., to make sure that the
arguments supplied have types appropriate to the format string
specified.
-Wimplicit-int
Warn when a declaration does not specify a type.
-Wimplicit-function-declarations
Warn whenever a function is used before being declared.
-Wimplicit
Same as '-Wimplicit-int' '-Wimplicit-function-declaration'.
-Wmain
Warn if the type of 'main' is suspicious. 'main' should be a
function with external linkage, returning int, taking either zero
arguments, two, or three arguments of appropriate types.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when
there is an assignment in a context where a truth value is expected,
or when operators are nested whose precedence people often get
confused about.
Also warn about constructions where there may be confusion to which
if statement an else branch belongs. Here is an example of such a
case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C, every else branch belongs to the innermost possible if statement, which
in this example is if (b). This is often not what the programmer expected, as
illustrated in the above example by indentation the programmer chose. When
there is the potential for this confusion, GNU C will issue a warning when
this flag is specified. To eliminate the warning, add explicit braces around
the innermost if statement so there is no way the else could belong to the
enclosing if. The resulting code would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
-Wreturn-type
Warn whenever a function is defined with a return-type that defaults
to int. Also warn about any return statement with no return-value
in a function whose return-type is not void.
-Wswitch
Warn whenever a switch statement has an index of enumeral type and
lacks a case for one or more of the named codes of that enumeration.
(The presence of a default label prevents this warning.) case
labels outside the enumeration range also provoke warnings when this
option is used.
-Wtrigraphs
Warn if any trigraphs are encountered (assuming they are enabled).
-Wunused
Warn whenever a variable is unused aside from its declaration,
whenever a function is declared static but never defined, whenever a
label is declared but not used, and whenever a statement computes a
result that is explicitly not used.
In order to get a warning about an unused function parameter, you
must specify both '-W' and '-Wunused'.
To suppress this warning for an expression, simply cast it to void.
For unused variables and parameters, use the 'unused' attribute (see
Variable Attributes).
-Wuninitialized
An automatic variable is used without first being initialized.
These warnings are possible only in optimizing compilation, because
they require data flow information that is computed only when
optimizing. If you don't specify '-O', you simply won't get these
warnings.
These warnings occur only for variables that are candidates for
register allocation. Therefore, they do not occur for a variable
that is declared volatile, or whose address is taken, or whose size
is other than 1, 2, 4 or 8 bytes. Also, they do not occur for
structures, unions or arrays, even when they are in registers.
Note that there may be no warning about a variable that is used only
to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the
warnings are printed.
These warnings are made optional because GNU CC is not smart enough
to see all the reasons why the code might be correct despite
appearing to have an error. Here is one example of how this can
happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of y is always 1, 2 or 3, then x is always initialized, but GNU
CC doesn't know this. Here is another common case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
┬╖┬╖┬╖
if (change_y) y = save_y;
}
This has no bug because save_y is used only if it is set.
Some spurious warnings can be avoided if you declare all the functions you use
that never return as noreturn. See Function Attributes.
-Wreorder (C++ only)
Warn when the order of member initializers given in the code does
not match the order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
Here the compiler will warn that the member initializers for 'i' and 'j' will
be rearranged to match the declaration order of the members.
-Wtemplate-debugging
When using templates in a C++ program, warn if debugging is not yet
fully available (C++ only).
-Wall
All of the above '-W' options combined. This enables all the
warnings about constructions that some users consider questionable,
and that are easy to avoid (or modify to prevent the warning), even
in conjunction with macros.
The following '-W┬╖┬╖┬╖' options are not implied by '-Wall'. Some of them warn
about constructions that users generally do not consider questionable, but
which occasionally you might wish to check for; others warn about
constructions that are necessary or hard to avoid in some cases, and there is
no simple way to modify the code to suppress the warning.
-W
Print extra warning messages for these events:
1. A nonvolatile automatic variable might be changed by a call
to longjmp. These warnings as well are possible only in
optimizing compilation.
The compiler sees only the calls to setjmp. It cannot know
where longjmp will be called; in fact, a signal handler could
call it at any point in the code. As a result, you may get a
warning even when there is in fact no problem because longjmp
cannot in fact be called at the place which would cause a
problem.
2. A function can return either with or without a value.
(Falling off the end of the function body is considered
returning without a value.) For example, this function would
evoke such a warning:
foo (a)
{
if (a > 0)
return a;
}
3. An expression-statement or the left-hand side of a comma
expression contains no side effects. To suppress the warning,
cast the unused expression to void. For example, an
expression such as 'x[i,j]' will cause a warning, but
'x[(void)i,j]' will not.
4. An unsigned value is compared against zero with '<' or '<='.
5. A comparison like 'x<=y<=z' appears; this is equivalent to
'(x<=y ? 1 : 0) <= z', which is a different interpretation
from that of ordinary mathematical notation.
6. Storage-class specifiers like static are not the first things
in a declaration. According to the C Standard, this usage is
obsolescent.
7. If '-Wall' or '-Wunused' is also specified, warn about unused
arguments.
8. A comparison between signed and unsigned values could produce
an incorrect result when the signed value is converted to
unsigned. (But do not warn if '-Wno-sign-compare' is also
specified.)
9. An aggregate has a partly bracketed initializer. For example,
the following code would evoke such a warning, because braces
are missing around the initializer for x.h:
struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };
-Wtraditional
Warn about certain constructs that behave differently in traditional
and ANSI C.
1. Macro arguments occurring within string constants in the
macro body. These would substitute the argument in
traditional C, but are part of the constant in ANSI C.
2. A function declared external in one block and then used after
the end of the block.
3. A switch statement has an operand of type long.
-Wundef
Warn if an undefined identifier is evaluated in an '#if' directive.
-Wshadow
Warn whenever a local variable shadows another local variable.
-Wid-clash-len
Warn whenever two distinct identifiers match in the first len
characters. This may help you prepare a program that will compile
with certain obsolete, brain-damaged compilers.
-Wlarger-than-len
Warn whenever an object of larger than len bytes is defined.
-Wpointer-arith
Warn about anything that depends on the ``size of'' a function type
or of void. GNU C assigns these types a size of 1, for convenience
in calculations with void * pointers and pointers to functions.
-Wbad-function-cast
Warn whenever a function call is cast to a non-matching type. For
example, warn if int malloc() is cast to anything *.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier
from the target type. For example, warn if a const char * is cast
to an ordinary char *.
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of
the target is increased. For example, warn if a char * is cast to
an int * on machines where integers can only be accessed at two- or
four-byte boundaries.
-Wwrite-strings
Give string constants the type const char[length] so that copying
the address of one into a non-const char * pointer will get a
warning. These warnings will help you find at compile time code
that can try to write into a string constant, but only if you have
been very careful about using const in declarations and prototypes.
Otherwise, it will just be a nuisance; this is why we did not make
'-Wall' request these warnings.
-Wconversion
Warn if a prototype causes a type conversion that is different from
what would happen to the same argument in the absence of a
prototype. This includes conversions of fixed point to floating and
vice versa, and conversions changing the width or signedness of a
fixed point argument except when the same as the default promotion.
Also, warn if a negative integer constant expression is implicitly
converted to an unsigned type. For example, warn about the
assignment x = -1 if x is unsigned. But do not warn about explicit
casts like (unsigned) -1.
-Wsign-compare
Warn when a comparison between signed and unsigned values could
produce an incorrect result when the signed value is converted to
unsigned. This warning is also enabled by '-W'; to get the other
warnings of '-W' without this warning, use '-W -Wno-sign-compare'.
-Waggregate-return
Warn if any functions that return structures or unions are defined
or called. (In languages where you can return an array, this also
elicits a warning.)
-Wstrict-prototypes
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted
without a warning if preceded by a declaration which specifies the
argument types.)
-Wmissing-prototypes
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. The aim is to detect global functions that
fail to be declared in header files.
-Wmissing-declarations
Warn if a global function is defined without a previous declaration.
Do so even if the definition itself provides a prototype. Use this
option to detect global functions that are not declared in header
files.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even
in cases where multiple declaration is valid and changes nothing.
-Wnested-externs
Warn if an extern declaration is encountered within an function.
-Winline
Warn if a function can not be inlined, and either it was declared as
inline, or else the '-finline-functions' option was given.
-Wold-style-cast
Warn if an old-style (C-style) cast is used within a program.
-Woverloaded-virtual
Warn when a derived class function declaration may be an error in
defining a virtual function (C++ only). In a derived class, the
definitions of virtual functions must match the type signature of a
virtual function declared in the base class. With this option, the
compiler warns when you define a function with the same name as a
virtual function, but with a type signature that does not match any
declarations from the base class.
-Wsynth (C++ only)
Warn when g++'s synthesis behavior does not match that of cfront.
For instance:
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, g++ will synthesize a default 'A& operator =
(const A&);', while cfront will use the user-defined 'operator ='.
-Werror
Make all warnings into errors.
ΓòÉΓòÉΓòÉ 4.7. Options for Debugging Your Program or GNU CC ΓòÉΓòÉΓòÉ
GNU CC has various special options that are used for debugging either your
program or GCC:
-g
Produce debugging information in the operating system's native
format (stabs, COFF, XCOFF, or DWARF). GDB can work with this
debugging information.
On most systems that use stabs format, '-g' enables use of extra
debugging information that only GDB can use; this extra information
makes debugging work better in GDB but will probably make other
debuggers crash or refuse to read the program. If you want to
control for certain whether to generate the extra information, use
'-gstabs+', '-gstabs', '-gxcoff+', '-gxcoff', '-gdwarf-1+', or
'-gdwarf-1' (see below).
Unlike most other C compilers, GNU CC allows you to use '-g' with
'-O'. The shortcuts taken by optimized code may occasionally
produce surprising results: some variables you declared may not
exist at all; flow of control may briefly move where you did not
expect it; some statements may not be executed because they compute
constant results or their values were already at hand; some
statements may execute in different places because they were moved
out of loops.
Nevertheless it proves possible to debug optimized output. This
makes it reasonable to use the optimizer for programs that might
have bugs.
The following options are useful when GNU CC is generated with the
capability for more than one debugging format.
-ggdb
Produce debugging information for use by GDB. This means to use the
most expressive format available (DWARF 2, stabs, or the native
format if neither of those are supported), including GDB extensions
if at all possible.
-gstabs
Produce debugging information in stabs format (if that is
supported), without GDB extensions. This is the format used by DBX
on most BSD systems. On MIPS, Alpha and System V Release 4 systems
this option produces stabs debugging output which is not understood
by DBX or SDB. On System V Release 4 systems this option requires
the GNU assembler.
-gstabs+
Produce debugging information in stabs format (if that is
supported), using GNU extensions understood only by the GNU debugger
(GDB). The use of these extensions is likely to make other
debuggers crash or refuse to read the program.
-gcoff
Produce debugging information in COFF format (if that is supported).
This is the format used by SDB on most System V systems prior to
System V Release 4.
-gxcoff
Produce debugging information in XCOFF format (if that is
supported). This is the format used by the DBX debugger on IBM
RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is
supported), using GNU extensions understood only by the GNU debugger
(GDB). The use of these extensions is likely to make other
debuggers crash or refuse to read the program, and may cause
assemblers other than the GNU assembler (GAS) to fail with an error.
-gdwarf
Produce debugging information in DWARF version 1 format (if that is
supported). This is the format used by SDB on most System V Release
4 systems.
-gdwarf+
Produce debugging information in DWARF version 1 format (if that is
supported), using GNU extensions understood only by the GNU debugger
(GDB). The use of these extensions is likely to make other
debuggers crash or refuse to read the program.
-gdwarf-2
Produce debugging information in DWARF version 2 format (if that is
supported). This is the format used by DBX on IRIX 6.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gdwarflevel
-gdwarf-2level
Request debugging information and also use level to specify how much
information. The default level is 2.
Level 1 produces minimal information, enough for making backtraces
in parts of the program that you don't plan to debug. This includes
descriptions of functions and external variables, but no information
about local variables and no line numbers.
Level 3 includes extra information, such as all the macro
definitions present in the program. Some debuggers support macro
expansion when you use '-g3'.
-p
Generate extra code to write profile information suitable for the
analysis program prof. You must use this option when compiling the
source files you want data about, and you must also use it when
linking.
-pg
Generate extra code to write profile information suitable for the
analysis program gprof. You must use this option when compiling the
source files you want data about, and you must also use it when
linking.
- a
Generate extra code to write profile information for basic blocks,
which will record the number of times each basic block is executed,
the basic block start address, and the function name containing the
basic block. If '-g' is used, the line number and filename of the
start of the basic block will also be recorded. If not overridden
by the machine description, the default action is to append to the
text file 'bb.out'.
This data could be analyzed by a program like tcov. Note, however,
that the format of the data is not what tcov expects. Eventually GNU
gprof should be extended to process this data.
-ax
Generate extra code to profile basic blocks. Your executable will
produce output that is a superset of that produced when '-a' is
used. Additional output is the source and target address of the
basic blocks where a jump takes place, the number of times a jump is
executed, and (optionally) the complete sequence of basic blocks
being executed. The output is appended to file 'bb.out'.
You can examine different profiling aspects without recompilation.
Your executable will read a list of function names from file
'bb.in'. Profiling starts when a function on the list is entered and
stops when that invocation is exited. To exclude a function from
profiling, prefix its name with `-'. If a function name is not
unique, you can disambiguate it by writing it in the form
'/path/filename.d:functionname'. Your executable will write the
available paths and filenames in file 'bb.out'.
Several function names have a special meaning:
__bb_jumps__
Write source, target and frequency of jumps to file
'bb.out'.
__bb_hidecall__
Exclude function calls from frequency count.
__bb_showret__
Include function returns in frequency count.
__bb_trace__
Write the sequence of basic blocks executed to file
'bbtrace.gz'. The file will be compressed using the
program 'gzip', which must exist in your PATH. On
systems without the 'popen' function, the file will be
named 'bbtrace' and will not be compressed. Profiling
for even a few seconds on these systems will produce a
very large file. Note: __bb_hidecall__ and
__bb_showret__ will not affect the sequence written to
'bbtrace.gz'.
Here's a short example using different profiling parameters in
file 'bb.in'. Assume function foo consists of basic blocks 1 and
2 and is called twice from block 3 of function main. After the
calls, block 3 transfers control to block 4 of main.
With __bb_trace__ and main contained in file 'bb.in', the
following sequence of blocks is written to file 'bbtrace.gz': 0 3
1 2 1 2 4. The return from block 2 to block 3 is not shown,
because the return is to a point inside the block and not to the
top. The block address 0 always indicates, that control is
transferred to the trace from somewhere outside the observed
functions. With '-foo' added to 'bb.in', the blocks of function
foo are removed from the trace, so only 0 3 4 remains.
With __bb_jumps__ and main contained in file 'bb.in', jump
frequencies will be written to file 'bb.out'. The frequencies are
obtained by constructing a trace of blocks and incrementing a
counter for every neighbouring pair of blocks in the trace. The
trace 0 3 1 2 1 2 4 displays the following frequencies:
Jump from block 0x0 to block 0x3 executed 1 time(s)
Jump from block 0x3 to block 0x1 executed 1 time(s)
Jump from block 0x1 to block 0x2 executed 2 time(s)
Jump from block 0x2 to block 0x1 executed 1 time(s)
Jump from block 0x2 to block 0x4 executed 1 time(s)
With __bb_hidecall__, control transfer due to call instructions is
removed from the trace, that is the trace is cut into three parts:
0 3 4, 0 1 2 and 0 1 2. With __bb_showret__, control transfer due
to return instructions is added to the trace. The trace becomes:
0 3 1 2 3 1 2 3 4. Note, that this trace is not the same, as the
sequence written to 'bbtrace.gz'. It is solely used for counting
jump frequencies.
-fprofile-arcs
Instrument arcs during compilation. For each function of your
program, GNU CC creates a program flow graph, then finds a spanning
tree for the graph. Only arcs that are not on the spanning tree
have to be instrumented: the compiler adds code to count the number
of times that these arcs are executed. When an arc is the only exit
or only entrance to a block, the instrumentation code can be added
to the block; otherwise, a new basic block must be created to hold
the instrumentation code.
Since not every arc in the program must be instrumented, programs
compiled with this option run faster than programs compiled with
'-a', which adds instrumentation code to every basic block in the
program. The tradeoff: since gcov does not have execution counts
for all branches, it must start with the execution counts for the
instrumented branches, and then iterate over the program flow graph
until the entire graph has been solved. Hence, gcov runs a little
more slowly than a program which uses information from '-a'.
'-fprofile-arcs' also makes it possible to estimate branch
probabilities, and to calculate basic block execution counts. In
general, basic block execution counts do not give enough information
to estimate all branch probabilities. When the compiled program
exits, it saves the arc execution counts to a file called
'sourcename.da'. Use the compiler option '-fbranch-probabilities'
(see Options that Control Optimization) when recompiling, to
optimize using estimated branch probabilities.
-ftest-coverage
Create data files for the gcov code-coverage utility (see gcov: a
GNU CC Test Coverage Program). The data file names begin with the
name of your source file:
sourcename.bb
A mapping from basic blocks to line numbers, which
gcov uses to associate basic block execution counts
with line numbers.
sourcename.bbg
A list of all arcs in the program flow graph. This
allows gcov to reconstruct the program flow graph, so
that it can compute all basic block and arc execution
counts from the information in the sourcename.da file
(this last file is the output from '-fprofile-arcs').
-Q
Makes the compiler print out each function name as it is compiled,
and print some statistics about each pass when it finishes.
-dletters
Says to make debugging dumps during compilation at times specified
by letters. This is used for debugging the compiler. The file
names for most of the dumps are made by appending a word to the
source file name (e.g. 'foo.c.rtl' or 'foo.c.jump'). Here are the
possible letters for use in letters, and their meanings:
'M'
Dump all macro definitions, at the end of
preprocessing, and write no output.
'N'
Dump all macro names, at the end of preprocessing.
'D'
Dump all macro definitions, at the end of
preprocessing, in addition to normal output.
'y'
Dump debugging information during parsing, to standard
error.
'r'
Dump after RTL generation, to 'file.rtl'.
'x'
Just generate RTL for a function instead of compiling
it. Usually used with 'r'.
'j'
Dump after first jump optimization, to 'file.jump'.
's'
Dump after CSE (including the jump optimization that
sometimes follows CSE), to 'file.cse'.
'D'
Dump after purging ADDRESSOF, to 'file.addressof'.
'L'
Dump after loop optimization, to 'file.loop'.
't'
Dump after the second CSE pass (including the jump
optimization that sometimes follows CSE), to
'file.cse2'.
'b'
Dump after computing branch probabilities, to
'file.bp'.
'f'
Dump after flow analysis, to 'file.flow'.
'c'
Dump after instruction combination, to the file
'file.combine'.
'S'
Dump after the first instruction scheduling pass, to
'file.sched'.
'l'
Dump after local register allocation, to 'file.lreg'.
'g'
Dump after global register allocation, to 'file.greg'.
'R'
Dump after the second instruction scheduling pass, to
'file.sched2'.
'J'
Dump after last jump optimization, to 'file.jump2'.
'd'
Dump after delayed branch scheduling, to 'file.dbr'.
'k'
Dump after conversion from registers to stack, to
'file.stack'.
'a'
Produce all the dumps listed above.
'm'
Print statistics on memory usage, at the end of the
run, to standard error.
'p'
Annotate the assembler output with a comment
indicating which pattern and alternative was used.
'A'
Annotate the assembler output with miscellaneous
debugging information.
-fpretend-float
When running a cross-compiler, pretend that the target machine uses
the same floating point format as the host machine. This causes
incorrect output of the actual floating constants, but the actual
instruction sequence will probably be the same as GNU CC would make
when running on the target machine.
-save-temps
Store the usual ``temporary'' intermediate files permanently; place
them in the current directory and name them based on the source
file. Thus, compiling 'foo.c' with '-c -save-temps' would produce
files 'foo.i' and 'foo.s', as well as 'foo.o'.
-print-file-name=library
Print the full absolute name of the library file library that would
be used when linking---and don't do anything else. With this
option, GNU CC does not compile or link anything; it just prints the
file name.
-print-prog-name=program
Like '-print-file-name', but searches for a program such as 'cpp'.
-print-libgcc-file-name
Same as '-print-file-name=libgcc.a'.
This is useful when you use '-nostdlib' or '-nodefaultlibs' but you
do want to link with 'libgcc.a'. You can do
gcc -nostdlib files┬╖┬╖┬╖ `gcc -print-libgcc-file-name`
-print-search-dirs
Print the name of the configured installation directory and a list
of program and library directories gcc will search---and don't do
anything else.
This is useful when gcc prints the error message 'installation
problem, cannot exec cpp: No such file or directory'. To resolve
this you either need to put 'cpp' and the other compiler components
where gcc expects to find them, or you can set the environment
variable GCC_EXEC_PREFIX to the directory where you installed them.
Don't forget the trailing '/'. See Environment Variables.
ΓòÉΓòÉΓòÉ 4.8. Options That Control Optimization ΓòÉΓòÉΓòÉ
These options control various sorts of optimizations:
-O
-O1
Optimize. Optimizing compilation takes somewhat more time, and a
lot more memory for a large function.
Without '-O', the compiler's goal is to reduce the cost of
compilation and to make debugging produce the expected results.
Statements are independent: if you stop the program with a
breakpoint between statements, you can then assign a new value to
any variable or change the program counter to any other statement in
the function and get exactly the results you would expect from the
source code.
Without '-O', the compiler only allocates variables declared
register in registers. The resulting compiled code is a little
worse than produced by PCC without '-O'.
With '-O', the compiler tries to reduce code size and execution
time.
When you specify '-O', the compiler turns on '-fthread-jumps' and
'-fdefer-pop' on all machines. The compiler turns on
'-fdelayed-branch' on machines that have delay slots, and
'-fomit-frame-pointer' on machines that can support debugging even
without a frame pointer. On some machines the compiler also turns
on other flags.
-O2
Optimize even more. GNU CC performs nearly all supported
optimizations that do not involve a space-speed tradeoff. The
compiler does not perform loop unrolling or function inlining when
you specify '-O2'. As compared to '-O', this option increases both
compilation time and the performance of the generated code.
'-O2' turns on all optional optimizations except for loop unrolling
and function inlining. It also turns on the '-fforce-mem' option on
all machines and frame pointer elimination on machines where doing
so does not interfere with debugging.
-O3
Optimize yet more. '-O3' turns on all optimizations specified by
'-O2' and also turns on the 'inline-functions' option.
-O0
Do not optimize.
If you use multiple '-O' options, with or without level numbers, the
last such option is the one that is effective.
Options of the form '-fflag' specify machine-independent flags. Most flags
have both positive and negative forms; the negative form of '-ffoo' would be
'-fno-foo'. In the table below, only one of the forms is listed---the one
which is not the default. You can figure out the other form by either removing
'no-' or adding it.
-ffloat-store
Do not store floating point variables in registers, and inhibit
other options that might change whether a floating point value is
taken from a register or memory.
This option prevents undesirable excess precision on machines such
as the 68000 where the floating registers (of the 68881) keep more
precision than a double is supposed to have. Similarly for the x86
architecture. For most programs, the excess precision does only
good, but a few programs rely on the precise definition of IEEE
floating point. Use '-ffloat-store' for such programs.
-fno-default-inline
Do not make member functions inline by default merely because they
are defined inside the class scope (C++ only). Otherwise, when you
specify '-O', member functions defined inside class scope are
compiled inline by default; i.e., you don't need to add 'inline' in
front of the member function name.
-fno-defer-pop
Always pop the arguments to each function call as soon as that
function returns. For machines which must pop arguments after a
function call, the compiler normally lets arguments accumulate on
the stack for several function calls and pops them all at once.
-fforce-mem
Force memory operands to be copied into registers before doing
arithmetic on them. This produces better code by making all memory
references potential common subexpressions. When they are not
common subexpressions, instruction combination should eliminate the
separate register-load. The '-O2' option turns on this option.
-fforce-addr
Force memory address constants to be copied into registers before
doing arithmetic on them. This may produce better code just as
'-fforce-mem' may.
-fomit-frame-pointer
Don't keep the frame pointer in a register for functions that don't
need one. This avoids the instructions to save, set up and restore
frame pointers; it also makes an extra register available in many
functions. It also makes debugging impossible on some machines.
On some machines, such as the Vax, this flag has no effect, because
the standard calling sequence automatically handles the frame
pointer and nothing is saved by pretending it doesn't exist. The
machine-description macro FRAME_POINTER_REQUIRED controls whether a
target machine supports this flag. See Section Register Usage of
Using and Porting GCC.
-fno-inline
Don't pay attention to the inline keyword. Normally this option is
used to keep the compiler from expanding any functions inline. Note
that if you are not optimizing, no functions can be expanded inline.
-finline-functions
Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be worth
integrating in this way.
If all calls to a given function are integrated, and the function is
declared static, then the function is normally not output as
assembler code in its own right.
-fkeep-inline-functions
Even if all calls to a given function are integrated, and the
function is declared static, nevertheless output a separate run-time
callable version of the function. This switch does not affect
extern inline functions.
-fkeep-static-consts
Emit variables declared static const when optimization isn't turned
on, even if the variables aren't referenced.
GNU CC enables this option by default. If you want to force the
compiler to check if the variable was referenced, regardless of
whether or not optimization is turned on, use the
'-fno-keep-static-consts' option.
-fno-function-cse
Do not put function addresses in registers; make each instruction
that calls a constant function contain the function's address
explicitly.
This option results in less efficient code, but some strange hacks
that alter the assembler output may be confused by the optimizations
performed when this option is not used.
-ffast-math
This option allows GCC to violate some ANSI or IEEE rules and/or
specifications in the interest of optimizing code for speed. For
example, it allows the compiler to assume arguments to the sqrt
function are non-negative numbers and that no floating-point values
are NaNs.
This option should never be turned on by any '-O' option since it
can result in incorrect output for programs which depend on an exact
implementation of IEEE or ANSI rules/specifications for math
functions.
The following options control specific optimizations. The '-O2' option turns
on all of these optimizations except '-funroll-loops' and
'-funroll-all-loops'. On most machines, the '-O' option turns on the
'-fthread-jumps' and '-fdelayed-branch' options, but specific machines may
handle it differently.
You can use the following flags in the rare cases when ``fine-tuning'' of
optimizations to be performed is desired.
-fstrength-reduce
Perform the optimizations of loop strength reduction and elimination
of iteration variables.
-fthread-jumps
Perform optimizations where we check to see if a jump branches to a
location where another comparison subsumed by the first is found.
If so, the first branch is redirected to either the destination of
the second branch or a point immediately following it, depending on
whether the condition is known to be true or false.
-fcse-follow-jumps
In common subexpression elimination, scan through jump instructions
when the target of the jump is not reached by any other path. For
example, when CSE encounters an if statement with an else clause,
CSE will follow the jump when the condition tested is false.
-fcse-skip-blocks
This is similar to '-fcse-follow-jumps', but causes CSE to follow
jumps which conditionally skip over blocks. When CSE encounters a
simple if statement with no else clause, '-fcse-skip-blocks' causes
CSE to follow the jump around the body of the if.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations has
been performed.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively
expensive.
-fdelayed-branch
If supported for the target machine, attempt to reorder instructions
to exploit instruction slots available after delayed branch
instructions.
-fschedule-insns
If supported for the target machine, attempt to reorder instructions
to eliminate execution stalls due to required data being
unavailable. This helps machines that have slow floating point or
memory load instructions by allowing other instructions to be issued
until the result of the load or floating point instruction is
required.
-fschedule-insns2
Similar to '-fschedule-insns', but requests an additional pass of
instruction scheduling after register allocation has been done.
This is especially useful on machines with a relatively small number
of registers and where memory load instructions take more than one
cycle.
-ffunction-sections
Place each function into its own section in the output file if the
target supports arbitrary sections. The function's name determines
the section's name in the output file.
Use this option on systems where the linker can perform
optimizations to improve locality of reference in the instruction
space. HPPA processors running HP-UX and Sparc processors running
Solaris 2 have linkers with such optimizations. Other systems using
the ELF object format as well as AIX may have these optimizations in
the future.
Only use this option when there are significant benefits from doing
so. When you specify this option, the assembler and linker will
create larger object and executable files and will also be slower.
You will not be able to use gprof on all systems if you specify this
option and you may have problems with debugging if you specify both
this option and '-g'.
-fcaller-saves
Enable values to be allocated in registers that will be clobbered by
function calls, by emitting extra instructions to save and restore
the registers around such calls. Such allocation is done only when
it seems to result in better code than would otherwise be produced.
This option is enabled by default on certain machines, usually those
which have no call-preserved registers to use instead.
-funroll-loops
Perform the optimization of loop unrolling. This is only done for
loops whose number of iterations can be determined at compile time
or run time. '-funroll-loop' implies both '-fstrength-reduce' and
'-frerun-cse-after-loop'.
-funroll-all-loops
Perform the optimization of loop unrolling. This is done for all
loops and usually makes programs run more slowly.
'-funroll-all-loops' implies '-fstrength-reduce' as well as
'-frerun-cse-after-loop'.
-fno-peephole
Disable any machine-specific peephole optimizations.
-fbranch-probabilities
After running a program compiled with '-fprofile-arcs' (see Options
for Debugging Your Program or gcc), you can compile it a second time
using '-fbranch-probabilities', to improve optimizations based on
guessing the path a branch might take.
ΓòÉΓòÉΓòÉ 4.9. Options Controlling the Preprocessor ΓòÉΓòÉΓòÉ
These options control the C preprocessor, which is run on each C source file
before actual compilation.
If you use the '-E' option, nothing is done except preprocessing. Some of
these options make sense only together with '-E' because they cause the
preprocessor output to be unsuitable for actual compilation.
-include file
Process file as input before processing the regular input file. In
effect, the contents of file are compiled first. Any '-D' and '-U'
options on the command line are always processed before '-include
file', regardless of the order in which they are written. All the
'-include' and '-imacros' options are processed in the order in
which they are written.
-imacros file
Process file as input, discarding the resulting output, before
processing the regular input file. Because the output generated
from file is discarded, the only effect of '-imacros file' is to
make the macros defined in file available for use in the main input.
Any '-D' and '-U' options on the command line are always processed
before '-imacros file', regardless of the order in which they are
written. All the '-include' and '-imacros' options are processed in
the order in which they are written.
-idirafter dir
Add the directory dir to the second include path. The directories
on the second include path are searched when a header file is not
found in any of the directories in the main include path (the one
that '-I' adds to).
-iprefix prefix
Specify prefix as the prefix for subsequent '-iwithprefix' options.
-iwithprefix dir
Add a directory to the second include path. The directory's name is
made by concatenating prefix and dir, where prefix was specified
previously with '-iprefix'. If you have not specified a prefix yet,
the directory containing the installed passes of the compiler is
used as the default.
-iwithprefixbefore dir
Add a directory to the main include path. The directory's name is
made by concatenating prefix and dir, as in the case of
'-iwithprefix'.
-isystem dir
Add a directory to the beginning of the second include path, marking
it as a system directory, so that it gets the same special treatment
as is applied to the standard system directories.
-nostdinc
Do not search the standard system directories for header files.
Only the directories you have specified with '-I' options (and the
current directory, if appropriate) are searched. See Directory
Options, for information on '-I'.
By using both '-nostdinc' and '-I-', you can limit the include-file
search path to only those directories you specify explicitly.
-undef
Do not predefine any nonstandard macros. (Including architecture
flags).
-E
Run only the C preprocessor. Preprocess all the C source files
specified and output the results to standard output or to the
specified output file.
-C
Tell the preprocessor not to discard comments. Used with the '-E'
option.
-P
Tell the preprocessor not to generate '#line' directives. Used with
the '-E' option.
-M
Tell the preprocessor to output a rule suitable for make describing
the dependencies of each object file. For each source file, the
preprocessor outputs one make-rule whose target is the object file
name for that source file and whose dependencies are all the
#include header files it uses. This rule may be a single line or
may be continued with '\'-newline if it is long. The list of rules
is printed on standard output instead of the preprocessed C program.
'-M' implies '-E'.
Another way to specify output of a make rule is by setting the
environment variable DEPENDENCIES_OUTPUT (see Environment
Variables).
-MM
Like '-M' but the output mentions only the user header files
included with '#include "file"'. System header files included with
'#include <file>' are omitted.
-MD
Like '-M' but the dependency information is written to a file made
by replacing ".c" with ".d" at the end of the input file names. This
is in addition to compiling the file as specified---'-MD' does not
inhibit ordinary compilation the way '-M' does.
In Mach, you can use the utility md to merge multiple dependency
files into a single dependency file suitable for using with the
'make' command.
-MMD
Like '-MD' except mention only user header files, not system header
files.
- MG
Treat missing header files as generated files and assume they live
in the same directory as the source file. If you specify '-MG', you
must also specify either '-M' or '-MM'. '-MG' is not supported with
'-MD' or '-MMD'.
-H
Print the name of each header file used, in addition to other normal
activities.
-Aquestion(answer)
Assert the answer answer for question, in case it is tested with a
preprocessing conditional such as '#if #question(answer)'. '-A-'
disables the standard assertions that normally describe the target
machine.
-Dmacro
Define macro macro with the string '1' as its definition.
-Dmacro=defn
Define macro macro as defn. All instances of '-D' on the command
line are processed before any '-U' options.
-Umacro
Undefine macro macro. '-U' options are evaluated after all '-D'
options, but before any '-include' and '-imacros' options.
-dM
Tell the preprocessor to output only a list of the macro definitions
that are in effect at the end of preprocessing. Used with the '-E'
option.
-dD
Tell the preprocessing to pass all macro definitions into the
output, in their proper sequence in the rest of the output.
-dN
Like '-dD' except that the macro arguments and contents are omitted.
Only '#define name' is included in the output.
-trigraphs
Support ANSI C trigraphs. The '-ansi' option also has this effect.
-Wp,option
Pass option as an option to the preprocessor. If option contains
commas, it is split into multiple options at the commas.
ΓòÉΓòÉΓòÉ 4.10. Passing Options to the Assembler ΓòÉΓòÉΓòÉ
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option contains
commas, it is split into multiple options at the commas.
ΓòÉΓòÉΓòÉ 4.11. Options for Linking ΓòÉΓòÉΓòÉ
These options come into play when the compiler links object files into an
executable output file. They are meaningless if the compiler is not doing a
link step.
object-file-name
A file name that does not end in a special recognized suffix is
considered to name an object file or library. (Object files are
distinguished from libraries by the linker according to the file
contents.) If linking is done, these object files are used as input
to the linker.
-c
-S
-E
If any of these options is used, then the linker is not run, and
object file names should not be used as arguments. See Overall
Options.
-llibrary
Search the library named library when linking.
It makes a difference where in the command you write this option;
the linker searches processes libraries and object files in the
order they are specified. Thus, 'foo.o -lz bar.o' searches library
'z' after file 'foo.o' but before 'bar.o'. If 'bar.o' refers to
functions in 'z', those functions may not be loaded.
The linker searches a standard list of directories for the library,
which is actually a file named 'liblibrary.a'. The linker then uses
this file as if it had been specified precisely by name.
The directories searched include several standard system directories
plus any that you specify with '-L'.
Normally the files found this way are library files---archive files
whose members are object files. The linker handles an archive file
by scanning through it for members which define symbols that have so
far been referenced but not defined. But if the file that is found
is an ordinary object file, it is linked in the usual fashion. The
only difference between using an '-l' option and specifying a file
name is that '-l' surrounds library with 'lib' and '.a' and searches
several directories.
-lobjc
You need this special case of the '-l' option in order to link an
Objective C program.
-nostartfiles
Do not use the standard system startup files when linking. The
standard system libraries are used normally, unless -nostdlib or
-nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the
libraries you specify will be passed to the linker. The standard
startup files are used normally, unless -nostartfiles is used.
-nostdlib
Do not use the standard system startup files or libraries when
linking. No startup files and only the libraries you specify will be
passed to the linker.
One of the standard libraries bypassed by '-nostdlib' and
'-nodefaultlibs' is 'libgcc.a', a library of internal subroutines
that GNU CC uses to overcome shortcomings of particular machines, or
special needs for some languages. (See Section Interfacing to GNU CC
Output of Porting GNU CC, for more discussion of 'libgcc.a'.) In
most cases, you need 'libgcc.a' even when you want to avoid other
standard libraries. In other words, when you specify '-nostdlib' or
'-nodefaultlibs' you should usually specify '-lgcc' as well. This
ensures that you have no unresolved references to internal GNU CC
library subroutines. (For example, '__main', used to ensure C++
constructors will be called; see collect2.)
-s
Remove all symbol table and relocation information from the
executable.
-static
On systems that support dynamic linking, this prevents linking with
the shared libraries. On other systems, this option has no effect.
-shared
Produce a shared object which can then be linked with other objects
to form an executable. Not all systems support this option. You
must also specify '-fpic' or '-fPIC' on some systems when you
specify this option.
-symbolic
Bind references to global symbols when building a shared object.
Warn about any unresolved references (unless overridden by the link
editor option '-Xlinker -z -Xlinker defs'). Only a few systems
support this option.
-Xlinker option
Pass option as an option to the linker. You can use this to supply
system-specific linker options which GNU CC does not know how to
recognize.
If you want to pass an option that takes an argument, you must use
'-Xlinker' twice, once for the option and once for the argument. For
example, to pass '-assert definitions', you must write '-Xlinker
-assert -Xlinker definitions'. It does not work to write '-Xlinker
"-assert definitions"', because this passes the entire string as a
single argument, which is not what the linker expects.
-Wl,option
Pass option as an option to the linker. If option contains commas,
it is split into multiple options at the commas.
-u symbol
Pretend the symbol symbol is undefined, to force linking of library
modules to define it. You can use '-u' multiple times with
different symbols to force loading of additional library modules.
ΓòÉΓòÉΓòÉ 4.12. Options for Directory Search ΓòÉΓòÉΓòÉ
These options specify directories to search for header files, for libraries
and for parts of the compiler:
-Idir
Add the directory dir to the head of the list of directories to be
searched for header files. This can be used to override a system
header file, substituting your own version, since these directories
are searched before the system header file directories. If you use
more than one '-I' option, the directories are scanned in
left-to-right order; the standard system directories come after.
-I-
Any directories you specify with '-I' options before the '-I-'
option are searched only for the case of '#include "file"'; they are
not searched for '#include <file>'.
If additional directories are specified with '-I' options after the
'-I-', these directories are searched for all '#include' directives.
(Ordinarily all '-I' directories are used this way.)
In addition, the '-I-' option inhibits the use of the current
directory (where the current input file came from) as the first
search directory for '#include "file"'. There is no way to override
this effect of '-I-'. With '-I.' you can specify searching the
directory which was current when the compiler was invoked. That is
not exactly the same as what the preprocessor does by default, but
it is often satisfactory.
'-I-' does not inhibit the use of the standard system directories
for header files. Thus, '-I-' and '-nostdinc' are independent.
-Ldir
Add directory dir to the list of directories to be searched for
'-l'.
-Bprefix
This option specifies where to find the executables, libraries,
include files, and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms
'cpp', 'cc1', 'as' and 'ld'. It tries prefix as a prefix for each
program it tries to run, both with and without 'machine/version/'
(see Target Options).
For each subprogram to be run, the compiler driver first tries the
'-B' prefix, if any. If that name is not found, or if '-B' was not
specified, the driver tries two standard prefixes, which are
'/usr/lib/gcc/' and '/usr/local/lib/gcc-lib/'. If neither of those
results in a file name that is found, the unmodified program name is
searched for using the directories specified in your 'PATH'
environment variable.
'-B' prefixes that effectively specify directory names also apply to
libraries in the linker, because the compiler translates these
options into '-L' options for the linker. They also apply to
includes files in the preprocessor, because the compiler translates
these options into '-isystem' options for the preprocessor. In this
case, the compiler appends 'include' to the prefix.
The run-time support file 'libgcc.a' can also be searched for using
the '-B' prefix, if needed. If it is not found there, the two
standard prefixes above are tried, and that is all. The file is
left out of the link if it is not found by those means.
Another way to specify a prefix much like the '-B' prefix is to use
the environment variable GCC_EXEC_PREFIX. See Environment
Variables.
-specs=file
Process file after the compiler reads in the standard 'specs' file,
in order to override the defaults that the 'gcc' driver program uses
when determining what switches to pass to 'cc1', 'cc1plus', 'as',
'ld', etc. More than one '-specs='file can be specified on the
command line, and they are processed in order, from left to right.
ΓòÉΓòÉΓòÉ 4.13. Specifying Target Machine and Compiler Version ΓòÉΓòÉΓòÉ
By default, GNU CC compiles code for the same type of machine that you are
using. However, it can also be installed as a cross-compiler, to compile for
some other type of machine. In fact, several different configurations of GNU
CC, for different target machines, can be installed side by side. Then you
specify which one to use with the '-b' option.
In addition, older and newer versions of GNU CC can be installed side by side.
One of them (probably the newest) will be the default, but you may sometimes
wish to use another.
-b machine
The argument machine specifies the target machine for compilation.
This is useful when you have installed GNU CC as a cross-compiler.
The value to use for machine is the same as was specified as the
machine type when configuring GNU CC as a cross-compiler. For
example, if a cross-compiler was configured with 'configure i386v',
meaning to compile for an 80386 running System V, then you would
specify '-b i386v' to run that cross compiler.
When you do not specify '-b', it normally means to compile for the
same type of machine that you are using.
-V version
The argument version specifies which version of GNU CC to run. This
is useful when multiple versions are installed. For example,
version might be '2.0', meaning to run GNU CC version 2.0.
The default version, when you do not specify '-V', is the last
version of GNU CC that you installed.
The '-b' and '-V' options actually work by controlling part of the file name
used for the executable files and libraries used for compilation. A given
version of GNU CC, for a given target machine, is normally kept in the
directory '/usr/local/lib/gcc-lib/machine/version'.
Thus, sites can customize the effect of '-b' or '-V' either by changing the
names of these directories or adding alternate names (or symbolic links). If
in directory '/usr/local/lib/gcc-lib/' the file '80386' is a link to the file
'i386v', then '-b 80386' becomes an alias for '-b i386v'.
In one respect, the '-b' or '-V' do not completely change to a different
compiler: the top-level driver program gcc that you originally invoked
continues to run and invoke the other executables (preprocessor, compiler per
se, assembler and linker) that do the real work. However, since no real work
is done in the driver program, it usually does not matter that the driver
program in use is not the one for the specified target and version.
The only way that the driver program depends on the target machine is in the
parsing and handling of special machine-specific options. However, this is
controlled by a file which is found, along with the other executables, in the
directory for the specified version and target machine. As a result, a single
installed driver program adapts to any specified target machine and compiler
version.
The driver program executable does control one significant thing, however: the
default version and target machine. Therefore, you can install different
instances of the driver program, compiled for different targets or versions,
under different names.
For example, if the driver for version 2.0 is installed as ogcc and that for
version 2.1 is installed as gcc, then the command gcc will use version 2.1 by
default, while ogcc will use 2.0 by default. However, you can choose either
version with either command with the '-V' option.
ΓòÉΓòÉΓòÉ 4.14. Hardware Models and Configurations ΓòÉΓòÉΓòÉ
Earlier we discussed the standard option '-b' which chooses among different
installed compilers for completely different target machines, such as Vax vs.
68000 vs. 80386.
In addition, each of these target machine types can have its own special
options, starting with '-m', to choose among various hardware models or
configurations---for example, 68010 vs 68020, floating coprocessor or none. A
single installed version of the compiler can compile for any model or
configuration, according to the options specified.
Some configurations of the compiler also support additional special options,
usually for compatibility with other compilers on the same platform.
M680x0 Options M680x0 Options
VAX Options VAX Options
SPARC Options SPARC Options
Convex Options Convex Options
AMD29K Options AMD29K Options
ARM Options ARM Options
MN10300 Options MN10300 Options
M32R/D Options M32R/D Options
M88K Options M88K Options
RS/6000 and PowerPC Options RS/6000 and PowerPC Options
RT Options RT Options
MIPS Options MIPS Options
i386 Options i386 Options
HPPA Options HPPA Options
Intel 960 Options Intel 960 Options
DEC Alpha Options DEC Alpha Options
Clipper Options Clipper Options
H8/300 Options H8/300 Options
SH Options SH Options
System V Options System V Options
V850 Options V850 Options
ΓòÉΓòÉΓòÉ 4.14.1. M680x0 Options ΓòÉΓòÉΓòÉ
These are the '-m' options defined for the 68000 series. The default values
for these options depends on which style of 68000 was selected when the
compiler was configured; the defaults for the most common choices are given
below.
-m68000
-mc68000
Generate output for a 68000. This is the default when the compiler
is configured for 68000-based systems.
-m68020
-mc68020
Generate output for a 68020. This is the default when the compiler
is configured for 68020-based systems.
-m68881
Generate output containing 68881 instructions for floating point.
This is the default for most 68020 systems unless '-nfp' was
specified when the compiler was configured.
-m68030
Generate output for a 68030. This is the default when the compiler
is configured for 68030-based systems.
-m68040
Generate output for a 68040. This is the default when the compiler
is configured for 68040-based systems.
This option inhibits the use of 68881/68882 instructions that have
to be emulated by software on the 68040. If your 68040 does not
have code to emulate those instructions, use '-m68040'.
-m68060
Generate output for a 68060. This is the default when the compiler
is configured for 68060-based systems.
This option inhibits the use of 68020 and 68881/68882 instructions
that have to be emulated by software on the 68060. If your 68060
does not have code to emulate those instructions, use '-m68060'.
-m5200
Generate output for a 520X "coldfire" family cpu. This is the
default when the compiler is configured for 520X-based systems.
-m68020-40
Generate output for a 68040, without using any of the new
instructions. This results in code which can run relatively
efficiently on either a 68020/68881 or a 68030 or a 68040. The
generated code does use the 68881 instructions that are emulated on
the 68040.
-m68020-60
Generate output for a 68060, without using any of the new
instructions. This results in code which can run relatively
efficiently on either a 68020/68881 or a 68030 or a 68040. The
generated code does use the 68881 instructions that are emulated on
the 68060.
-mfpa
Generate output containing Sun FPA instructions for floating point.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all m68k
targets. Normally the facilities of the machine's usual C compiler
are used, but this can't be done directly in cross-compilation. You
must make your own arrangements to provide suitable library
functions for cross-compilation. The embedded targets 'm68k-*-aout'
and 'm68k-*-coff' do provide software floating point support.
-mshort
Consider type int to be 16 bits wide, like short int.
-mnobitfield
Do not use the bit-field instructions. The '-m68000' option implies
'-mnobitfield'.
-mbitfield
Do use the bit-field instructions. The '-m68020' option implies
'-mbitfield'. This is the default if you use a configuration
designed for a 68020.
-mrtd
Use a different function-calling convention, in which functions that
take a fixed number of arguments return with the rtd instruction,
which pops their arguments while returning. This saves one
instruction in the caller since there is no need to pop the
arguments there.
This calling convention is incompatible with the one normally used
on Unix, so you cannot use it if you need to call libraries compiled
with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including printf); otherwise
incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
The rtd instruction is supported by the 68010, 68020, 68030, 68040,
and 68060 processors, but not by the 68000 or 5200.
-malign-int
-mno-align-int
Control whether GNU CC aligns int, long, long long, float, double,
and long double variables on a 32-bit boundary ('-malign-int') or a
16-bit boundary ('-mno-align-int'). Aligning variables on 32-bit
boundaries produces code that runs somewhat faster on processors
with 32-bit busses at the expense of more memory.
Warning: if you use the '-malign-int' switch, GNU CC will align
structures containing the above types differently than most
published application binary interface specifications for the m68k.
ΓòÉΓòÉΓòÉ 4.14.2. VAX Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for the Vax:
-munix
Do not output certain jump instructions (aobleq and so on) that the
Unix assembler for the Vax cannot handle across long ranges.
-mgnu
Do output those jump instructions, on the assumption that you will
assemble with the GNU assembler.
-mg
Output code for g-format floating point numbers instead of d-format.
ΓòÉΓòÉΓòÉ 4.14.3. SPARC Options ΓòÉΓòÉΓòÉ
These '-m' switches are supported on the SPARC:
-mno-app-regs
-mapp-regs
Specify '-mapp-regs' to generate output using the global registers 2
through 4, which the SPARC SVR4 ABI reserves for applications. This
is the default.
To be fully SVR4 ABI compliant at the cost of some performance loss,
specify '-mno-app-regs'. You should compile libraries and system
software with this option.
-mfpu
-mhard-float
Generate output containing floating point instructions. This is the
default.
-mno-fpu
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all SPARC
targets. Normally the facilities of the machine's usual C compiler
are used, but this cannot be done directly in cross-compilation.
You must make your own arrangements to provide suitable library
functions for cross-compilation. The embedded targets
'sparc-*-aout' and 'sparclite-*-*' do provide software floating
point support.
'-msoft-float' changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile 'libgcc.a', the
library that comes with GNU CC, with '-msoft-float' in order for
this to work.
-mhard-quad-float
Generate output containing quad-word (long double) floating point
instructions.
-msoft-quad-float
Generate output containing library calls for quad-word (long double)
floating point instructions. The functions called are those
specified in the SPARC ABI. This is the default.
As of this writing, there are no sparc implementations that have
hardware support for the quad-word floating point instructions.
They all invoke a trap handler for one of these instructions, and
then the trap handler emulates the effect of the instruction.
Because of the trap handler overhead, this is much slower than
calling the ABI library routines. Thus the '-msoft-quad-float'
option is the default.
-mno-epilogue
-mepilogue
With '-mepilogue' (the default), the compiler always emits code for
function exit at the end of each function. Any function exit in the
middle of the function (such as a return statement in C) will
generate a jump to the exit code at the end of the function.
With '-mno-epilogue', the compiler tries to emit exit code inline at
every function exit.
-mno-flat
-mflat
With '-mflat', the compiler does not generate save/restore
instructions and will use a "flat" or single register window calling
convention. This model uses %i7 as the frame pointer and is
compatible with the normal register window model. Code from either
may be intermixed. The local registers and the input registers (0-5)
are still treated as "call saved" registers and will be saved on the
stack as necessary.
With '-mno-flat' (the default), the compiler emits save/restore
instructions (except for leaf functions) and is the normal mode of
operation.
-mno-unaligned-doubles
-munaligned-doubles
Assume that doubles have 8 byte alignment. This is the default.
With '-munaligned-doubles', GNU CC assumes that doubles have 8 byte
alignment only if they are contained in another type, or if they
have an absolute address. Otherwise, it assumes they have 4 byte
alignment. Specifying this option avoids some rare compatibility
problems with code generated by other compilers. It is not the
default because it results in a performance loss, especially for
floating point code.
-mv8
-msparclite
These two options select variations on the SPARC architecture.
By default (unless specifically configured for the Fujitsu
SPARClite), GCC generates code for the v7 variant of the SPARC
architecture.
'-mv8' will give you SPARC v8 code. The only difference from v7
code is that the compiler emits the integer multiply and integer
divide instructions which exist in SPARC v8 but not in SPARC v7.
'-msparclite' will give you SPARClite code. This adds the integer
multiply, integer divide step and scan (ffs) instructions which
exist in SPARClite but not in SPARC v7.
These options are deprecated and will be deleted in GNU CC 2.9. They
have been replaced with '-mcpu=xxx'.
-mcypress
-msupersparc
These two options select the processor for which the code is
optimised.
With '-mcypress' (the default), the compiler optimizes code for the
Cypress CY7C602 chip, as used in the SparcStation/SparcServer 3xx
series. This is also appropriate for the older SparcStation 1, 2,
IPX etc.
With '-msupersparc' the compiler optimizes code for the SuperSparc
cpu, as used in the SparcStation 10, 1000 and 2000 series. This flag
also enables use of the full SPARC v8 instruction set.
These options are deprecated and will be deleted in GNU CC 2.9. They
have been replaced with '-mcpu=xxx'.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling
parameters for machine type cpu_type. Supported values for cpu_type
are 'v7', 'cypress', 'v8', 'supersparc', 'sparclite', 'f930',
'f934', 'sparclet', 'tsc701', 'v9', and 'ultrasparc'.
Default instruction scheduling parameters are used for values that
select an architecture and not an implementation. These are 'v7',
'v8', 'sparclite', 'sparclet', 'v9'.
Here is a list of each supported architecture and their supported
implementations.
v7: cypress
v8: supersparc
sparclite: f930, f934
sparclet: tsc701
v9: ultrasparc
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type,
but do not set the instruction set or register set that the option
'-mcpu='cpu_type would.
The same values for '-mcpu='cpu_type are used for '-mtune='cpu_type,
though the only useful values are those that select a particular cpu
implementation: 'cypress', 'supersparc', 'f930', 'f934', 'tsc701',
'ultrasparc'.
-malign-loops=num
Align loops to a 2 raised to a num byte boundary. If
'-malign-loops' is not specified, the default is 2.
-malign-jumps=num
Align instructions that are only jumped to to a 2 raised to a num
byte boundary. If '-malign-jumps' is not specified, the default is
2.
-malign-functions=num
Align the start of functions to a 2 raised to num byte boundary. If
'-malign-functions' is not specified, the default is 2 if compiling
for 32 bit sparc, and 5 if compiling for 64 bit sparc.
These '-m' switches are supported in addition to the above on the SPARCLET
processor.
-mlittle-endian
Generate code for a processor running in little-endian mode.
-mlive-g0
Treat register %g0 as a normal register. GCC will continue to
clobber it as necessary but will not assume it always reads as 0.
-mbroken-saverestore
Generate code that does not use non-trivial forms of the save and
restore instructions. Early versions of the SPARCLET processor do
not correctly handle save and restore instructions used with
arguments. They correctly handle them used without arguments. A
save instruction used without arguments increments the current
window pointer but does not allocate a new stack frame. It is
assumed that the window overflow trap handler will properly handle
this case as will interrupt handlers.
These '-m' switches are supported in addition to the above on SPARC V9
processors in 64 bit environments.
-mlittle-endian
Generate code for a processor running in little-endian mode.
-m32
-m64
Generate code for a 32 bit or 64 bit environment. The 32 bit
environment sets int, long and pointer to 32 bits. The 64 bit
environment sets int to 32 bits and long and pointer to 64 bits.
- mcmodel=medlow
Generate code for the Medium/Low code model: the program must be
linked in the low 32 bits of the address space. Pointers are 64
bits. Programs can be statically or dynamically linked.
-mcmodel=medmid
Generate code for the Medium/Middle code model: the program must be
linked in the low 44 bits of the address space, the text segment
must be less than 2G bytes, and data segment must be within 2G of
the text segment. Pointers are 64 bits.
-mcmodel=medany
Generate code for the Medium/Anywhere code model: the program may be
linked anywhere in the address space, the text segment must be less
than 2G bytes, and data segment must be within 2G of the text
segment. Pointers are 64 bits.
-mcmodel=embmedany
Generate code for the Medium/Anywhere code model for embedded
systems: assume a 32 bit text and a 32 bit data segment, both
starting anywhere (determined at link time). Register %g4 points to
the base of the data segment. Pointers still 64 bits. Programs are
statically linked, PIC is not supported.
-mstack-bias
-mno-stack-bias
With '-mstack-bias', GNU CC assumes that the stack pointer, and
frame pointer if present, are offset by -2047 which must be added
back when making stack frame references. Otherwise, assume no such
offset is present.
ΓòÉΓòÉΓòÉ 4.14.4. Convex Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for Convex:
-mc1
Generate output for C1. The code will run on any Convex machine.
The preprocessor symbol __convex__c1__ is defined.
-mc2
Generate output for C2. Uses instructions not available on C1.
Scheduling and other optimizations are chosen for max performance on
C2. The preprocessor symbol __convex_c2__ is defined.
-mc32
Generate output for C32xx. Uses instructions not available on C1.
Scheduling and other optimizations are chosen for max performance on
C32. The preprocessor symbol __convex_c32__ is defined.
-mc34
Generate output for C34xx. Uses instructions not available on C1.
Scheduling and other optimizations are chosen for max performance on
C34. The preprocessor symbol __convex_c34__ is defined.
-mc38
Generate output for C38xx. Uses instructions not available on C1.
Scheduling and other optimizations are chosen for max performance on
C38. The preprocessor symbol __convex_c38__ is defined.
-margcount
Generate code which puts an argument count in the word preceding
each argument list. This is compatible with regular CC, and a few
programs may need the argument count word. GDB and other
source-level debuggers do not need it; this info is in the symbol
table.
-mnoargcount
Omit the argument count word. This is the default.
-mvolatile-cache
Allow volatile references to be cached. This is the default.
-mvolatile-nocache
Volatile references bypass the data cache, going all the way to
memory. This is only needed for multi-processor code that does not
use standard synchronization instructions. Making non-volatile
references to volatile locations will not necessarily work.
-mlong32
Type long is 32 bits, the same as type int. This is the default.
-mlong64
Type long is 64 bits, the same as type long long. This option is
useless, because no library support exists for it.
ΓòÉΓòÉΓòÉ 4.14.5. AMD29K Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for the AMD Am29000:
-mdw
Generate code that assumes the DW bit is set, i.e., that byte and
halfword operations are directly supported by the hardware. This is
the default.
-mndw
Generate code that assumes the DW bit is not set.
-mbw
Generate code that assumes the system supports byte and halfword
write operations. This is the default.
-mnbw
Generate code that assumes the systems does not support byte and
halfword write operations. '-mnbw' implies '-mndw'.
-msmall
Use a small memory model that assumes that all function addresses
are either within a single 256 KB segment or at an absolute address
of less than 256k. This allows the call instruction to be used
instead of a const, consth, calli sequence.
-mnormal
Use the normal memory model: Generate call instructions only when
calling functions in the same file and calli instructions otherwise.
This works if each file occupies less than 256 KB but allows the
entire executable to be larger than 256 KB. This is the default.
-mlarge
Always use calli instructions. Specify this option if you expect a
single file to compile into more than 256 KB of code.
-m29050
Generate code for the Am29050.
-m29000
Generate code for the Am29000. This is the default.
-mkernel-registers
Generate references to registers gr64-gr95 instead of to registers
gr96-gr127. This option can be used when compiling kernel code that
wants a set of global registers disjoint from that used by user-mode
code.
Note that when this option is used, register names in '-f' flags
must use the normal, user-mode, names.
-muser-registers
Use the normal set of global registers, gr96-gr127. This is the
default.
-mstack-check
-mno-stack-check
Insert (or do not insert) a call to __msp_check after each stack
adjustment. This is often used for kernel code.
-mstorem-bug
-mno-storem-bug
'-mstorem-bug' handles 29k processors which cannot handle the
separation of a mtsrim insn and a storem instruction (most 29000
chips to date, but not the 29050).
-mno-reuse-arg-regs
-mreuse-arg-regs
'-mno-reuse-arg-regs' tells the compiler to only use incoming
argument registers for copying out arguments. This helps detect
calling a function with fewer arguments than it was declared with.
-mno-impure-text
-mimpure-text
'-mimpure-text', used in addition to '-shared', tells the compiler
to not pass '-assert pure-text' to the linker when linking a shared
object.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GNU CC. Normally
the facilities of the machine's usual C compiler are used, but this
can't be done directly in cross-compilation. You must make your own
arrangements to provide suitable library functions for
cross-compilation.
ΓòÉΓòÉΓòÉ 4.14.6. ARM Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for Advanced RISC Machines (ARM) architectures:
-mapcs-frame
Generate a stack frame that is compliant with the ARM Procedure Call
Standard for all functions, even if this is not strictly necessary
for correct execution of the code.
-mapcs-26
Generate code for a processor running with a 26-bit program counter,
and conforming to the function calling standards for the APCS 26-bit
option. This option replaces the '-m2' and '-m3' options of
previous releases of the compiler.
-mapcs-32
Generate code for a processor running with a 32-bit program counter,
and conforming to the function calling standards for the APCS 32-bit
option. This option replaces the '-m6' option of previous releases
of the compiler.
-mhard-float
Generate output containing floating point instructions. This is the
default.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all ARM
targets. Normally the facilities of the machine's usual C compiler
are used, but this cannot be done directly in cross-compilation.
You must make your own arrangements to provide suitable library
functions for cross-compilation.
'-msoft-float' changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile 'libgcc.a', the
library that comes with GNU CC, with '-msoft-float' in order for
this to work.
-mlittle-endian
Generate code for a processor running in little-endian mode. This
is the default for all standard configurations.
-mbig-endian
Generate code for a processor running in big-endian mode; the
default is to compile code for a little-endian processor.
-mwords-little-endian
This option only applies when generating code for big-endian
processors. Generate code for a little-endian word order but a
big-endian byte order. That is, a byte order of the form
'32107654'. Note: this option should only be used if you require
compatibility with code for big-endian ARM processors generated by
versions of the compiler prior to 2.8.
-mshort-load-bytes
Do not try to load half-words (eg 'short's) by loading a word from
an unaligned address. For some targets the MMU is configured to
trap unaligned loads; use this option to generate code that is safe
in these environments.
-mno-short-load-bytes
Use unaligned word loads to load half-words (eg 'short's). This
option produces more efficient code, but the MMU is sometimes
configured to trap these instructions.
-mbsd
This option only applies to RISC iX. Emulate the native BSD-mode
compiler. This is the default if '-ansi' is not specified.
-mxopen
This option only applies to RISC iX. Emulate the native X/Open-mode
compiler.
-mno-symrename
This option only applies to RISC iX. Do not run the assembler
post-processor, 'symrename', after code has been assembled. Normally
it is necessary to modify some of the standard symbols in
preparation for linking with the RISC iX C library; this option
suppresses this pass. The post-processor is never run when the
compiler is built for cross-compilation.
ΓòÉΓòÉΓòÉ 4.14.7. MN10300 Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for Matsushita MN10300 architectures:
-mmult-bug
Generate code to avoid bugs in the multiply instructions for the
MN10300 processors. This is the default.
-mno-mult-bug
Do not generate code to avoid bugs in the multiply instructions for
the MN10300 processors.
ΓòÉΓòÉΓòÉ 4.14.8. M32R/D Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for Mitsubishi M32R/D architectures:
-mcode-model=small
Assume all objects live in the lower 16MB of memory (so that their
addresses can be loaded with the ld24 instruction), and assume all
subroutines are reachable with the bl instruction. This is the
default.
The addressability of a particular object can be set with the model
attribute.
-mcode-model=medium
Assume objects may be anywhere in the 32 bit address space (the
compiler will generate seth/add3 instructions to load their
addresses), and assume all subroutines are reachable with the bl
instruction.
-mcode-model=large
Assume objects may be anywhere in the 32 bit address space (the
compiler will generate seth/add3 instructions to load their
addresses), and assume subroutines may not be reachable with the bl
instruction (the compiler will generate the much slower seth/add3/jl
instruction sequence).
-msdata=none
Disable use of the small data area. Variables will be put into one
of '.data', 'bss', or '.rodata' (unless the section attribute has
been specified). This is the default.
The small data area consists of sections '.sdata' and '.sbss'.
Objects may be explicitly put in the small data area with the
section attribute using one of these sections.
-msdata=sdata
Put small global and static data in the small data area, but do not
generate special code to reference them.
-msdata=use
Put small global and static data in the small data area, and
generate special instructions to reference them.
-G num
Put global and static objects less than or equal to num bytes into
the small data or bss sections instead of the normal data or bss
sections. The default value of num is 8. The '-msdata' option must
be set to one of 'sdata' or 'use' for this option to have any
effect.
All modules should be compiled with the same '-G num' value.
Compiling with different values of num may or may not work; if it
doesn't the linker will give an error message - incorrect code will
not be generated.
-mlongcall
Normally the compiler produces single-instruction, 26 bit, direct
calls. In order to access functions that may lie anywhere in the 32
bit address space we need to call through a function pointer.
Because indirect calls are more expensive we would like to make
direct calls wherever possible. With '-mlongcall' the compiler uses
a conservative heuristic to decide whether to make a direct (26)
call or an indirect (32 bit) call: it generates a direct call if the
target function is non public; or if its definition has already been
seen; or if it is declared with the attribute "shortcall" (See
Function Attributes). Otherwise it generates an indirect call. An
underlying assumption is that individual translation units span less
than 32MB so that it is always safe to make direct calls to
functions in the same module.
Here is an example:
static void f ();
void g () { /* do something */ }
extern void h ();
void test ()
{
f ();
g ();
h ();
}
If this example is compiled with -mlongcall, the function 'test' will contain
direct calls to 'f' (non-public) and 'g' (definition seen before it is called)
and an indirect call to 'h'.
ΓòÉΓòÉΓòÉ 4.14.9. M88K Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for Motorola 88k architectures:
-m88000
Generate code that works well on both the m88100 and the m88110.
-m88100
Generate code that works best for the m88100, but that also runs on
the m88110.
-m88110
Generate code that works best for the m88110, and may not run on the
m88100.
-mbig-pic
Obsolete option to be removed from the next revision. Use '-fPIC'.
-midentify-revision
Include an ident directive in the assembler output recording the
source file name, compiler name and version, timestamp, and
compilation flags used.
-mno-underscores
In assembler output, emit symbol names without adding an underscore
character at the beginning of each name. The default is to use an
underscore as prefix on each name.
-mocs-debug-info
-mno-ocs-debug-info
Include (or omit) additional debugging information (about registers
used in each stack frame) as specified in the 88open Object
Compatibility Standard, ``OCS''. This extra information allows
debugging of code that has had the frame pointer eliminated. The
default for DG/UX, SVr4, and Delta 88 SVr3.2 is to include this
information; other 88k configurations omit this information by
default.
-mocs-frame-position
When emitting COFF debugging information for automatic variables and
parameters stored on the stack, use the offset from the canonical
frame address, which is the stack pointer (register 31) on entry to
the function. The DG/UX, SVr4, Delta88 SVr3.2, and BCS
configurations use '-mocs-frame-position'; other 88k configurations
have the default '-mno-ocs-frame-position'.
-mno-ocs-frame-position
When emitting COFF debugging information for automatic variables and
parameters stored on the stack, use the offset from the frame
pointer register (register 30). When this option is in effect, the
frame pointer is not eliminated when debugging information is
selected by the -g switch.
-moptimize-arg-area
-mno-optimize-arg-area
Control how function arguments are stored in stack frames.
'-moptimize-arg-area' saves space by optimizing them, but this
conflicts with the 88open specifications. The opposite alternative,
'-mno-optimize-arg-area', agrees with 88open standards. By default
GNU CC does not optimize the argument area.
-mshort-data-num
num Generate smaller data references by making them relative to r0,
which allows loading a value using a single instruction (rather than
the usual two). You control which data references are affected by
specifying num with this option. For example, if you specify
'-mshort-data-512', then the data references affected are those
involving displacements of less than 512 bytes. '-mshort-data-num'
is not effective for num greater than 64k.
-mserialize-volatile
-mno-serialize-volatile
Do, or don't, generate code to guarantee sequential consistency of
volatile memory references. By default, consistency is guaranteed.
The order of memory references made by the MC88110 processor does
not always match the order of the instructions requesting those
references. In particular, a load instruction may execute before a
preceding store instruction. Such reordering violates sequential
consistency of volatile memory references, when there are multiple
processors. When consistency must be guaranteed, GNU C generates
special instructions, as needed, to force execution in the proper
order.
The MC88100 processor does not reorder memory references and so
always provides sequential consistency. However, by default, GNU C
generates the special instructions to guarantee consistency even
when you use '-m88100', so that the code may be run on an MC88110
processor. If you intend to run your code only on the MC88100
processor, you may use '-mno-serialize-volatile'.
The extra code generated to guarantee consistency may affect the
performance of your application. If you know that you can safely
forgo this guarantee, you may use '-mno-serialize-volatile'.
-msvr4
-msvr3
Turn on ('-msvr4') or off ('-msvr3') compiler extensions related to
System V release 4 (SVr4). This controls the following:
.Which variant of the assembler syntax to emit.
.'-msvr4' makes the C preprocessor recognize '#pragma weak' that
is used on System V release 4.
.'-msvr4' makes GNU CC issue additional declaration directives
used in SVr4.
'-msvr4' is the default for the m88k-motorola-sysv4 and
m88k-dg-dgux m88k configurations. '-msvr3' is the default for all
other m88k configurations.
-mversion-03.00
This option is obsolete, and is ignored.
-mno-check-zero-division
-mcheck-zero-division
Do, or don't, generate code to guarantee that integer division by
zero will be detected. By default, detection is guaranteed.
Some models of the MC88100 processor fail to trap upon integer
division by zero under certain conditions. By default, when
compiling code that might be run on such a processor, GNU C
generates code that explicitly checks for zero-valued divisors and
traps with exception number 503 when one is detected. Use of
mno-check-zero-division suppresses such checking for code generated
to run on an MC88100 processor.
GNU C assumes that the MC88110 processor correctly detects all
instances of integer division by zero. When '-m88110' is specified,
both '-mcheck-zero-division' and '-mno-check-zero-division' are
ignored, and no explicit checks for zero-valued divisors are
generated.
-muse-div-instruction
Use the div instruction for signed integer division on the MC88100
processor. By default, the div instruction is not used.
On the MC88100 processor the signed integer division instruction
div) traps to the operating system on a negative operand. The
operating system transparently completes the operation, but at a
large cost in execution time. By default, when compiling code that
might be run on an MC88100 processor, GNU C emulates signed integer
division using the unsigned integer division instruction divu),
thereby avoiding the large penalty of a trap to the operating
system. Such emulation has its own, smaller, execution cost in both
time and space. To the extent that your code's important signed
integer division operations are performed on two nonnegative
operands, it may be desirable to use the div instruction directly.
On the MC88110 processor the div instruction (also known as the divs
instruction) processes negative operands without trapping to the
operating system. When '-m88110' is specified,
'-muse-div-instruction' is ignored, and the div instruction is used
for signed integer division.
Note that the result of dividing INT_MIN by -1 is undefined. In
particular, the behavior of such a division with and without
'-muse-div-instruction' may differ.
-mtrap-large-shift
-mhandle-large-shift
Include code to detect bit-shifts of more than 31 bits;
respectively, trap such shifts or emit code to handle them properly.
By default GNU CC makes no special provision for large bit shifts.
-mwarn-passed-structs
Warn when a function passes a struct as an argument or result.
Structure-passing conventions have changed during the evolution of
the C language, and are often the source of portability problems.
By default, GNU CC issues no such warning.
ΓòÉΓòÉΓòÉ 4.14.10. IBM RS/6000 and PowerPC Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for the IBM RS/6000 and PowerPC:
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
GNU CC supports two related instruction set architectures for the
RS/6000 and PowerPC. The POWER instruction set are those
instructions supported by the 'rios' chip set used in the original
RS/6000 systems and the PowerPC instruction set is the architecture
of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and the IBM
4xx microprocessors.
Neither architecture is a subset of the other. However there is a
large common subset of instructions supported by both. An MQ
register is included in processors supporting the POWER
architecture.
You use these options to specify which instructions are available on
the processor you are using. The default value of these options is
determined when configuring GNU CC. Specifying the '-mcpu=cpu_type'
overrides the specification of these options. We recommend you use
the '-mcpu=cpu_type' option rather than the options listed above.
The '-mpower' option allows GNU CC to generate instructions that are
found only in the POWER architecture and to use the MQ register.
Specifying '-mpower2' implies '-power' and also allows GNU CC to
generate instructions that are present in the POWER2 architecture
but not the original POWER architecture.
The '-mpowerpc' option allows GNU CC to generate instructions that
are found only in the 32-bit subset of the PowerPC architecture.
Specifying '-mpowerpc-gpopt' implies '-mpowerpc' and also allows GNU
CC to use the optional PowerPC architecture instructions in the
General Purpose group, including floating-point square root.
Specifying '-mpowerpc-gfxopt' implies '-mpowerpc' and also allows
GNU CC to use the optional PowerPC architecture instructions in the
Graphics group, including floating-point select.
If you specify both '-mno-power' and '-mno-powerpc', GNU CC will use
only the instructions in the common subset of both architectures
plus some special AIX common-mode calls, and will not use the MQ
register. Specifying both '-mpower' and '-mpowerpc' permits GNU CC
to use any instruction from either architecture and to allow use of
the MQ register; specify this for the Motorola MPC601.
-mnew-mnemonics
-mold-mnemonics
Select which mnemonics to use in the generated assembler code.
'-mnew-mnemonics' requests output that uses the assembler mnemonics
defined for the PowerPC architecture, while '-mold-mnemonics'
requests the assembler mnemonics defined for the POWER architecture.
Instructions defined in only one architecture have only one
mnemonic; GNU CC uses that mnemonic irrespective of which of these
options is specified.
PowerPC assemblers support both the old and new mnemonics, as will
later POWER assemblers. Current POWER assemblers only support the
old mnemonics. Specify '-mnew-mnemonics' if you have an assembler
that supports them, otherwise specify '-mold-mnemonics'.
The default value of these options depends on how GNU CC was
configured. Specifying '-mcpu=cpu_type' sometimes overrides the
value of these option. Unless you are building a cross-compiler,
you should normally not specify either '-mnew-mnemonics' or
'-mold-mnemonics', but should instead accept the default.
-mcpu=cpu_type
Set architecture type, register usage, choice of mnemonics, and
instruction scheduling parameters for machine type cpu_type.
Supported values for cpu_type are 'rs6000', 'rios1', 'rios2', 'rsc',
'601', '602', '603', '603e', '604', '604e', '620', 'power',
'power2', 'powerpc', '403', '505', '801', '821', '823', and '860'
and 'common'. '-mcpu=power', '-mcpu=power2', and '-mcpu=powerpc'
specify generic POWER, POWER2 and pure PowerPC (i.e., not MPC601)
architecture machine types, with an appropriate, generic processor
model assumed for scheduling purposes.
Specifying any of the following options: '-mcpu=rios1',
'-mcpu=rios2', '-mcpu=rsc', '-mcpu=power', or '-mcpu=power2' enables
the '-mpower' option and disables the '-mpowerpc' option;
'-mcpu=601' enables both the '-mpower' and '-mpowerpc' options. All
of '-mcpu=602', '-mcpu=603', '-mcpu=603e', '-mcpu=604', '-mcpu=620',
enable the '-mpowerpc' option and disable the '-mpower' option.
Exactly similarly, all of '-mcpu=403', '-mcpu=505', '-mcpu=821',
'-mcpu=860' and '-mcpu=powerpc' enable the '-mpowerpc' option and
disable the '-mpower' option. '-mcpu=common' disables both the
'-mpower' and '-mpowerpc' options. AIX versions 4 or greater selects
'-mcpu=common' by default, so that code will operate on all members
of the RS/6000 and PowerPC families. In that case, GNU CC will use
only the instructions in the common subset of both architectures
plus some special AIX common-mode calls, and will not use the MQ
register. GNU CC assumes a generic processor model for scheduling
purposes.
Specifying any of the options '-mcpu=rios1', '-mcpu=rios2',
'-mcpu=rsc', '-mcpu=power', or '-mcpu=power2' also disables the
'new-mnemonics' option. Specifying '-mcpu=601', '-mcpu=602',
'-mcpu=603', '-mcpu=603e', '-mcpu=604', '620', '403', or
'-mcpu=powerpc' also enables the 'new-mnemonics' option.
Specifying '-mcpu=403', '-mcpu=821', or '-mcpu=860' also enables the
'-msoft-float' option.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type,
but do not set the architecture type, register usage, choice of
mnemonics like '-mcpu='cpu_type would. The same values for cpu_type
are used for '-mtune='cpu_type as for '-mcpu='cpu_type. The
'-mtune='cpu_type option overrides the '-mcpu='cpu_type option in
terms of instruction scheduling parameters.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created
for every executable file. The '-mfull-toc' option is selected by
default. In that case, GNU CC will allocate at least one TOC entry
for each unique non-automatic variable reference in your program.
GNU CC will also place floating-point constants in the TOC.
However, only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have
overflowed the available TOC space, you can reduce the amount of TOC
space used with the '-mno-fp-in-toc' and '-mno-sum-in-toc' options.
'-mno-fp-in-toc' prevents GNU CC from putting floating-point
constants in the TOC and '-mno-sum-in-toc' forces GNU CC to generate
code to calculate the sum of an address and a constant at run-time
instead of putting that sum into the TOC. You may specify one or
both of these options. Each causes GNU CC to produce very slightly
slower and larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both
of these options, specify '-mminimal-toc' instead. This option
causes GNU CC to make only one TOC entry for every file. When you
specify this option, GNU CC will produce code that is slower and
larger but which uses extremely little TOC space. You may wish to
use this option only on files that contain less frequently executed
code.
-mxl-call
-mno-xl-call
On AIX, pass floating-point arguments to prototyped functions beyond
the register save area (RSA) on the stack in addition to argument
FPRs. The AIX calling convention was extended but not initially
documented to handle an obscure K&R C case of calling a function
that takes the address of its arguments with fewer arguments than
declared. AIX XL compilers assume that floating point arguments
which do not fit in the RSA are on the stack when they compile a
subroutine without optimization. Because always storing
floating-point arguments on the stack is inefficient and rarely
needed, this option is not enabled by default and only is necessary
when calling subroutines compiled by AIX XL compilers without
optimization.
-mthreads
Support AIX Threads. Link an application written to use pthreads
with special libraries and startup code to enable the application to
run.
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an
application written to use message passing with special startup code
to enable the application to run. The system must have PE installed
in the standard location ('/usr/lpp/ppe.poe/'), or the 'specs' file
must be overridden with the '-specs=' option to specify the
appropriate directory location. The Parallel Environment does not
support threads, so the '-mpe' option and the '-mthreads' option are
incompatible.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register
set. Software floating point emulation is provided if you use the
'-msoft-float' option, and pass the option to GNU CC when linking.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word
instructions and the store multiple word instructions. These
instructions are generated by default on POWER systems, and not
generated on PowerPC systems. Do not use '-mmultiple' on little
endian PowerPC systems, since those instructions do not work when
the processor is in little endian mode.
-mstring
-mno-string
Generate code that uses (does not use) the load string instructions
and the store string word instructions to save multiple registers
and do small block moves. These instructions are generated by
default on POWER systems, and not generated on PowerPC systems. Do
not use '-mstring' on little endian PowerPC systems, since those
instructions do not work when the processor is in little endian
mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store
instructions that update the base register to the address of the
calculated memory location. These instructions are generated by
default. If you use '-mno-update', there is a small window between
the time that the stack pointer is updated and the address of the
previous frame is stored, which means code that walks the stack
frame across interrupts or signals may get corrupted data.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating point multiply
and accumulate instructions. These instructions are generated by
default if hardware floating is used.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force
structures and unions that contain bit fields to be aligned to the
base type of the bit field.
For example, by default a structure containing nothing but 8
unsigned bitfields of length 1 would be aligned to a 4 byte boundary
and have a size of 4 bytes. By using '-mno-bit-align', the
structure would be aligned to a 1 byte boundary and be one byte in
size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that
unaligned memory references will be handled by the system.
-mrelocatable
-mno-relocatable
On embedded PowerPC systems generate code that allows (does not
allow) the program to be relocated to a different address at
runtime. If you use '-mrelocatable' on any module, all objects
linked together must be compiled with '-mrelocatable' or
'-mrelocatable-lib'.
-mrelocatable-lib
-mno-relocatable-lib
On embedded PowerPC systems generate code that allows (does not
allow) the program to be relocated to a different address at
runtime. Modules compiled with '-mrelocatable-lib' can be linked
with either modules compiled without '-mrelocatable' and
'-mrelocatable-lib' or with modules compiled with the
'-mrelocatable' options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that
register 2 contains a pointer to a global area pointing to the
addresses used in the program.
-mno-traceback
-mtraceback
On embedded PowerPC systems do not (do) generate a traceback tag
before the start of the function. This tag can be used by the
debugger to identify where the start of a function is.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the
processor in little endian mode. The '-mlittle-endian' option is
the same as '-mlittle'.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the
processor in big endian mode. The '-mbig-endian' option is the same
as '-mbig'.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using
calling conventions that adheres to the March 1995 draft of the
System V Application Binary Interface, PowerPC processor supplement.
This is the default unless you configured GCC using
'powerpc-*-eabiaix'.
-mcall-sysv-eabi
Specify both '-mcall-sysv' and '-meabi' options.
-mcall-sysv-noeabi
Specify both '-mcall-sysv' and '-mno-eabi' options.
-mcall-aix
On System V.4 and embedded PowerPC systems compile code using
calling conventions that are similar to those used on AIX. This is
the default if you configured GCC using 'powerpc-*-eabiaix'.
-mcall-solaris
On System V.4 and embedded PowerPC systems compile code for the
Solaris operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the
Linux-based GNU system.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to
variable argument functions are properly prototyped. Otherwise, the
compiler must insert an instruction before every non prototyped call
to set or clear bit 6 of the condition code register (CR) to
indicate whether floating point values were passed in the floating
point registers in case the function takes a variable arguments.
With '-mprototype', only calls to prototyped variable argument
functions will set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is
called 'sim-crt0.o' and that the standard C libraries are 'libsim.a'
and 'libc.a'. This is the default for 'powerpc-*-eabisim'.
configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is
called 'crt0.o' and the standard C libraries are 'libmvme.a' and
'libc.a'.
-mads
On embedded PowerPC systems, assume that the startup module is
called 'crt0.o' and the standard C libraries are 'libads.a' and
'libc.a'.
-myellowknife
On embedded PowerPC systems, assume that the startup module is
called 'crt0.o' and the standard C libraries are 'libyk.a' and
'libc.a'.
-memb
On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags
header to indicate that 'eabi' extended relocations are used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to the
Embedded Applications Binary Interface (eabi) which is a set of
modifications to the System V.4 specifications. Selecting -meabi
means that the stack is aligned to an 8 byte boundary, a function
__eabi is called to from main to set up the eabi environment, and
the '-msdata' option can use both r2 and r13 to point to two
separate small data areas. Selecting -mno-eabi means that the stack
is aligned to a 16 byte boundary, do not call an initialization
function from main, and the '-msdata' option will only use r13 to
point to a single small data area. The '-meabi' option is on by
default if you configured GCC using one of the 'powerpc*-*-eabi*'
options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized
const global and static data in the '.sdata2' section, which is
pointed to by register r2. Put small initialized non-const global
and static data in the '.sdata' section, which is pointed to by
register r13. Put small uninitialized global and static data in the
'.sbss' section, which is adjacent to the '.sdata' section. The
'-msdata=eabi' option is incompatible with the '-mrelocatable'
option. The '-msdata=eabi' option also sets the '-memb' option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and
static data in the '.sdata' section, which is pointed to by register
r13. Put small uninitialized global and static data in the '.sbss'
section, which is adjacent to the '.sdata' section. The
'-msdata=sysv' option is incompatible with the '-mrelocatable'
option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if '-meabi' is used,
compile code the same as '-msdata=eabi', otherwise compile code the
same as '-msdata=sysv'.
-msdata-data
On System V.4 and embedded PowerPC systems, put small global and
static data in the '.sdata' section. Put small uninitialized global
and static data in the '.sbss' section. Do not use register r13 to
address small data however. This is the default behavior unless
other '-msdata' options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static
data in the '.data' section, and all uninitialized data in the
'.bss' section.
-G num
On embedded PowerPC systems, put global and static items less than
or equal to num bytes into the small data or bss sections instead of
the normal data or bss section. By default, num is 8. The '-G num'
switch is also passed to the linker. All modules should be compiled
with the same '-G num' value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit register
names in the assembly language output using symbolic forms.
ΓòÉΓòÉΓòÉ 4.14.11. IBM RT Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for the IBM RT PC:
-min-line-mul
Use an in-line code sequence for integer multiplies. This is the
default.
-mcall-lib-mul
Call lmul$$ for integer multiples.
-mfull-fp-blocks
Generate full-size floating point data blocks, including the minimum
amount of scratch space recommended by IBM. This is the default.
-mminimum-fp-blocks
Do not include extra scratch space in floating point data blocks.
This results in smaller code, but slower execution, since scratch
space must be allocated dynamically.
-mfp-arg-in-fpregs
Use a calling sequence incompatible with the IBM calling convention
in which floating point arguments are passed in floating point
registers. Note that varargs.h and stdargs.h will not work with
floating point operands if this option is specified.
-mfp-arg-in-gregs
Use the normal calling convention for floating point arguments.
This is the default.
-mhc-struct-return
Return structures of more than one word in memory, rather than in a
register. This provides compatibility with the MetaWare HighC (hc)
compiler. Use the option '-fpcc-struct-return' for compatibility
with the Portable C Compiler (pcc).
-mnohc-struct-return
Return some structures of more than one word in registers, when
convenient. This is the default. For compatibility with the
IBM-supplied compilers, use the option '-fpcc-struct-return' or the
option '-mhc-struct-return'.
ΓòÉΓòÉΓòÉ 4.14.12. MIPS Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for the MIPS family of computers:
-mcpu=cpu type
Assume the defaults for the machine type cpu type when scheduling
instructions. The choices for cpu type are 'r2000', 'r3000',
'r4000', 'r4400', 'r4600', and 'r6000'. While picking a specific
cpu type will schedule things appropriately for that particular
chip, the compiler will not generate any code that does not meet
level 1 of the MIPS ISA (instruction set architecture) without the
'-mips2' or '-mips3' switches being used.
-mips1
Issue instructions from level 1 of the MIPS ISA. This is the
default. 'r3000' is the default cpu type at this ISA level.
-mips2
Issue instructions from level 2 of the MIPS ISA (branch likely,
square root instructions). 'r6000' is the default cpu type at this
ISA level.
-mips3
Issue instructions from level 3 of the MIPS ISA (64 bit
instructions). 'r4000' is the default cpu type at this ISA level.
This option does not change the sizes of any of the C data types.
-mfp32
Assume that 32 32-bit floating point registers are available. This
is the default.
-mfp64
Assume that 32 64-bit floating point registers are available. This
is the default when the '-mips3' option is used.
-mgp32
Assume that 32 32-bit general purpose registers are available. This
is the default.
-mgp64
Assume that 32 64-bit general purpose registers are available. This
is the default when the '-mips3' option is used.
-mint64
Types long, int, and pointer are 64 bits. This works only if
'-mips3' is also specified.
-mlong64
Types long and pointer are 64 bits, and type int is 32 bits. This
works only if '-mips3' is also specified.
-mmips-as
Generate code for the MIPS assembler, and invoke 'mips-tfile' to add
normal debug information. This is the default for all platforms
except for the OSF/1 reference platform, using the OSF/rose object
format. If the either of the '-gstabs' or '-gstabs+' switches are
used, the 'mips-tfile' program will encapsulate the stabs within
MIPS ECOFF.
-mgas
Generate code for the GNU assembler. This is the default on the
OSF/1 reference platform, using the OSF/rose object format. Also,
this is the default if the configure option '--with-gnu-as' is used.
-msplit-addresses
-mno-split-addresses
Generate code to load the high and low parts of address constants
separately. This allows gcc to optimize away redundant loads of the
high order bits of addresses. This optimization requires GNU as and
GNU ld. This optimization is enabled by default for some embedded
targets where GNU as and GNU ld are standard.
-mrnames
-mno-rnames
The '-mrnames' switch says to output code using the MIPS software
names for the registers, instead of the hardware names (ie, a0
instead of $4). The only known assembler that supports this option
is the Algorithmics assembler.
-mgpopt
-mno-gpopt
The '-mgpopt' switch says to write all of the data declarations
before the instructions in the text section, this allows the MIPS
assembler to generate one word memory references instead of using
two words for short global or static data items. This is on by
default if optimization is selected.
-mstats
-mno-stats
For each non-inline function processed, the '-mstats' switch causes
the compiler to emit one line to the standard error file to print
statistics about the program (number of registers saved, stack size,
etc.).
-mmemcpy
-mno-memcpy
The '-mmemcpy' switch makes all block moves call the appropriate
string function ('memcpy' or 'bcopy') instead of possibly generating
inline code.
-mmips-tfile
-mno-mips-tfile
The '-mno-mips-tfile' switch causes the compiler not postprocess the
object file with the 'mips-tfile' program, after the MIPS assembler
has generated it to add debug support. If 'mips-tfile' is not run,
then no local variables will be available to the debugger. In
addition, 'stage2' and 'stage3' objects will have the temporary file
names passed to the assembler embedded in the object file, which
means the objects will not compare the same. The '-mno-mips-tfile'
switch should only be used when there are bugs in the 'mips-tfile'
program that prevents compilation.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GNU CC. Normally
the facilities of the machine's usual C compiler are used, but this
can't be done directly in cross-compilation. You must make your own
arrangements to provide suitable library functions for
cross-compilation.
-mhard-float
Generate output containing floating point instructions. This is the
default if you use the unmodified sources.
-mabicalls
-mno-abicalls
Emit (or do not emit) the pseudo operations '.abicalls', '.cpload',
and '.cprestore' that some System V.4 ports use for position
independent code.
-mlong-calls
-mno-long-calls
Do all calls with the 'JALR' instruction, which requires loading up
a function's address into a register before the call. You need to
use this switch, if you call outside of the current 512 megabyte
segment to functions that are not through pointers.
-mhalf-pic
-mno-half-pic
Put pointers to extern references into the data section and load
them up, rather than put the references in the text section.
-membedded-pic
-mno-embedded-pic
Generate PIC code suitable for some embedded systems. All calls are
made using PC relative address, and all data is addressed using the
$gp register. This requires GNU as and GNU ld which do most of the
work.
-membedded-data
-mno-embedded-data
Allocate variables to the read-only data section first if possible,
then next in the small data section if possible, otherwise in data.
This gives slightly slower code than the default, but reduces the
amount of RAM required when executing, and thus may be preferred for
some embedded systems.
-msingle-float
-mdouble-float
The '-msingle-float' switch tells gcc to assume that the floating
point coprocessor only supports single precision operations, as on
the 'r4650' chip. The '-mdouble-float' switch permits gcc to use
double precision operations. This is the default.
-mmad
-mno-mad
Permit use of the 'mad', 'madu' and 'mul' instructions, as on the
'r4650' chip.
-m4650
Turns on '-msingle-float', '-mmad', and, at least for now,
'-mcpu=r4650'.
-EL
Compile code for the processor in little endian mode. The requisite
libraries are assumed to exist.
-EB
Compile code for the processor in big endian mode. The requisite
libraries are assumed to exist.
- G num
Put global and static items less than or equal to num bytes into the
small data or bss sections instead of the normal data or bss
section. This allows the assembler to emit one word memory
reference instructions based on the global pointer (gp or $28),
instead of the normal two words used. By default, num is 8 when the
MIPS assembler is used, and 0 when the GNU assembler is used. The
'-G num' switch is also passed to the assembler and linker. All
modules should be compiled with the same '-G num' value.
-nocpp
Tell the MIPS assembler to not run it's preprocessor over user
assembler files (with a '.s' suffix) when assembling them.
ΓòÉΓòÉΓòÉ 4.14.13. Intel 386 Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for the i386 family of computers:
-mcpu=cpu type
Assume the defaults for the machine type cpu type when scheduling
instructions. The choices for cpu type are: 'i386', 'i486', 'i586'
('pentium'), 'pentium', 'i686' ('pentiumpro') and 'pentiumpro'.
While picking a specific cpu type will schedule things appropriately
for that particular chip, the compiler will not generate any code
that does not run on the i386 without the '-march=cpu type' option
being used.
-march=cpu type
Generate instructions for the machine type cpu type. The choices
for cpu type are: 'i386', 'i486', 'pentium', and 'pentiumpro'.
Specifying '-march=cpu type' implies '-mcpu=cpu type'.
-m386
-m486
-mpentium
-mpentiumpro
Synonyms for -mcpu=i386, -mcpu=i486, -mcpu=pentium, and
-mcpu=pentiumpro respectively.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating point
comparisons. These handle correctly the case where the result of a
comparison is unordered.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GNU CC. Normally
the facilities of the machine's usual C compiler are used, but this
can't be done directly in cross-compilation. You must make your own
arrangements to provide suitable library functions for
cross-compilation.
On machines where a function returns floating point results in the
80387 register stack, some floating point opcodes may be emitted
even if '-msoft-float' is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types
float and double in an FPU register, even if there is no FPU. The
idea is that the operating system should emulate an FPU.
The option '-mno-fp-ret-in-387' causes such values to be returned in
ordinary CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the sin, cos and sqrt instructions
for the 387. Specify this option to avoid generating those
instructions. This option is the default on FreeBSD. As of revision
2.6.1, these instructions are not generated unless you also use the
'-ffast-math' switch.
-malign-double
-mno-align-double
Control whether GNU CC aligns double, long double, and long long
variables on a two word boundary or a one word boundary. Aligning
double variables on a two word boundary will produce code that runs
somewhat faster on a 'Pentium' at the expense of more memory.
Warning: if you use the '-malign-double' switch, structures
containing the above types will be aligned differently than the
published application binary interface specifications for the 386.
-msvr3-shlib
-mno-svr3-shlib
Control whether GNU CC places uninitialized locals into bss or data.
'-msvr3-shlib' places these locals into bss. These options are
meaningful only on System V Release 3.
-mno-wide-multiply
-mwide-multiply
Control whether GNU CC uses the mul and imul that produce 64 bit
results in eax:edx from 32 bit operands to do long long multiplies
and 32-bit division by constants.
-mrtd
Use a different function-calling convention, in which functions that
take a fixed number of arguments return with the ret num
instruction, which pops their arguments while returning. This saves
one instruction in the caller since there is no need to pop the
arguments there.
You can specify that an individual function is called with this
calling sequence with the function attribute 'stdcall'. You can
also override the '-mrtd' option by using the function attribute
'cdecl'. See Function Attributes
Warning: this calling convention is incompatible with the one
normally used on Unix, so you cannot use it if you need to call
libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including printf); otherwise
incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
-mreg-alloc=regs
Control the default allocation order of integer registers. The
string regs is a series of letters specifying a register. The
supported letters are: a allocate EAX; b allocate EBX; c allocate
ECX; d allocate EDX; S allocate ESI; D allocate EDI; B allocate EBP.
-mregparm=num
Control how many registers are used to pass integer arguments. By
default, no registers are used to pass arguments, and at most 3
registers can be used. You can control this behavior for a specific
function by using the function attribute 'regparm'. See Function
Attributes
Warning: if you use this switch, and num is nonzero, then you must
build all modules with the same value, including any libraries.
This includes the system libraries and startup modules.
-malign-loops=num
Align loops to a 2 raised to a num byte boundary. If
'-malign-loops' is not specified, the default is 2.
-malign-jumps=num
Align instructions that are only jumped to to a 2 raised to a num
byte boundary. If '-malign-jumps' is not specified, the default is
2 if optimizing for a 386, and 4 if optimizing for a 486.
-malign-functions=num
Align the start of functions to a 2 raised to num byte boundary. If
'-malign-functions' is not specified, the default is 2 if optimizing
for a 386, and 4 if optimizing for a 486.
ΓòÉΓòÉΓòÉ 4.14.14. HPPA Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for the HPPA family of computers:
-mpa-risc-1-0
Generate code for a PA 1.0 processor.
-mpa-risc-1-1
Generate code for a PA 1.1 processor.
-mbig-switch
Generate code suitable for big switch tables. Use this option only
if the assembler/linker complain about out of range branches within
a switch table.
-mjump-in-delay
Fill delay slots of function calls with unconditional jump
instructions by modifying the return pointer for the function call
to be the target of the conditional jump.
-mdisable-fpregs
Prevent floating point registers from being used in any manner.
This is necessary for compiling kernels which perform lazy context
switching of floating point registers. If you use this option and
attempt to perform floating point operations, the compiler will
abort.
-mdisable-indexing
Prevent the compiler from using indexing address modes. This avoids
some rather obscure problems when compiling MIG generated code under
MACH.
-mno-space-regs
Generate code that assumes the target has no space registers. This
allows GCC to generate faster indirect calls and use unscaled index
address modes.
Such code is suitable for level 0 PA systems and kernels.
-mfast-indirect-calls
Generate code that assumes calls never cross space boundaries. This
allows GCC to emit code which performs faster indirect calls.
This option will not work in the presense of shared libraries or
nested functions.
-mspace
Optimize for space rather than execution time. Currently this only
enables out of line function prologues and epilogues. This option
is incompatible with PIC code generation and profiling.
-mlong-load-store
Generate 3-instruction load and store sequences as sometimes
required by the HP-UX 10 linker. This is equivalent to the '+k'
option to the HP compilers.
-mportable-runtime
Use the portable calling conventions proposed by HP for ELF systems.
-mgas
Enable the use of assembler directives only GAS understands.
-mschedule=cpu type
Schedule code according to the constraints for the machine type cpu
type. The choices for cpu type are '700' for 7n0 machines, '7100'
for 7n5 machines, and '7100' for 7n2 machines. '7100' is the
default for cpu type.
Note the '7100LC' scheduling information is incomplete and using
'7100LC' often leads to bad schedules. For now it's probably best
to use '7100' instead of '7100LC' for the 7n2 machines.
-mlinker-opt
Enable the optimization pass in the HPUX linker. Note this makes
symbolic debugging impossible. It also triggers a bug in the HPUX 8
and HPUX 9 linkers in which they give bogus error messages when
linking some programs.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all HPPA
targets. Normally the facilities of the machine's usual C compiler
are used, but this cannot be done directly in cross-compilation.
You must make your own arrangements to provide suitable library
functions for cross-compilation. The embedded target
'hppa1.1-*-pro' does provide software floating point support.
'-msoft-float' changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile 'libgcc.a', the
library that comes with GNU CC, with '-msoft-float' in order for
this to work.
ΓòÉΓòÉΓòÉ 4.14.15. Intel 960 Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for the Intel 960 implementations:
-mcpu type
Assume the defaults for the machine type cpu type for some of the
other options, including instruction scheduling, floating point
support, and addressing modes. The choices for cpu type are 'ka',
'kb', 'mc', 'ca', 'cf', 'sa', and 'sb'. The default is 'kb'.
-mnumerics
-msoft-float
The '-mnumerics' option indicates that the processor does support
floating-point instructions. The '-msoft-float' option indicates
that floating-point support should not be assumed.
-mleaf-procedures
-mno-leaf-procedures
Do (or do not) attempt to alter leaf procedures to be callable with
the bal instruction as well as call. This will result in more
efficient code for explicit calls when the bal instruction can be
substituted by the assembler or linker, but less efficient code in
other cases, such as calls via function pointers, or using a linker
that doesn't support this optimization.
-mtail-call
-mno-tail-call
Do (or do not) make additional attempts (beyond those of the
machine-independent portions of the compiler) to optimize
tail-recursive calls into branches. You may not want to do this
because the detection of cases where this is not valid is not
totally complete. The default is '-mno-tail-call'.
-mcomplex-addr
-mno-complex-addr
Assume (or do not assume) that the use of a complex addressing mode
is a win on this implementation of the i960. Complex addressing
modes may not be worthwhile on the K-series, but they definitely are
on the C-series. The default is currently '-mcomplex-addr' for all
processors except the CB and CC.
-mcode-align
-mno-code-align
Align code to 8-byte boundaries for faster fetching (or don't
bother). Currently turned on by default for C-series implementations
only.
-mic-compat
-mic2.0-compat
-mic3.0-compat
Enable compatibility with iC960 v2.0 or v3.0.
-masm-compat
-mintel-asm
Enable compatibility with the iC960 assembler.
-mstrict-align
-mno-strict-align
Do not permit (do permit) unaligned accesses.
-mold-align
Enable structure-alignment compatibility with Intel's gcc release
version 1.3 (based on gcc 1.37). This option implies
'-mstrict-align'.
ΓòÉΓòÉΓòÉ 4.14.16. DEC Alpha Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for the DEC Alpha implementations:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for
floating-point operations. When -msoft-float is specified,
functions in 'libgcc1.c' will be used to perform floating-point
operations. Unless they are replaced by routines that emulate the
floating-point operations, or compiled in such a way as to call such
emulations routines, these routines will issue floating-point
operations. If you are compiling for an Alpha without
floating-point operations, you must ensure that the library is built
so as not to call them.
Note that Alpha implementations without floating-point operations
are required to have floating-point registers.
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-point register
set. -mno-fp-regs implies -msoft-float. If the floating-point
register set is not used, floating point operands are passed in
integer registers as if they were integers and floating-point
results are passed in $0 instead of $f0. This is a non-standard
calling sequence, so any function with a floating-point argument or
return value called by code compiled with -mno-fp-regs must also be
compiled with that option.
A typical use of this option is building a kernel that does not use,
and hence need not save and restore, any floating-point registers.
-mieee
The Alpha architecture implements floating-point hardware optimized
for maximum performance. It is mostly compliant with the IEEE
floating point standard. However, for full compliance, software
assistance is required. This option generates code fully IEEE
compliant code except that the inexact flag is not maintained (see
below). If this option is turned on, the CPP macro _IEEE_FP is
defined during compilation. The option is a shorthand for:
'-D_IEEE_FP -mfp-trap-mode=su -mtrap-precision=i -mieee-conformant'.
The resulting code is less efficient but is able to correctly
support denormalized numbers and exceptional IEEE values such as
not-a-number and plus/minus infinity. Other Alpha compilers call
this option -ieee_with_no_inexact.
-mieee-with-inexact
This is like '-mieee' except the generated code also maintains the
IEEE inexact flag. Turning on this option causes the generated code
to implement fully-compliant IEEE math. The option is a shorthand
for '-D_IEEE_FP -D_IEEE_FP_INEXACT' plus the three following:
'-mieee-conformant', '-mfp-trap-mode=sui', and '-mtrap-precision=i'.
On some Alpha implementations the resulting code may execute
significantly slower than the code generated by default. Since
there is very little code that depends on the inexact flag, you
should normally not specify this option. Other Alpha compilers call
this option '-ieee_with_inexact'.
-mfp-trap-mode=trap mode
This option controls what floating-point related traps are enabled.
Other Alpha compilers call this option '-fptm 'trap mode. The trap
mode can be set to one of four values:
'n'
This is the default (normal) setting. The only traps
that are enabled are the ones that cannot be disabled
in software (e.g., division by zero trap).
'u'
In addition to the traps enabled by 'n', underflow
traps are enabled as well.
'su'
Like 'su', but the instructions are marked to be safe
for software completion (see Alpha architecture manual
for details).
'sui'
Like 'su', but inexact traps are enabled as well.
-mfp-rounding-mode=rounding mode
Selects the IEEE rounding mode. Other Alpha compilers call this
option '-fprm 'rounding mode. The rounding mode can be one of:
'n'
Normal IEEE rounding mode. Floating point numbers are
rounded towards the nearest machine number or towards
the even machine number in case of a tie.
'm'
Round towards minus infinity.
'c'
Chopped rounding mode. Floating point numbers are
rounded towards zero.
'd'
Dynamic rounding mode. A field in the floating point
control register (fpcr, see Alpha architecture
reference manual) controls the rounding mode in
effect. The C library initializes this register for
rounding towards plus infinity. Thus, unless your
program modifies the fpcr, 'd' corresponds to round
towards plus infinity.
-mtrap-precision=trap precision
In the Alpha architecture, floating point traps are imprecise. This
means without software assistance it is impossible to recover from a
floating trap and program execution normally needs to be terminated.
GNU CC can generate code that can assist operating system trap
handlers in determining the exact location that caused a floating
point trap. Depending on the requirements of an application,
different levels of precisions can be selected:
'p'
Program precision. This option is the default and
means a trap handler can only identify which program
caused a floating point exception.
'f'
Function precision. The trap handler can determine
the function that caused a floating point exception.
'i'
Instruction precision. The trap handler can determine
the exact instruction that caused a floating point
exception.
Other Alpha compilers provide the equivalent options called
'-scope_safe' and '-resumption_safe'.
-mieee-conformant
This option marks the generated code as IEEE conformant. You must
not use this option unless you also specify '-mtrap-precision=i' and
either '-mfp-trap-mode=su' or '-mfp-trap-mode=sui'. Its only effect
is to emit the line '.eflag 48' in the function prologue of the
generated assembly file. Under DEC Unix, this has the effect that
IEEE-conformant math library routines will be linked in.
-mbuild-constants
Normally GNU CC examines a 32- or 64-bit integer constant to see if
it can construct it from smaller constants in two or three
instructions. If it cannot, it will output the constant as a
literal and generate code to load it from the data segment at
runtime.
Use this option to require GNU CC to construct all integer constants
using code, even if it takes more instructions (the maximum is six).
You would typically use this option to build a shared library
dynamic loader. Itself a shared library, it must relocate itself in
memory before it can find the variables and constants in its own
data segment.
-malpha-as
-mgas
Select whether to generate code to be assembled by the
vendor-supplied assembler ('-malpha-as') or by the GNU assembler
'-mgas'.
-mbwx
-mno-bwx
-mcix
-mno-cix
-mmax
-mno-max
Indicate whether GNU CC should generate code to use the optional
BWX, CIX, and MAX instruction sets. The default is to use the
instruction sets supported by the CPU type specified via '-mcpu='
option or that of the CPU on which GNU CC was built if none was
specified.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling
parameters for machine type cpu_type. You can specify either the
'EV' style name or the corresponding chip number. GNU CC supports
scheduling parameters for the EV4 and EV5 family of processors and
will choose the default values for the instruction set from the
processor you specify. If you do not specify a processor type, GNU
CC will default to the processor on which the compiler was built.
Supported values for cpu_type are
'ev4'
'21064'
Schedules as an EV4 and has no instruction set
extensions.
'ev5'
'21164'
Schedules as an EV5 and has no instruction set
extensions.
'ev56'
'21164a'
Schedules as an EV5 and supports the BWX extension.
'pca56'
'21164PC'
Schedules as an EV5 and supports the BWX and MAX
extensions.
'ev6'
'21264'
Schedules as an EV5 (until Digital releases the
scheduling parameters for the EV6) and supports the
BWX, CIX, and MAX extensions.
ΓòÉΓòÉΓòÉ 4.14.17. Clipper Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for the Clipper implementations:
-mc300
Produce code for a C300 Clipper processor. This is the default.
-mc400
Produce code for a C400 Clipper processor i.e. use floating point
registers f8┬╖┬╖f15.
ΓòÉΓòÉΓòÉ 4.14.18. H8/300 Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for the H8/300 implementations:
-mrelax
Shorten some address references at link time, when possible; uses
the linker option '-relax'. See Section ld and the H8/300 of Using
ld, for a fuller description.
-mh
Generate code for the H8/300H.
-ms
Generate code for the H8/S.
-mint32
Make int data 32 bits by default.
-malign-300
On the h8/300h, use the same alignment rules as for the h8/300. The
default for the h8/300h is to align longs and floats on 4 byte
boundaries. '-malign-300' causes them to be aligned on 2 byte
boundaries. This option has no effect on the h8/300.
ΓòÉΓòÉΓòÉ 4.14.19. SH Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for the SH implementations:
-m1
Generate code for the SH1.
-m2
Generate code for the SH2.
-m3
Generate code for the SH3.
-m3e
Generate code for the SH3e.
-mb
Compile code for the processor in big endian mode.
-ml
Compile code for the processor in little endian mode.
-mrelax
Shorten some address references at link time, when possible; uses
the linker option '-relax'.
ΓòÉΓòÉΓòÉ 4.14.20. Options for System V ΓòÉΓòÉΓòÉ
These additional options are available on System V Release 4 for compatibility
with other compilers on those systems:
-G
Create a shared object. It is recommended that '-symbolic' or
'-shared' be used instead.
-Qy
Identify the versions of each tool used by the compiler, in a .ident
assembler directive in the output.
-Qn
Refrain from adding .ident directives to the output file (this is
the default).
-YP,dirs
Search the directories dirs, and no others, for libraries specified
with '-l'.
-Ym,dir
Look in the directory dir to find the M4 preprocessor. The assembler
uses this option.
ΓòÉΓòÉΓòÉ 4.14.21. V850 Options ΓòÉΓòÉΓòÉ
These '-m' options are defined for V850 implementations:
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to
be far away, the compiler will always load the functions address up
into a register, and call indirect through the pointer.
-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use the same index
pointer 4 or more times to copy pointer into the ep register, and
use the shorter sld and sst instructions. The '-mep' option is on
by default if you optimize.
-mno-prolog-function
-mprolog-function
Do not use (do use) external functions to save and restore registers
at the prolog and epilog of a function. The external functions are
slower, but use less code space if more than one function saves the
same number of registers. The '-mprolog-function' option is on by
default if you optimize.
-mspace
Try to make the code as small as possible. At present, this just
turns on the '-mep' and '-mprolog-function' options.
-mtda=n
Put static or global variables whose size is n bytes or less into
the tiny data area that register ep points to. The tiny data area
can hold up to 256 bytes in total (128 bytes for byte references).
-msda=n
Put static or global variables whose size is n bytes or less into
the small data area that register gp points to. The small data area
can hold up to 64 kilobytes.
-mzda=n
Put static or global variables whose size is n bytes or less into
the first 32 kilobytes of memory.
-mv850
Specify that the target processor is the V850.
-mbig-switch
Generate code suitable for big switch tables. Use this option only
if the assembler/linker complain about out of range branches within
a switch table.
ΓòÉΓòÉΓòÉ 4.15. Options for Code Generation Conventions ΓòÉΓòÉΓòÉ
These machine-independent options control the interface conventions used in
code generation.
Most of them have both positive and negative forms; the negative form of
'-ffoo' would be '-fno-foo'. In the table below, only one of the forms is
listed---the one which is not the default. You can figure out the other form
by either removing 'no-' or adding it.
-fexceptions
Enable exception handling, and generate extra code needed to
propagate exceptions. If you do not specify this option, GNU CC
enables it by default for languages like C++ that normally require
exception handling, and disabled for languages like C that do not
normally require it. However, when compiling C code that needs to
interoperate properly with exception handlers written in C++, you
may need to enable this option. You may also wish to disable this
option is you are compiling older C++ programs that don't use
exception handling.
-funwind-tables
Similar to -fexceptions, except that it will just generate any
needed static data, but will not affect the generated code in any
other way. You will normally not enable this option; instead, a
language processor that needs this handling would enable it on your
behalf.
-fpcc-struct-return
Return ``short'' struct and union values in memory like longer ones,
rather than in registers. This convention is less efficient, but it
has the advantage of allowing intercallability between GNU
CC-compiled files and files compiled with other compilers.
The precise convention for returning structures in memory depends on
the target configuration macros.
Short structures and unions are those whose size and alignment match
that of some integer type.
-freg-struct-return
Use the convention that struct and union values are returned in
registers when possible. This is more efficient for small
structures than '-fpcc-struct-return'.
If you specify neither '-fpcc-struct-return' nor its contrary
'-freg-struct-return', GNU CC defaults to whichever convention is
standard for the target. If there is no standard convention, GNU CC
defaults to '-fpcc-struct-return', except on targets where GNU CC is
the principal compiler. In those cases, we can choose the standard,
and we chose the more efficient register return alternative.
-fshort-enums
Allocate to an enum type only as many bytes as it needs for the
declared range of possible values. Specifically, the enum type will
be equivalent to the smallest integer type which has enough room.
-fshort-double
Use the same size for double as for float.
-fshared-data
Requests that the data and non-const variables of this compilation
be shared data rather than private data. The distinction makes
sense only on certain operating systems, where shared data is shared
between processes running the same program, while private data
exists in one copy per process.
-fno-common
Allocate even uninitialized global variables in the bss section of
the object file, rather than generating them as common blocks. This
has the effect that if the same variable is declared ( without
extern) in two different compilations, you will get an error when
you link them. The only reason this might be useful is if you wish
to verify that the program will work on other systems which always
work this way.
-fno-ident
Ignore the '#ident' directive.
-fno-gnu-linker
Do not output global initializations (such as C++ constructors and
destructors) in the form used by the GNU linker (on systems where
the GNU linker is the standard method of handling them). Use this
option when you want to use a non-GNU linker, which also requires
using the collect2 program to make sure the system linker includes
constructors and destructors. (collect2 is included in the GNU CC
distribution.) For systems which must use collect2, the compiler
driver gcc is configured to do this automatically.
-finhibit-size-directive
Don't output a .size assembler directive, or anything else that
would cause trouble if the function is split in the middle, and the
two halves are placed at locations far apart in memory. This option
is used when compiling 'crtstuff.c'; you should not need to use it
for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code to
make it more readable. This option is generally only of use to
those who actually need to read the generated assembly code (perhaps
while debugging the compiler itself).
'-fno-verbose-asm', the default, causes the extra information to be
omitted and is useful when comparing two assembler files.
-fvolatile
Consider all memory references through pointers to be volatile.
-fvolatile-global
Consider all memory references to extern and global data items to be
volatile.
-fpic
Generate position-independent code (PIC) suitable for use in a
shared library, if supported for the target machine. Such code
accesses all constant addresses through a global offset table (GOT).
The dynamic loader resolves the GOT entries when the program starts
(the dynamic loader is not part of GNU CC; it is part of the
operating system). If the GOT size for the linked executable
exceeds a machine-specific maximum size, you get an error message
from the linker indicating that '-fpic' does not work; in that case,
recompile with '-fPIC' instead. (These maximums are 16k on the
m88k, 8k on the Sparc, and 32k on the m68k and RS/6000. The 386 has
no such limit.)
Position-independent code requires special support, and therefore
works only on certain machines. For the 386, GNU CC supports PIC
for System V but not for the Sun 386i. Code generated for the IBM
RS/6000 is always position-independent.
-fPIC
If supported for the target machine, emit position-independent code,
suitable for dynamic linking and avoiding any limit on the size of
the global offset table. This option makes a difference on the
m68k, m88k, and the Sparc.
Position-independent code requires special support, and therefore
works only on certain machines.
-ffixed-reg
Treat the register named reg as a fixed register; generated code
should never refer to it ( except perhaps as a stack pointer, frame
pointer or in some other fixed role).
reg must be the name of a register. The register names accepted are
machine-specific and are defined in the REGISTER_NAMES macro in the
machine description macro file.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is
clobbered by function calls. It may be allocated for temporaries or
variables that do not live across a call. Functions compiled this
way will not save and restore the register reg.
Use of this flag for a register that has a fixed pervasive role in
the machine's execution model, such as the stack pointer or frame
pointer, will produce disastrous results.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved by
functions. It may be allocated even for temporaries or variables
that live across a call. Functions compiled this way will save and
restore the register reg if they use it.
Use of this flag for a register that has a fixed pervasive role in
the machine's execution model, such as the stack pointer or frame
pointer, will produce disastrous results.
A different sort of disaster will result from the use of this flag
for a register in which function values may be returned.
This flag does not have a negative form, because it specifies a
three-way choice.
-fpack-struct
Pack all structure members together without holes. Usually you
would not want to use this option, since it makes the code
suboptimal, and the offsets of structure members won't agree with
system libraries.
-fcheck-memory-usage
Generate extra code to check each memory access. GNU CC will
generate code that is suitable for a detector of bad memory accesses
such as 'Checker'. If you specify this option, you can not use the
asm or __asm__ keywords.
You must also specify this option when you compile functions you
call that have side effects. If you do not, you may get erroneous
messages from the detector. Normally, you should compile all your
code with this option. If you use functions from a library that have
side-effects (such as read), you may not be able to recompile the
library and specify this option. In that case, you can enable the
'-fprefix-function-name' option, which requests GNU CC to
encapsulate your code and make other functions look as if they were
compiled with '-fcheck-memory-usage'. This is done by calling
``stubs'', which are provided by the detector. If you cannot find
or build stubs for every function you call, you may have to specify
'-fcheck-memory-usage' without '-fprefix-function-name'.
-fprefix-function-name
Request GNU CC to add a prefix to the symbols generated for function
names. GNU CC adds a prefix to the names of functions defined as
well as functions called. Code compiled with this option and code
compiled without the option can't be linked together, unless or
stubs are used.
If you compile the following code with '-fprefix-function-name'
extern void bar (int);
void
foo (int a)
{
return bar (a + 5);
}
GNU CC will compile the code as if it was written:
extern void prefix_bar (int);
void
prefix_foo (int a)
{
return prefix_bar (a + 5);
}
This option is designed to be used with '-fcheck-memory-usage'.
-fstack-check
Generate code to verify that you do not go beyond the boundary of
the stack. You should specify this flag if you are running in an
environment with multiple threads, but only rarely need to specify
it in a single-threaded environment since stack overflow is
automatically detected on nearly all systems if there is only one
stack.
+e0
+e1
Control whether virtual function definitions in classes are used to
generate code, or only to define interfaces for their callers. (C++
only).
These options are provided for compatibility with cfront 1.x usage;
the recommended alternative GNU C++ usage is in flux. See
Declarations and Definitions in One Header.
With '+e0', virtual function definitions in classes are declared
extern; the declaration is used only as an interface specification,
not to generate code for the virtual functions (in this
compilation).
With '+e1', G++ actually generates the code implementing virtual
functions defined in the code, and makes them publicly visible.
ΓòÉΓòÉΓòÉ 4.16. Environment Variables Affecting GNU CC ΓòÉΓòÉΓòÉ
This section describes several environment variables that affect how GNU CC
operates. They work by specifying directories or prefixes to use when
searching for various kinds of files.
Note that you can also specify places to search using options such as '-B',
'-I' and '-L' (see Directory Options). These take precedence over places
specified using environment variables, which in turn take precedence over
those specified by the configuration of GNU CC.
TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary
files. GNU CC uses temporary files to hold the output of one stage
of compilation which is to be used as input to the next stage: for
example, the output of the preprocessor, which is the input to the
compiler proper.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names
of the subprograms executed by the compiler. No slash is added when
this prefix is combined with the name of a subprogram, but you can
specify a prefix that ends with a slash if you wish.
If GNU CC cannot find the subprogram using the specified prefix, it
tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is 'prefix/lib/gcc-lib/' where
prefix is the value of prefix when you ran the 'configure' script.
Other prefixes specified with '-B' take precedence over this prefix.
This prefix is also used for finding files such as 'crt0.o' that are
used for linking.
In addition, the prefix is used in an unusual way in finding the
directories to search for header files. For each of the standard
directories whose name normally begins with '/usr/local/lib/gcc-lib'
(more precisely, with the value of GCC_INCLUDE_DIR), GNU CC tries
replacing that beginning with the specified prefix to produce an
alternate directory name. Thus, with '-Bfoo/', GNU CC will search
'foo/bar' where it would normally search '/usr/local/lib/bar'. These
alternate directories are searched first; the standard directories
come next.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directories,
much like PATH. GNU CC tries the directories thus specified when
searching for subprograms, if it can't find the subprograms using
GCC_EXEC_PREFIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories,
much like PATH. When configured as a native compiler, GNU CC tries
the directories thus specified when searching for special linker
files, if it can't find them using GCC_EXEC_PREFIX. Linking using
GNU CC also uses these directories when searching for ordinary
libraries for the '-l' option (but directories specified with '-L'
come first).
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
These environment variables pertain to particular languages. Each
variable's value is a colon-separated list of directories, much like
PATH. When GNU CC searches for header files, it tries the
directories listed in the variable for the language you are using,
after the directories specified with '-I' but before the standard
header file directories.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output
dependencies for Make based on the header files processed by the
compiler. This output looks much like the output from the '-M'
option (see Preprocessor Options), but it goes to a separate file,
and is in addition to the usual results of compilation.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which
case the Make rules are written to that file, guessing the target
name from the source file name. Or the value can have the form
'file target', in which case the rules are written to file file
using target as the target name.
ΓòÉΓòÉΓòÉ 4.17. Running Protoize ΓòÉΓòÉΓòÉ
The program protoize is an optional part of GNU C. You can use it to add
prototypes to a program, thus converting the program to ANSI C in one respect.
The companion program unprotoize does the reverse: it removes argument types
from any prototypes that are found.
When you run these programs, you must specify a set of source files as command
line arguments. The conversion programs start out by compiling these files to
see what functions they define. The information gathered about a file foo is
saved in a file named 'foo.X'.
After scanning comes actual conversion. The specified files are all eligible
to be converted; any files they include (whether sources or just headers) are
eligible as well.
But not all the eligible files are converted. By default, protoize and
unprotoize convert only source and header files in the current directory. You
can specify additional directories whose files should be converted with the
'-d directory' option. You can also specify particular files to exclude with
the '-x file' option. A file is converted if it is eligible, its directory
name matches one of the specified directory names, and its name within the
directory has not been excluded.
Basic conversion with protoize consists of rewriting most function definitions
and function declarations to specify the types of the arguments. The only
ones not rewritten are those for varargs functions.
protoize optionally inserts prototype declarations at the beginning of the
source file, to make them available for any calls that precede the function's
definition. Or it can insert prototype declarations with block scope in the
blocks where undeclared functions are called.
Basic conversion with unprotoize consists of rewriting most function
declarations to remove any argument types, and rewriting function definitions
to the old-style pre-ANSI form.
Both conversion programs print a warning for any function declaration or
definition that they can't convert. You can suppress these warnings with
'-q'.
The output from protoize or unprotoize replaces the original source file. The
original file is renamed to a name ending with '.save'. If the '.save' file
already exists, then the source file is simply discarded.
protoize and unprotoize both depend on GNU CC itself to scan the program and
collect information about the functions it uses. So neither of these programs
will work until GNU CC is installed.
Here is a table of the options you can use with protoize and unprotoize. Each
option works with both programs unless otherwise stated.
-B directory
Look for the file 'SYSCALLS.c.X' in directory, instead of the usual
directory (normally '/usr/local/lib'). This file contains prototype
information about standard system functions. This option applies
only to protoize.
-c compilation-options
Use compilation-options as the options when running gcc to produce
the '.X' files. The special option '-aux-info' is always passed in
addition, to tell gcc to write a '.X' file.
Note that the compilation options must be given as a single argument
to protoize or unprotoize. If you want to specify several gcc
options, you must quote the entire set of compilation options to
make them a single word in the shell.
There are certain gcc arguments that you cannot use, because they
would produce the wrong kind of output. These include '-g', '-O',
'-c', '-S', and '-o' If you include these in the
compilation-options, they are ignored.
-C
Rename files to end in '.C' instead of '.c'. This is convenient if
you are converting a C program to C++. This option applies only to
protoize.
-g
Add explicit global declarations. This means inserting explicit
declarations at the beginning of each source file for each function
that is called in the file and was not declared. These declarations
precede the first function definition that contains a call to an
undeclared function. This option applies only to protoize.
-i string
Indent old-style parameter declarations with the string string. This
option applies only to protoize.
unprotoize converts prototyped function definitions to old-style
function definitions, where the arguments are declared between the
argument list and the initial '{'. By default, unprotoize uses five
spaces as the indentation. If you want to indent with just one
space instead, use '-i " "'.
-k
Keep the '.X' files. Normally, they are deleted after conversion is
finished.
-l
Add explicit local declarations. protoize with '-l' inserts a
prototype declaration for each function in each block which calls
the function without any declaration. This option applies only to
protoize.
-n
Make no real changes. This mode just prints information about the
conversions that would have been done without '-n'.
-N
Make no '.save' files. The original files are simply deleted. Use
this option with caution.
-p program
Use the program program as the compiler. Normally, the name 'gcc'
is used.
-q
Work quietly. Most warnings are suppressed.
-v
Print the version number, just like '-v' for gcc.
If you need special compiler options to compile one of your program's source
files, then you should generate that file's '.X' file specially, by running
gcc on that source file with the appropriate options and the option
'-aux-info'. Then run protoize on the entire set of files. protoize will use
the existing '.X' file because it is newer than the source file. For example:
gcc -Dfoo=bar file1.c -aux-info
protoize *.c
You need to include the special files along with the rest in the protoize
command, even though their '.X' files already exist, because otherwise they
won't get converted.
See Protoize Caveats, for more information on how to use protoize
successfully.
ΓòÉΓòÉΓòÉ 5. Installing GNU CC ΓòÉΓòÉΓòÉ
Configurations Configurations Supported by GNU CC.
Other Dir Compiling in a separate directory (not where the
source is).
Cross-Compiler Building and installing a cross-compiler.
Sun Install See below for installation on the Sun.
VMS Install See below for installation on VMS.
Collect2 How collect2 works; how it finds ld.
Header Dirs Understanding the standard header file
directories.
Here is the procedure for installing GNU CC on a Unix system. See VMS
Install, for VMS systems. In this section we assume you compile in the same
directory that contains the source files; see Other Dir, to find out how to
compile in a separate directory on Unix systems.
You cannot install GNU C by itself on MSDOS; it will not compile under any
MSDOS compiler except itself. You need to get the complete compilation
package DJGPP, which includes binaries as well as sources, and includes all
the necessary compilation tools and libraries.
1. If you have built GNU CC previously in the same directory for a different
target machine, do 'make distclean' to delete all files that might be
invalid. One of the files this deletes is 'Makefile'; if 'make
distclean' complains that 'Makefile' does not exist, it probably means
that the directory is already suitably clean.
2. On a System V release 4 system, make sure '/usr/bin' precedes '/usr/ucb'
in PATH. The cc command in '/usr/ucb' uses libraries which have bugs.
3. Specify the host, build and target machine configurations. You do this
by running the file 'configure'.
The build machine is the system which you are using, the host machine is
the system where you want to run the resulting compiler (normally the
build machine), and the target machine is the system for which you want
the compiler to generate code.
If you are building a compiler to produce code for the machine it runs on
(a native compiler), you normally do not need to specify any operands to
'configure'; it will try to guess the type of machine you are on and use
that as the build, host and target machines. So you don't need to
specify a configuration when building a native compiler unless
'configure' cannot figure out what your configuration is or guesses
wrong.
In those cases, specify the build machine's configuration name with the
'--host' option; the host and target will default to be the same as the
host machine. (If you are building a cross-compiler, see
Cross-Compiler.)
Here is an example:
┬╖/configure --build=sparc-sun-sunos4.1
A configuration name may be canonical or it may be more or less abbreviated.
A canonical configuration name has three parts, separated by dashes. It looks
like this: 'cpu-company-system'. (The three parts may themselves contain
dashes; 'configure' can figure out which dashes serve which purpose.) For
example, 'm68k-sun-sunos4.1' specifies a Sun 3.
You can also replace parts of the configuration by nicknames or aliases. For
example, 'sun3' stands for 'm68k-sun', so 'sun3-sunos4.1' is another way to
specify a Sun 3. You can also use simply 'sun3-sunos', since the version of
SunOS is assumed by default to be version 4.
You can specify a version number after any of the system types, and some of
the CPU types. In most cases, the version is irrelevant, and will be ignored.
So you might as well specify the version if you know it.
See Configurations, for a list of supported configuration names and notes on
many of the configurations. You should check the notes in that section before
proceeding any further with the installation of GNU CC.
There are four additional options you can specify independently to describe
variant hardware and software configurations. These are '--with-gnu-as',
'--with-gnu-ld', '--with-stabs' and '--nfp'.
'--with-gnu-as'
If you will use GNU CC with the GNU assembler (GAS), you
should declare this by using the '--with-gnu-as' option
when you run 'configure'.
Using this option does not install GAS. It only modifies
the output of GNU CC to work with GAS. Building and
installing GAS is up to you.
Conversely, if you do not wish to use GAS and do not
specify '--with-gnu-as' when building GNU CC, it is up to
you to make sure that GAS is not installed. GNU CC
searches for a program named as in various directories; if
the program it finds is GAS, then it runs GAS. If you are
not sure where GNU CC finds the assembler it is using, try
specifying '-v' when you run it.
The systems where it makes a difference whether you use
GAS are
'hppa1.0-any-any', 'hppa1.1-any-any', 'i386-any-sysv',
'i386-any-isc',
'i860-any-bsd', 'm68k-bull-sysv',
'm68k-hp-hpux', 'm68k-sony-bsd',
'm68k-altos-sysv', 'm68000-hp-hpux',
'm68000-att-sysv', 'any-lynx-lynxos', and 'mips-any'). On
any other system, '--with-gnu-as' has no effect.
On the systems listed above (except for the HP-PA, for ISC
on the 386, and for 'mips-sgi-irix5.*'), if you use GAS,
you should also use the GNU linker (and specify
'--with-gnu-ld').
'--with-gnu-ld'
Specify the option '--with-gnu-ld' if you plan to use the
GNU linker with GNU CC.
This option does not cause the GNU linker to be installed;
it just modifies the behavior of GNU CC to work with the
GNU linker. Specifically, it inhibits the installation of
collect2, a program which otherwise serves as a front-end
for the system's linker on most configurations.
'--with-stabs'
On MIPS based systems and on Alphas, you must specify
whether you want GNU CC to create the normal ECOFF
debugging format, or to use BSD-style stabs passed through
the ECOFF symbol table. The normal ECOFF debug format
cannot fully handle languages other than C. BSD stabs
format can handle other languages, but it only works with
the GNU debugger GDB.
Normally, GNU CC uses the ECOFF debugging format by
default; if you prefer BSD stabs, specify '--with-stabs'
when you configure GNU CC.
No matter which default you choose when you configure GNU
CC, the user can use the '-gcoff' and '-gstabs+' options
to specify explicitly the debug format for a particular
compilation.
'--with-stabs' is meaningful on the ISC system on the 386,
also, if '--with-gas' is used. It selects use of stabs
debugging information embedded in COFF output. This kind
of debugging information supports C++ well; ordinary COFF
debugging information does not.
'--with-stabs' is also meaningful on 386 systems running
SVR4. It selects use of stabs debugging information
embedded in ELF output. The C++ compiler currently
(2.6.0) does not support the DWARF debugging information
normally used on 386 SVR4 platforms; stabs provide a
workable alternative. This requires gas and gdb, as the
normal SVR4 tools can not generate or interpret stabs.
'--nfp'
On certain systems, you must specify whether the machine
has a floating point unit. These systems include
'm68k-sun-sunosn' and 'm68k-isi-bsd'. On any other
system, '--nfp' currently has no effect, though perhaps
there are other systems where it could usefully make a
difference.
'--enable-threads=type'
Certain systems, notably Linux-based GNU systems, can't be
relied on to supply a threads facility for the Objective C
runtime and so will default to single-threaded runtime.
They may, however, have a library threads implementation
available, in which case threads can be enabled with this
option by supplying a suitable type, probably 'posix'.
The possibilities for type are 'single', 'posix', 'win32',
'solaris', 'irix' and 'mach'.
The 'configure' script searches subdirectories of the source directory for
other compilers that are to be integrated into GNU CC. The GNU compiler for
C++, called G++ is in a subdirectory named 'cp'. 'configure' inserts rules
into 'Makefile' to build all of those compilers.
Here we spell out what files will be set up by configure. Normally you
need not be concerned with these files.
A file named 'config.h' is created that contains a '#include' of the
top-level config file for the machine you will run the compiler on
(see Section The Configuration File of Using and Porting GCC). This
file is responsible for defining information about the host machine.
It includes 'tm.h'. The top-level config file is located in the
subdirectory 'config'. Its name is always 'xm-something.h'; usually
'xm-machine.h', but there are some exceptions.
If your system does not support symbolic links, you might want to
set up 'config.h' to contain a '#include' command which refers to
the appropriate file.
A file named 'tconfig.h' is created which includes the top-level
config file for your target machine. This is used for compiling
certain programs to run on that machine.
A file named 'tm.h' is created which includes the
machine-description macro file for your target machine. It should
be in the subdirectory 'config' and its name is often 'machine.h'.
The command file 'configure' also constructs the file 'Makefile' by
adding some text to the template file 'Makefile.in'. The additional
text comes from files in the 'config' directory, named 't-target'
and 'x-host'. If these files do not exist, it means nothing needs
to be added for a given target or host.
4. The standard directory for installing GNU CC is '/usr/local/lib'. If you
want to install its files somewhere else, specify '--prefix=dir' when you
run 'configure'. Here dir is a directory name to use instead of
'/usr/local' for all purposes with one exception: the directory
'/usr/local/include' is searched for header files no matter where you
install the compiler. To override this name, use the --local-prefix
option below.
5. Specify '--local-prefix=dir' if you want the compiler to search directory
'dir/include' for locally installed header files instead of
'/usr/local/include'.
You should specify '--local-prefix' only if your site has a different
convention (not '/usr/local') for where to put site-specific files.
The default value for '--local-prefix' is '/usr/local' regardless of the
value of '--prefix'. Specifying '--prefix' has no effect on which
directory GNU CC searches for local header files. This may seem
counterintuitive, but actually it is logical.
The purpose of '--prefix' is to specify where to install GNU CC. The
local header files in '/usr/local/include'---if you put any in that
directory---are not part of GNU CC. They are part of other
programs---perhaps many others. (GNU CC installs its own header files in
another directory which is based on the '--prefix' value.)
Do not specify '/usr' as the '--local-prefix'! The directory you use for
'--local-prefix' must not contain any of the system's standard header
files. If it did contain them, certain programs would be miscompiled
(including GNU Emacs, on certain targets), because this would override
and nullify the header file corrections made by the fixincludes script.
Indications are that people who use this option use it based on mistaken
ideas of what it is for. People use it as if it specified where to
install part of GNU CC. Perhaps they make this assumption because
installing GNU CC creates the directory.
6. Make sure the Bison parser generator is installed. (This is unnecessary
if the Bison output files 'c-parse.c' and 'cexp.c' are more recent than
'c-parse.y' and 'cexp.y' and you do not plan to change the '.y' files.)
Bison versions older than Sept 8, 1988 will produce incorrect output for
'c-parse.c'.
7. If you have chosen a configuration for GNU CC which requires other GNU
tools (such as GAS or the GNU linker) instead of the standard system
tools, install the required tools in the build directory under the names
'as', 'ld' or whatever is appropriate. This will enable the compiler to
find the proper tools for compilation of the program 'enquire'.
Alternatively, you can do subsequent compilation using a value of the
PATH environment variable such that the necessary GNU tools come before
the standard system tools.
8. Build the compiler. Just type 'make LANGUAGES=c' in the compiler
directory.
'LANGUAGES=c' specifies that only the C compiler should be compiled. The
makefile normally builds compilers for all the supported languages;
currently, C, C++ and Objective C. However, C is the only language that
is sure to work when you build with other non-GNU C compilers. In
addition, building anything but C at this stage is a waste of time.
In general, you can specify the languages to build by typing the argument
'LANGUAGES="list"', where list is one or more words from the list 'c',
'c++', and 'objective-c'. If you have any additional GNU compilers as
subdirectories of the GNU CC source directory, you may also specify their
names in this list.
Ignore any warnings you may see about ``statement not reached'' in
'insn-emit.c'; they are normal. Also, warnings about ``unknown escape
sequence'' are normal in 'genopinit.c' and perhaps some other files.
Likewise, you should ignore warnings about ``constant is so large that it
is unsigned'' in 'insn-emit.c' and 'insn-recog.c' and a warning about a
comparison always being zero in 'enquire.o'. Any other compilation
errors may represent bugs in the port to your machine or operating
system, and should be investigated and reported (see Bugs).
Some commercial compilers fail to compile GNU CC because they have bugs
or limitations. For example, the Microsoft compiler is said to run out
of macro space. Some Ultrix compilers run out of expression space; then
you need to break up the statement where the problem happens.
9. If you are building a cross-compiler, stop here. See Cross-Compiler.
10. Move the first-stage object files and executables into a subdirectory
with this command:
make stage1
The files are moved into a subdirectory named 'stage1'. Once installation
is complete, you may wish to delete these files with rm -r stage1.
11. If you have chosen a configuration for GNU CC which requires other GNU
tools (such as GAS or the GNU linker) instead of the standard system
tools, install the required tools in the 'stage1' subdirectory under the
names 'as', 'ld' or whatever is appropriate. This will enable the stage
1 compiler to find the proper tools in the following stage.
Alternatively, you can do subsequent compilation using a value of the
PATH environment variable such that the necessary GNU tools come before
the standard system tools.
12. Recompile the compiler with itself, with this command:
make CC="stage1/xgcc -Bstage1/" CFLAGS="-g -O2"
This is called making the stage 2 compiler.
The command shown above builds compilers for all the supported languages.
If you don't want them all, you can specify the languages to build by
typing the argument 'LANGUAGES="list"'. list should contain one or more
words from the list 'c', 'c++', 'objective-c', and 'proto'. Separate the
words with spaces. 'proto' stands for the programs protoize and unprotoize;
they are not a separate language, but you use LANGUAGES to enable or
disable their installation.
If you are going to build the stage 3 compiler, then you might want to
build only the C language in stage 2.
Once you have built the stage 2 compiler, if you are short of disk space,
you can delete the subdirectory 'stage1'.
On a 68000 or 68020 system lacking floating point hardware, unless you have
selected a 'tm.h' file that expects by default that there is no such
hardware, do this instead:
make CC="stage1/xgcc -Bstage1/" CFLAGS="-g -O2 -msoft-float"
13. If you wish to test the compiler by compiling it with itself one more
time, install any other necessary GNU tools (such as GAS or the GNU
linker) in the 'stage2' subdirectory as you did in the 'stage1'
subdirectory, then do this:
make stage2
make CC="stage2/xgcc -Bstage2/" CFLAGS="-g -O2"
This is called making the stage 3 compiler. Aside from the '-B' option,
the compiler options should be the same as when you made the stage 2
compiler. But the LANGUAGES option need not be the same. The command
shown above builds compilers for all the supported languages; if you don't
want them all, you can specify the languages to build by typing the
argument 'LANGUAGES="list"', as described above.
If you do not have to install any additional GNU tools, you may use the
command
make bootstrap LANGUAGES=language-list BOOT_CFLAGS=option-list
instead of making 'stage1', 'stage2', and performing the two compiler
builds.
14. Then compare the latest object files with the stage 2 object files---they
ought to be identical, aside from time stamps (if any).
On some systems, meaningful comparison of object files is impossible;
they always appear ``different.'' This is currently true on Solaris and
some systems that use ELF object file format. On some versions of Irix
on SGI machines and DEC Unix (OSF/1) on Alpha systems, you will not be
able to compare the files without specifying '-save-temps'; see the
description of individual systems above to see if you get comparison
failures. You may have similar problems on other systems.
Use this command to compare the files:
make compare
This will mention any object files that differ between stage 2 and stage 3.
Any difference, no matter how innocuous, indicates that the stage 2
compiler has compiled GNU CC incorrectly, and is therefore a potentially
serious bug which you should investigate and report (see Bugs).
If your system does not put time stamps in the object files, then this is a
faster way to compare them (using the Bourne shell):
for file in *.o; do
cmp $file stage2/$file
done
If you have built the compiler with the '-mno-mips-tfile' option on MIPS
machines, you will not be able to compare the files.
15. Install the compiler driver, the compiler's passes and run-time support
with 'make install'. Use the same value for CC, CFLAGS and LANGUAGES
that you used when compiling the files that are being installed. One
reason this is necessary is that some versions of Make have bugs and
recompile files gratuitously when you do this step. If you use the same
variable values, those files will be recompiled properly.
For example, if you have built the stage 2 compiler, you can use the
following command:
make install CC="stage2/xgcc -Bstage2/" CFLAGS="-g -O" LANGUAGES="list"
This copies the files 'cc1', 'cpp' and 'libgcc.a' to files 'cc1', 'cpp' and
'libgcc.a' in the directory '/usr/local/lib/gcc-lib/target/version', which
is where the compiler driver program looks for them. Here target is the
canonicalized form of target machine type specified when you ran
'configure', and version is the version number of GNU CC. This naming
scheme permits various versions and/or cross-compilers to coexist. It also
copies the executables for compilers for other languages (e.g., 'cc1plus'
for C++) to the same directory.
This also copies the driver program 'xgcc' into '/usr/local/bin/gcc', so
that it appears in typical execution search paths. It also copies 'gcc.1'
into '/usr/local/man/man1' and info pages into '/usr/local/info'.
On some systems, this command causes recompilation of some files. This is
usually due to bugs in make. You should either ignore this problem, or use
GNU Make.
Warning: there is a bug in alloca in the Sun library. To avoid this bug,
be sure to install the executables of GNU CC that were compiled by GNU CC.
(That is, the executables from stage 2 or 3, not stage 1.) They use alloca
as a built-in function and never the one in the library.
(It is usually better to install GNU CC executables from stage 2 or 3,
since they usually run faster than the ones compiled with some other
compiler.)
16. If you're going to use C++, it's likely that you need to also install a
C++ runtime library. Just as GNU C does not distribute a C runtime
library, it also does not include a C++ runtime library. All I/O
functionality, special class libraries, etc., are provided by the C++
runtime library.
The standard C++ runtime library for GNU CC is called 'libstdc++'. An
obsolescent library 'libg++' may also be available, but it's necessary
only for older software that hasn't been converted yet; if you don't know
whether you need 'libg++' then you probably don't need it.
Here's one way to build and install 'libstdc++' for GNU CC:
Build and install GNU CC, so that invoking 'gcc' obtains the GNU CC
that was just built.
Obtain a copy of a compatible 'libstdc++' distribution. For
example, the 'libstdc++-2.8.0.tar.gz' distribution should be
compatible with GCC 2.8.0. GCC distributors normally distribute
'libstdc++' as well.
Set the 'CXX' environment variable to 'gcc' while running the
'libstdc++' distribution's 'configure' command. Use the same
'configure' options that you used when you invoked GCC's 'configure'
command.
Invoke 'make' to build the C++ runtime.
Invoke 'make install' to install the C++ runtime.
To summarize, after building and installing GNU CC, invoke the following
shell commands in the topmost directory of the C++ library distribution.
For configure-options, use the same options that you used to configure GNU
CC.
$ CXX=gcc ./configure configure-options
$ make
$ make install
17. GNU CC includes a runtime library for Objective-C because it is an
integral part of the language. You can find the files associated with
the library in the subdirectory 'objc'. The GNU Objective-C Runtime
Library requires header files for the target's C library in order to be
compiled,and also requires the header files for the target's thread
library if you want thread support. See Cross-Compilers and Header
Files: Cross-Compilers and Header Files, for discussion about header
files issues for cross-compilation.
When you run 'configure', it picks the appropriate Objective-C thread
implementation file for the target platform. In some situations, you may
wish to choose a different back-end as some platforms support multiple
thread implementations or you may wish to disable thread support
completely. You do this by specifying a value for the OBJC_THREAD_FILE
makefile variable on the command line when you run make, for example:
make CC="stage2/xgcc -Bstage2/" CFLAGS="-g -O2" OBJC_THREAD_FILE=thr-single
Below is a list of the currently available back-ends.
thr-single
Disable thread support, should work for all platforms.
thr-decosf1
DEC OSF/1 thread support.
thr-irix
SGI IRIX thread support.
thr-mach
Generic MACH thread support, known to work on NEXTSTEP.
thr-os2
IBM OS/2 thread support.
thr-posix
Generix POSIX thread support.
thr-pthreads
PCThreads on Linux-based GNU systems.
thr-solaris
SUN Solaris thread support.
thr-win32
Microsoft Win32 API thread support.
ΓòÉΓòÉΓòÉ 5.1. Configurations Supported by GNU CC ΓòÉΓòÉΓòÉ
Here are the possible CPU types:
1750a, a29k, alpha, arm, cn, clipper, dsp16xx, elxsi, h8300,
hppa1.0, hppa1.1, i370, i386, i486, i586, i860, i960, m32r, m68000,
m68k, m88k, mips, mipsel, mips64, mips64el, ns32k, powerpc,
powerpcle, pyramid, romp, rs6000, sh, sparc, sparclite, sparc64,
vax, we32k.
Here are the recognized company names. As you can see, customary
abbreviations are used rather than the longer official names.
acorn, alliant, altos, apollo, apple, att, bull, cbm, convergent,
convex, crds, dec, dg, dolphin, elxsi, encore, harris, hitachi, hp,
ibm, intergraph, isi, mips, motorola, ncr, next, ns, omron, plexus,
sequent, sgi, sony, sun, tti, unicom, wrs.
The company name is meaningful only to disambiguate when the rest of the
information supplied is insufficient. You can omit it, writing just
'cpu-system', if it is not needed. For example, 'vax-ultrix4.2' is equivalent
to 'vax-dec-ultrix4.2'.
Here is a list of system types:
386bsd, aix, acis, amigaos, aos, aout, aux, bosx, bsd, clix, coff,
ctix, cxux, dgux, dynix, ebmon, ecoff, elf, esix, freebsd, hms,
genix, gnu, linux-gnu, hiux, hpux, iris, irix, isc, luna, lynxos,
mach, minix, msdos, mvs, netbsd, newsos, nindy, ns, osf, osfrose,
ptx, riscix, riscos, rtu, sco, sim, solaris, sunos, sym, sysv, udi,
ultrix, unicos, uniplus, unos, vms, vsta, vxworks, winnt, xenix.
You can omit the system type; then 'configure' guesses the operating system
from the CPU and company.
You can add a version number to the system type; this may or may not make a
difference. For example, you can write 'bsd4.3' or 'bsd4.4' to distinguish
versions of BSD. In practice, the version number is most needed for 'sysv3'
and 'sysv4', which are often treated differently.
If you specify an impossible combination such as 'i860-dg-vms', then you may
get an error message from 'configure', or it may ignore part of the
information and do the best it can with the rest. 'configure' always prints
the canonical name for the alternative that it used. GNU CC does not support
all possible alternatives.
Often a particular model of machine has a name. Many machine names are
recognized as aliases for CPU/company combinations. Thus, the machine name
'sun3', mentioned above, is an alias for 'm68k-sun'. Sometimes we accept a
company name as a machine name, when the name is popularly used for a
particular machine. Here is a table of the known machine names:
3300, 3b1, 3bn, 7300, altos3068, altos, apollo68, att-7300, balance,
convex-cn, crds, decstation-3100, decstation, delta, encore, fx2800,
gmicro, hp7nn, hp8nn, hp9k2nn, hp9k3nn, hp9k7nn, hp9k8nn, iris4d,
iris, isi68, m3230, magnum, merlin, miniframe, mmax, news-3600,
news800, news, next, pbd, pc532, pmax, powerpc, powerpcle, ps2,
risc-news, rtpc, sun2, sun386i, sun386, sun3, sun4, symmetry,
tower-32, tower.
Remember that a machine name specifies both the cpu type and the company name.
If you want to install your own homemade configuration files, you can use
'local' as the company name to access them. If you use configuration
'cpu-local', the configuration name without the cpu prefix is used to form the
configuration file names.
Thus, if you specify 'm68k-local', configuration uses files 'm68k.md',
'local.h', 'm68k.c', 'xm-local.h', 't-local', and 'x-local', all in the
directory 'config/m68k'.
Here is a list of configurations that have special treatment or special things
you must know:
'1750a-*-*'
MIL-STD-1750A processors.
The MIL-STD-1750A cross configuration produces output for as1750, an
assembler/linker available under the GNU Public License for the
1750A. as1750 can be obtained at
ftp://ftp.fta-berlin.de/pub/crossgcc/1750gals/. A similarly licensed
simulator for the 1750A is available from same address.
You should ignore a fatal error during the building of libgcc
(libgcc is not yet implemented for the 1750A.)
The as1750 assembler requires the file 'ms1750.inc', which is found
in the directory 'config/1750a'.
GNU CC produced the same sections as the Fairchild F9450 C Compiler,
namely:
Normal
The program code section.
Static
The read/write (RAM) data section.
Konst
The read-only (ROM) constants section.
Init
Initialization section (code to copy KREL to SREL).
The smallest addressable unit is 16 bits (BITS_PER_UNIT is 16).
This means that type `char' is represented with a 16-bit word per
character. The 1750A's " Load/Store Upper/Lower Byte" instructions
are not used by GNU CC.
'alpha-*-osf1'
Systems using processors that implement the DEC Alpha architecture
and are running the DEC Unix (OSF/1) operating system, for example
the DEC Alpha AXP systems.CC.)
GNU CC writes a '.verstamp' directive to the assembler output file
unless it is built as a cross-compiler. It gets the version to use
from the system header file '/usr/include/stamp.h'. If you install
a new version of DEC Unix, you should rebuild GCC to pick up the new
version stamp.
Note that since the Alpha is a 64-bit architecture, cross-compilers
from 32-bit machines will not generate code as efficient as that
generated when the compiler is running on a 64-bit machine because
many optimizations that depend on being able to represent a word on
the target in an integral value on the host cannot be performed.
Building cross-compilers on the Alpha for 32-bit machines has only
been tested in a few cases and may not work properly.
make compare may fail on old versions of DEC Unix unless you add
'-save-temps' to CFLAGS. On these systems, the name of the
assembler input file is stored in the object file, and that makes
comparison fail if it differs between the stage1 and stage2
compilations. The option '-save-temps' forces a fixed name to be
used for the assembler input file, instead of a randomly chosen name
in '/tmp'. Do not add '-save-temps' unless the comparisons fail
without that option. If you add '-save-temps', you will have to
manually delete the '.i' and '.s' files after each series of
compilations.
GNU CC now supports both the native (ECOFF) debugging format used by
DBX and GDB and an encapsulated STABS format for use only with GDB.
See the discussion of the '--with-stabs' option of 'configure' above
for more information on these formats and how to select them.
There is a bug in DEC's assembler that produces incorrect line
numbers for ECOFF format when the '.align' directive is used. To
work around this problem, GNU CC will not emit such alignment
directives while writing ECOFF format debugging information even if
optimization is being performed. Unfortunately, this has the very
undesirable side-effect that code addresses when '-O' is specified
are different depending on whether or not '-g' is also specified.
To avoid this behavior, specify '-gstabs+' and use GDB instead of
DBX. DEC is now aware of this problem with the assembler and hopes
to provide a fix shortly.
'arc-*-elf'
Argonaut ARC processor. This configuration is intended for embedded
systems.
'arm-*-aout'
Advanced RISC Machines ARM-family processors. These are often used
in embedded applications. There are no standard Unix
configurations. This configuration corresponds to the basic
instruction sequences and will produce 'a.out' format object
modules.
You may need to make a variant of the file 'arm.h' for your
particular configuration.
'arm-*-linuxaout'
Any of the ARM family processors running the Linux-based GNU system
with the 'a.out' binary format (ELF is not yet supported). You must
use version 2.8.1.0.7 or later of the GNU/Linux binutils, which you
can download from 'sunsite.unc.edu:/pub/Linux/GCC' and other mirror
sites for Linux-based GNU systems.
'arm-*-riscix'
The ARM2 or ARM3 processor running RISC iX, Acorn's port of BSD
Unix. If you are running a version of RISC iX prior to 1.2 then you
must specify the version number during configuration. Note that the
assembler shipped with RISC iX does not support stabs debugging
information; a new version of the assembler, with stabs support
included, is now available from Acorn and via ftp
'ftp.acorn.com:/pub/riscix/as+xterm.tar.Z'. To enable stabs
debugging, pass '--with-gnu-as' to configure.
You will need to install GNU 'sed' before you can run configure.
'a29k'
AMD Am29k-family processors. These are normally used in embedded
applications. There are no standard Unix configurations. This
configuration corresponds to AMD's standard calling sequence and
binary interface and is compatible with other 29k tools.
You may need to make a variant of the file 'a29k.h' for your
particular configuration.
'a29k-*-bsd'
AMD Am29050 used in a system running a variant of BSD Unix.
'decstation-*'
MIPS-based DECstations can support three different personalities:
Ultrix, DEC OSF/1, and OSF/rose. (Alpha-based DECstation products
have a configuration name beginning with 'alpha-dec'.) To configure
GCC for these platforms use the following configurations:
'decstation-ultrix'
Ultrix configuration.
'decstation-osf1'
Dec's version of OSF/1.
'decstation-osfrose'
Open Software Foundation reference port of OSF/1 which
uses the OSF/rose object file format instead of ECOFF.
Normally, you would not select this configuration.
The MIPS C compiler needs to be told to increase its table size
for switch statements with the '-Wf,-XNg1500' option in order to
compile 'cp/parse.c'. If you use the '-O2' optimization option,
you also need to use '-Olimit 3000'. Both of these options are
automatically generated in the 'Makefile' that the shell script
'configure' builds. If you override the CC make variable and use
the MIPS compilers, you may need to add '-Wf,-XNg1500 -Olimit
3000'.
'elxsi-elxsi-bsd'
The Elxsi's C compiler has known limitations that prevent it from
compiling GNU C. Please contact mrs@cygnus.com for more details.
'dsp16xx'
A port to the AT&T DSP1610 family of processors.
'h8300-*-*'
Hitachi H8/300 series of processors.
The calling convention and structure layout has changed in release
2.6. All code must be recompiled. The calling convention now passes
the first three arguments in function calls in registers.
Structures are no longer a multiple of 2 bytes.
'hppa*-*-*'
There are several variants of the HP-PA processor which run a
variety of operating systems. GNU CC must be configured to use the
correct processor type and operating system, or GNU CC will not
function correctly. The easiest way to handle this problem is to not
specify a target when configuring GNU CC, the 'configure' script
will try to automatically determine the right processor type and
operating system.
'-g' does not work on HP-UX, since that system uses a peculiar
debugging format which GNU CC does not know about. However, '-g'
will work if you also use GAS and GDB in conjunction with GCC. We
highly recommend using GAS for all HP-PA configurations.
You should be using GAS-2.6 (or later) along with GDB-4.16 (or
later). These can be retrieved from all the traditional GNU ftp
archive sites.
GAS will need to be installed into a directory before /bin,
/usr/bin, and /usr/ccs/bin in your search path. You should install
GAS before you build GNU CC.
To enable debugging, you must configure GNU CC with the
'--with-gnu-as' option before building.
'i370-*-*'
This port is very preliminary and has many known bugs. We hope to
have a higher-quality port for this machine soon.
'i386-*-linux-gnuoldld'
Use this configuration to generate 'a.out' binaries on Linux-based
GNU systems if you do not have gas/binutils version 2.5.2 or later
installed. This is an obsolete configuration.
'i386-*-linux-gnuaout'
Use this configuration to generate 'a.out' binaries on Linux-based
GNU systems. This configuration is being superseded. You must use
gas/binutils version 2.5.2 or later.
'i386-*-linux-gnu'
Use this configuration to generate ELF binaries on Linux-based GNU
systems. You must use gas/binutils version 2.5.2 or later.
'i386-*-sco'
Compilation with RCC is recommended. Also, it may be a good idea to
link with GNU malloc instead of the malloc that comes with the
system.
'i386-*-sco3.2v4'
Use this configuration for SCO release 3.2 version 4.
'i386-*-sco3.2v5*'
Use this for the SCO OpenServer Release family including 5.0.0,
5.0.2, 5.0.4, Internet FastStart 1.0, and Internet FastStart 1.1.
GNU CC can generate either ELF or COFF binaries. ELF is the
default. To get COFF output, you must specify '-mcoff' on the
command line. For 5.0.0 and 5.0.2, you must install TLS597 from
ftp.sco.com/TLS. 5.0.4 and later do not require this patch.
NOTE: You must follow the instructions about invoking 'make
bootstrap' because the native OpenServer compiler builds a 'cc1plus'
that will not correctly parse many valid C++ programs. You must do a
'make bootstrap' if you are building with the native compiler.
'i386-*-isc'
It may be a good idea to link with GNU malloc instead of the malloc
that comes with the system.
In ISC version 4.1, 'sed' core dumps when building 'deduced.h'. Use
the version of 'sed' from version 4.0.
'i386-*-esix'
It may be good idea to link with GNU malloc instead of the malloc
that comes with the system.
'i386-ibm-aix'
You need to use GAS version 2.1 or later, and LD from GNU binutils
version 2.2 or later.
'i386-sequent-bsd'
Go to the Berkeley universe before compiling.
'i386-sequent-ptx1*'
Sequent DYNIX/ptx 1.x.
'i386-sequent-ptx2*'
Sequent DYNIX/ptx 2.x.
'i386-sun-sunos4'
You may find that you need another version of GNU CC to begin
bootstrapping with, since the current version when built with the
system's own compiler seems to get an infinite loop compiling part
of 'libgcc2.c'. GNU CC version 2 compiled with GNU CC (any version)
seems not to have this problem.
See Sun Install, for information on installing GNU CC on Sun
systems.
'i[345]86-*-winnt3.5'
This version requires a GAS that has not yet been released. Until
it is, you can get a prebuilt binary version via anonymous ftp from
'cs.washington.edu:pub/gnat' or 'cs.nyu.edu:pub/gnat'. You must also
use the Microsoft header files from the Windows NT 3.5 SDK. Find
these on the CDROM in the '/mstools/h' directory dated 9/4/94. You
must use a fixed version of Microsoft linker made especially for NT
3.5, which is also is available on the NT 3.5 SDK CDROM. If you do
not have this linker, can you also use the linker from Visual C/C++
1.0 or 2.0.
Installing GNU CC for NT builds a wrapper linker, called 'ld.exe',
which mimics the behaviour of Unix 'ld' in the specification of
libraries ('-L' and '-l'). 'ld.exe' looks for both Unix and
Microsoft named libraries. For example, if you specify '-lfoo',
'ld.exe' will look first for 'libfoo.a' and then for 'foo.lib'.
You may install GNU CC for Windows NT in one of two ways, depending
on whether or not you have a Unix-like shell and various Unix-like
utilities.
.If you do not have a Unix-like shell and few Unix-like
utilities, you will use a DOS style batch script called
'configure.bat'. Invoke it as configure winnt from an MSDOS
console window or from the program manager dialog box.
'configure.bat' assumes you have already installed and have in
your path a Unix-like 'sed' program which is used to create a
working 'Makefile' from 'Makefile.in'.
'Makefile' uses the Microsoft Nmake program maintenance utility
and the Visual C/C++ V8.00 compiler to build GNU CC. You need
only have the utilities 'sed' and 'touch' to use this
installation method, which only automatically builds the
compiler itself. You must then examine what 'fixinc.winnt'
does, edit the header files by hand and build 'libgcc.a'
manually.
.The second type of installation assumes you are running a
Unix-like shell, have a complete suite of Unix-like utilities
in your path, and have a previous version of GNU CC already
installed, either through building it via the above
installation method or acquiring a pre-built binary. In this
case, use the 'configure' script in the normal fashion.
'i860-intel-osf1'
This is the Paragon. If you have version 1.0 of the operating
system, see Installation Problems, for special things you need to do
to compensate for peculiarities in the system.
'*-lynx-lynxos'
LynxOS 2.2 and earlier comes with GNU CC 1.x already installed as
'/bin/gcc'. You should compile with this instead of '/bin/cc'. You
can tell GNU CC to use the GNU assembler and linker, by specifying
'--with-gnu-as --with-gnu-ld' when configuring. These will produce
COFF format object files and executables; otherwise GNU CC will use
the installed tools, which produce 'a.out' format executables.
'm32r-*-elf'
Mitsubishi M32R processor. This configuration is intended for
embedded systems.
'm68000-hp-bsd'
HP 9000 series 200 running BSD. Note that the C compiler that comes
with this system cannot compile GNU CC; contact law@cygnus.com to
get binaries of GNU CC for bootstrapping.
'm68k-altos'
Altos 3068. You must use the GNU assembler, linker and debugger.
Also, you must fix a kernel bug. Details in the file
'README.ALTOS'.
'm68k-apple-aux'
Apple Macintosh running A/UX. You may configure GCC to use either
the system assembler and linker or the GNU assembler and linker.
You should use the GNU configuration if you can, especially if you
also want to use GNU C++. You enabled that configuration with + the
'--with-gnu-as' and '--with-gnu-ld' options to configure.
Note the C compiler that comes with this system cannot compile GNU
CC. You can fine binaries of GNU CC for bootstrapping on
jagubox.gsfc.nasa.gov. You will also a patched version of '/bin/ld'
there that raises some of the arbitrary limits found in the
original.
'm68k-att-sysv'
AT&T 3b1, a.k.a. 7300 PC. Special procedures are needed to compile
GNU CC with this machine's standard C compiler, due to bugs in that
compiler. You can bootstrap it more easily with previous versions
of GNU CC if you have them.
Installing GNU CC on the 3b1 is difficult if you do not already have
GNU CC running, due to bugs in the installed C compiler. However,
the following procedure might work. We are unable to test it.
.Comment out the '#include "config.h"' line near the start of
'cccp.c' and do 'make cpp'. This makes a preliminary version
of GNU cpp.
.Save the old '/lib/cpp' and copy the preliminary GNU cpp to
that file name.
.Undo your change in 'cccp.c', or reinstall the original
version, and do 'make cpp' again.
.Copy this final version of GNU cpp into '/lib/cpp'.
.Replace every occurrence of obstack_free in the file 'tree.c'
with _obstack_free.
.Run make to get the first-stage GNU CC.
.Reinstall the original version of '/lib/cpp'.
.Now you can compile GNU CC with itself and install it in the
normal fashion.
'm68k-bull-sysv'
Bull DPX/2 series 200 and 300 with BOS-2.00.45 up to BOS-2.01. GNU
CC works either with native assembler or GNU assembler. You can use
GNU assembler with native coff generation by providing
'--with-gnu-as' to the configure script or use GNU assembler with
dbx-in-coff encapsulation by providing '--with-gnu-as --stabs'. For
any problem with native assembler or for availability of the DPX/2
port of GAS, contact F.Pierresteguy@frcl.bull.fr.
'm68k-crds-unox'
Use 'configure unos' for building on Unos.
The Unos assembler is named casm instead of as. For some strange
reason linking '/bin/as' to '/bin/casm' changes the behavior, and
does not work. So, when installing GNU CC, you should install the
following script as 'as' in the subdirectory where the passes of GCC
are installed:
#!/bin/sh
casm $*
The default Unos library is named 'libunos.a' instead of 'libc.a'.
To allow GNU CC to function, either change all references to '-lc'
in 'gcc.c' to '-lunos' or link '/lib/libc.a' to '/lib/libunos.a'.
When compiling GNU CC with the standard compiler, to overcome bugs
in the support of alloca, do not use '-O' when making stage 2.
Then use the stage 2 compiler with '-O' to make the stage 3
compiler. This compiler will have the same characteristics as the
usual stage 2 compiler on other systems. Use it to make a stage 4
compiler and compare that with stage 3 to verify proper
compilation.
(Perhaps simply defining ALLOCA in 'x-crds' as described in the
comments there will make the above paragraph superfluous. Please
inform us of whether this works.)
Unos uses memory segmentation instead of demand paging, so you
will need a lot of memory. 5 Mb is barely enough if no other
tasks are running. If linking 'cc1' fails, try putting the object
files into a library and linking from that library.
'm68k-hp-hpux'
HP 9000 series 300 or 400 running HP-UX. HP-UX version 8.0 has a
bug in the assembler that prevents compilation of GNU CC. To fix
it, get patch PHCO_4484 from HP.
In addition, if you wish to use gas '--with-gnu-as' you must use gas
version 2.1 or later, and you must use the GNU linker version 2.1 or
later. Earlier versions of gas relied upon a program which
converted the gas output into the native HP/UX format, but that
program has not been kept up to date. gdb does not understand that
native HP/UX format, so you must use gas if you wish to use gdb.
'm68k-sun'
Sun 3. We do not provide a configuration file to use the Sun FPA by
default, because programs that establish signal handlers for
floating point traps inherently cannot work with the FPA.
See Sun Install, for information on installing GNU CC on Sun
systems.
'm88k-*-svr3'
Motorola m88k running the AT&T/Unisoft/Motorola V.3 reference port.
These systems tend to use the Green Hills C, revision 1.8.5, as the
standard C compiler. There are apparently bugs in this compiler
that result in object files differences between stage 2 and stage 3.
If this happens, make the stage 4 compiler and compare it to the
stage 3 compiler. If the stage 3 and stage 4 object files are
identical, this suggests you encountered a problem with the standard
C compiler; the stage 3 and 4 compilers may be usable.
It is best, however, to use an older version of GNU CC for
bootstrapping if you have one.
'm88k-*-dgux'
Motorola m88k running DG/UX. To build 88open BCS native or cross
compilers on DG/UX, specify the configuration name as
'm88k-*-dguxbcs' and build in the 88open BCS software development
environment. To build ELF native or cross compilers on DG/UX,
specify 'm88k-*-dgux' and build in the DG/UX ELF development
environment. You set the software development environment by issuing
'sde-target' command and specifying either 'm88kbcs' or
'm88kdguxelf' as the operand.
If you do not specify a configuration name, 'configure' guesses the
configuration based on the current software development environment.
'm88k-tektronix-sysv3'
Tektronix XD88 running UTekV 3.2e. Do not turn on optimization
while building stage1 if you bootstrap with the buggy Green Hills
compiler. Also, The bundled LAI System V NFS is buggy so if you
build in an NFS mounted directory, start from a fresh reboot, or
avoid NFS all together. Otherwise you may have trouble getting clean
comparisons between stages.
'mips-mips-bsd'
MIPS machines running the MIPS operating system in BSD mode. It's
possible that some old versions of the system lack the functions
memcpy, memcmp, and memset. If your system lacks these, you must
remove or undo the definition of TARGET_MEM_FUNCTIONS in
'mips-bsd.h'.
The MIPS C compiler needs to be told to increase its table size for
switch statements with the '-Wf,-XNg1500' option in order to compile
'cp/parse.c'. If you use the '-O2' optimization option, you also
need to use '-Olimit 3000'. Both of these options are automatically
generated in the 'Makefile' that the shell script 'configure'
builds. If you override the CC make variable and use the MIPS
compilers, you may need to add '-Wf,-XNg1500 -Olimit 3000'.
'mips-mips-riscos*'
The MIPS C compiler needs to be told to increase its table size for
switch statements with the '-Wf,-XNg1500' option in order to compile
'cp/parse.c'. If you use the '-O2' optimization option, you also
need to use '-Olimit 3000'. Both of these options are automatically
generated in the 'Makefile' that the shell script 'configure'
builds. If you override the CC make variable and use the MIPS
compilers, you may need to add '-Wf,-XNg1500 -Olimit 3000'.
MIPS computers running RISC-OS can support four different
personalities: default, BSD 4.3, System V.3, and System V.4 (older
versions of RISC-OS don't support V.4). To configure GCC for these
platforms use the following configurations:
'mips-mips-riscosrev'
Default configuration for RISC-OS, revision rev.
'mips-mips-riscosrevbsd'
BSD 4.3 configuration for RISC-OS, revision rev.
'mips-mips-riscosrevsysv4'
System V.4 configuration for RISC-OS, revision rev.
'mips-mips-riscosrevsysv'
System V.3 configuration for RISC-OS, revision rev.
The revision rev mentioned above is the revision of RISC-OS to
use. You must reconfigure GCC when going from a RISC-OS revision
4 to RISC-OS revision 5. This has the effect of avoiding a linker
bug (see Installation Problems, for more details).
'mips-sgi-*'
In order to compile GCC on an SGI running IRIX 4, the "c.hdr.lib"
option must be installed from the CD-ROM supplied from Silicon
Graphics. This is found on the 2nd CD in release 4.0.1.
In order to compile GCC on an SGI running IRIX 5, the
"compiler_dev.hdr" subsystem must be installed from the IDO CD-ROM
supplied by Silicon Graphics.
make compare may fail on version 5 of IRIX unless you add
'-save-temps' to CFLAGS. On these systems, the name of the
assembler input file is stored in the object file, and that makes
comparison fail if it differs between the stage1 and stage2
compilations. The option '-save-temps' forces a fixed name to be
used for the assembler input file, instead of a randomly chosen name
in '/tmp'. Do not add '-save-temps' unless the comparisons fail
without that option. If you do you '-save-temps', you will have to
manually delete the '.i' and '.s' files after each series of
compilations.
The MIPS C compiler needs to be told to increase its table size for
switch statements with the '-Wf,-XNg1500' option in order to compile
'cp/parse.c'. If you use the '-O2' optimization option, you also
need to use '-Olimit 3000'. Both of these options are automatically
generated in the 'Makefile' that the shell script 'configure'
builds. If you override the CC make variable and use the MIPS
compilers, you may need to add '-Wf,-XNg1500 -Olimit 3000'.
On Irix version 4.0.5F, and perhaps on some other versions as well,
there is an assembler bug that reorders instructions incorrectly.
To work around it, specify the target configuration
'mips-sgi-irix4loser'. This configuration inhibits assembler
optimization.
In a compiler configured with target 'mips-sgi-irix4', you can turn
off assembler optimization by using the '-noasmopt' option. This
compiler option passes the option '-O0' to the assembler, to inhibit
reordering.
The '-noasmopt' option can be useful for testing whether a problem
is due to erroneous assembler reordering. Even if a problem does
not go away with '-noasmopt', it may still be due to assembler
reordering---perhaps GNU CC itself was miscompiled as a result.
To enable debugging under Irix 5, you must use GNU as 2.5 or later,
and use the '--with-gnu-as' configure option when configuring gcc.
GNU as is distributed as part of the binutils package.
'mips-sony-sysv'
Sony MIPS NEWS. This works in NEWSOS 5.0.1, but not in 5.0.2 (which
uses ELF instead of COFF). Support for 5.0.2 will probably be
provided soon by volunteers. In particular, the linker does not
like the code generated by GCC when shared libraries are linked in.
'ns32k-encore'
Encore ns32000 system. Encore systems are supported only under BSD.
'ns32k-*-genix'
National Semiconductor ns32000 system. Genix has bugs in alloca and
malloc; you must get the compiled versions of these from GNU Emacs.
'ns32k-sequent'
Go to the Berkeley universe before compiling.
'ns32k-utek'
UTEK ns32000 system (``merlin''). The C compiler that comes with
this system cannot compile GNU CC; contact 'tektronix!reed!mason' to
get binaries of GNU CC for bootstrapping.
'romp-*-aos'
'romp-*-mach'
The only operating systems supported for the IBM RT PC are AOS and
MACH. GNU CC does not support AIX running on the RT. We recommend
you compile GNU CC with an earlier version of itself; if you compile
GNU CC with hc, the Metaware compiler, it will work, but you will
get mismatches between the stage 2 and stage 3 compilers in various
files. These errors are minor differences in some floating-point
constants and can be safely ignored; the stage 3 compiler is
correct.
'rs6000-*-aix'
'powerpc-*-aix'
Various early versions of each release of the IBM XLC compiler will
not bootstrap GNU CC. Symptoms include differences between the
stage2 and stage3 object files, and errors when compiling 'libgcc.a'
or 'enquire'. Known problematic releases include: xlc-1.2.1.8,
xlc-1.3.0.0 (distributed with AIX 3.2.5), and xlc-1.3.0.19. Both
xlc-1.2.1.28 and xlc-1.3.0.24 (PTF 432238) are known to produce
working versions of GNU CC, but most other recent releases correctly
bootstrap GNU CC.
Release 4.3.0 of AIX and ones prior to AIX 3.2.4 include a version
of the IBM assembler which does not accept debugging directives:
assembler updates are available as PTFs. Also, if you are using AIX
3.2.5 or greater and the GNU assembler, you must have a version
modified after October 16th, 1995 in order for the GNU C compiler to
build. See the file 'README.RS6000' for more details on any of
these problems.
GNU CC does not yet support the 64-bit PowerPC instructions.
Objective C does not work on this architecture because it makes
assumptions that are incompatible with the calling conventions.
AIX on the RS/6000 provides support (NLS) for environments outside
of the United States. Compilers and assemblers use NLS to support
locale-specific representations of various objects including
floating-point numbers ("." vs "," for separating decimal
fractions). There have been problems reported where the library
linked with GNU CC does not produce the same floating-point formats
that the assembler accepts. If you have this problem, set the LANG
environment variable to "C" or "En_US".
Due to changes in the way that GNU CC invokes the binder (linker)
for AIX 4.1, you may now receive warnings of duplicate symbols from
the link step that were not reported before. The assembly files
generated by GNU CC for AIX have always included multiple symbol
definitions for certain global variable and function declarations in
the original program. The warnings should not prevent the linker
from producing a correct library or runnable executable.
By default, AIX 4.1 produces code that can be used on either Power
or PowerPC processors.
You can specify a default version for the '-mcpu='cpu_type switch by
using the configure option '--with-cpu-'cpu_type.
'powerpc-*-elf'
'powerpc-*-sysv4'
PowerPC system in big endian mode, running System V.4.
You can specify a default version for the '-mcpu='cpu_type switch by
using the configure option '--with-cpu-'cpu_type.
'powerpc-*-linux-gnu'
PowerPC system in big endian mode, running the Linux-based GNU
system.
You can specify a default version for the '-mcpu='cpu_type switch by
using the configure option '--with-cpu-'cpu_type.
'powerpc-*-eabiaix'
Embedded PowerPC system in big endian mode with -mcall-aix selected
as the default.
You can specify a default version for the '-mcpu='cpu_type switch by
using the configure option '--with-cpu-'cpu_type.
'powerpc-*-eabisim'
Embedded PowerPC system in big endian mode for use in running under
the PSIM simulator.
You can specify a default version for the '-mcpu='cpu_type switch by
using the configure option '--with-cpu-'cpu_type.
'powerpc-*-eabi'
Embedded PowerPC system in big endian mode.
You can specify a default version for the '-mcpu='cpu_type switch by
using the configure option '--with-cpu-'cpu_type.
'powerpcle-*-elf'
'powerpcle-*-sysv4'
PowerPC system in little endian mode, running System V.4.
You can specify a default version for the '-mcpu='cpu_type switch by
using the configure option '--with-cpu-'cpu_type.
'powerpcle-*-solaris2*'
PowerPC system in little endian mode, running Solaris 2.5.1 or
higher.
You can specify a default version for the '-mcpu='cpu_type switch by
using the configure option '--with-cpu-'cpu_type. Beta versions of
the Sun 4.0 compiler do not seem to be able to build GNU CC
correctly. There are also problems with the host assembler and
linker that are fixed by using the GNU versions of these tools.
'powerpcle-*-eabisim'
Embedded PowerPC system in little endian mode for use in running
under the PSIM simulator.
'powerpcle-*-eabi'
Embedded PowerPC system in little endian mode.
You can specify a default version for the '-mcpu='cpu_type switch by
using the configure option '--with-cpu-'cpu_type.
'powerpcle-*-winnt'
'powerpcle-*-pe'
PowerPC system in little endian mode running Windows NT.
You can specify a default version for the '-mcpu='cpu_type switch by
using the configure option '--with-cpu-'cpu_type.
'vax-dec-ultrix'
Don't try compiling with Vax C (vcc). It produces incorrect code in
some cases (for example, when alloca is used).
Meanwhile, compiling 'cp/parse.c' with pcc does not work because of
an internal table size limitation in that compiler. To avoid this
problem, compile just the GNU C compiler first, and use it to
recompile building all the languages that you want to run.
'sparc-sun-*'
See Sun Install, for information on installing GNU CC on Sun
systems.
'vax-dec-vms'
See VMS Install, for details on how to install GNU CC on VMS.
'we32k-*-*'
These computers are also known as the 3b2, 3b5, 3b20 and other
similar names. (However, the 3b1 is actually a 68000; see
Configurations.)
Don't use '-g' when compiling with the system's compiler. The
system's linker seems to be unable to handle such a large program
with debugging information.
The system's compiler runs out of capacity when compiling 'stmt.c'
in GNU CC. You can work around this by building 'cpp' in GNU CC
first, then use that instead of the system's preprocessor with the
system's C compiler to compile 'stmt.c'. Here is how:
mv /lib/cpp /lib/cpp.att
cp cpp /lib/cpp.gnu
echo '/lib/cpp.gnu -traditional ${1+"$@"}' > /lib/cpp
chmod +x /lib/cpp
The system's compiler produces bad code for some of the GNU CC
optimization files. So you must build the stage 2 compiler
without optimization. Then build a stage 3 compiler with
optimization. That executable should work. Here are the necessary
commands:
make LANGUAGES=c CC=stage1/xgcc CFLAGS="-Bstage1/ -g"
make stage2
make CC=stage2/xgcc CFLAGS="-Bstage2/ -g -O"
You may need to raise the ULIMIT setting to build a C++ compiler,
as the file 'cc1plus' is larger than one megabyte.
ΓòÉΓòÉΓòÉ 5.2. Compilation in a Separate Directory ΓòÉΓòÉΓòÉ
If you wish to build the object files and executables in a directory other
than the one containing the source files, here is what you must do
differently:
1. Make sure you have a version of Make that supports the VPATH feature.
(GNU Make supports it, as do Make versions on most BSD systems.)
2. If you have ever run 'configure' in the source directory, you must undo
the configuration. Do this by running:
make distclean
3. Go to the directory in which you want to build the compiler before
running 'configure':
mkdir gcc-sun3
cd gcc-sun3
On systems that do not support symbolic links, this directory must be on the
same file system as the source code directory.
4. Specify where to find 'configure' when you run it:
┬╖┬╖/gcc/configure ┬╖┬╖┬╖
This also tells configure where to find the compiler sources; configure takes
the directory from the file name that was used to invoke it. But if you want
to be sure, you can specify the source directory with the '--srcdir' option,
like this:
┬╖┬╖/gcc/configure --srcdir=┬╖┬╖/gcc other options
The directory you specify with '--srcdir' need not be the same as the one that
configure is found in.
Now, you can run make in that directory. You need not repeat the
configuration steps shown above, when ordinary source files change. You must,
however, run configure again when the configuration files change, if your
system does not support symbolic links.
ΓòÉΓòÉΓòÉ 5.3. Building and Installing a Cross-Compiler ΓòÉΓòÉΓòÉ
GNU CC can function as a cross-compiler for many machines, but not all.
Cross-compilers for the Mips as target using the Mips assembler currently
do not work, because the auxiliary programs 'mips-tdump.c' and
'mips-tfile.c' can't be compiled on anything but a Mips. It does work to
cross compile for a Mips if you use the GNU assembler and linker.
Cross-compilers between machines with different floating point formats
have not all been made to work. GNU CC now has a floating point emulator
with which these can work, but each target machine description needs to
be updated to take advantage of it.
Cross-compilation between machines of different word sizes is somewhat
problematic and sometimes does not work.
Since GNU CC generates assembler code, you probably need a cross-assembler
that GNU CC can run, in order to produce object files. If you want to link on
other than the target machine, you need a cross-linker as well. You also need
header files and libraries suitable for the target machine that you can
install on the host machine.
Steps of Cross Using a cross-compiler involves several steps
that may be carried out on different machines.
Configure Cross Configuring a cross-compiler.
Tools and Libraries Where to put the linker and assembler, and the C
library.
Cross Headers Finding and installing header files for a
cross-compiler.
Cross Runtime Supplying arithmetic runtime routines
('libgcc1.a').
Build Cross Actually compiling the cross-compiler.
ΓòÉΓòÉΓòÉ 5.3.1. Steps of Cross-Compilation ΓòÉΓòÉΓòÉ
To compile and run a program using a cross-compiler involves several steps:
Run the cross-compiler on the host machine to produce assembler files for
the target machine. This requires header files for the target machine.
Assemble the files produced by the cross-compiler. You can do this
either with an assembler on the target machine, or with a cross-assembler
on the host machine.
Link those files to make an executable. You can do this either with a
linker on the target machine, or with a cross-linker on the host machine.
Whichever machine you use, you need libraries and certain startup files
(typically 'crt┬╖┬╖┬╖.o') for the target machine.
It is most convenient to do all of these steps on the same host machine, since
then you can do it all with a single invocation of GNU CC. This requires a
suitable cross-assembler and cross-linker. For some targets, the GNU
assembler and linker are available.
ΓòÉΓòÉΓòÉ 5.3.2. Configuring a Cross-Compiler ΓòÉΓòÉΓòÉ
To build GNU CC as a cross-compiler, you start out by running 'configure'.
Use the '--target=target' to specify the target type. If 'configure' was
unable to correctly identify the system you are running on, also specify the
'--build=build' option. For example, here is how to configure for a
cross-compiler that produces code for an HP 68030 system running BSD on a
system that 'configure' can correctly identify:
┬╖/configure --target=m68k-hp-bsd4.3
ΓòÉΓòÉΓòÉ 5.3.3. Tools and Libraries for a Cross-Compiler ΓòÉΓòÉΓòÉ
If you have a cross-assembler and cross-linker available, you should install
them now. Put them in the directory '/usr/local/target/bin'. Here is a table
of the tools you should put in this directory:
as
This should be the cross-assembler.
ld
This should be the cross-linker.
ar
This should be the cross-archiver: a program which can manipulate
archive files (linker libraries) in the target machine's format.
ranlib
This should be a program to construct a symbol table in an archive
file.
The installation of GNU CC will find these programs in that directory, and
copy or link them to the proper place to for the cross-compiler to find them
when run later.
The easiest way to provide these files is to build the Binutils package and
GAS. Configure them with the same '--host' and '--target' options that you
use for configuring GNU CC, then build and install them. They install their
executables automatically into the proper directory. Alas, they do not
support all the targets that GNU CC supports.
If you want to install libraries to use with the cross-compiler, such as a
standard C library, put them in the directory '/usr/local/target/lib';
installation of GNU CC copies all the files in that subdirectory into the
proper place for GNU CC to find them and link with them. Here's an example of
copying some libraries from a target machine:
ftp target-machine
lcd /usr/local/target/lib
cd /lib
get libc.a
cd /usr/lib
get libg.a
get libm.a
quit
The precise set of libraries you'll need, and their locations on the target
machine, vary depending on its operating system.
Many targets require ``start files'' such as 'crt0.o' and 'crtn.o' which are
linked into each executable; these too should be placed in
'/usr/local/target/lib'. There may be several alternatives for 'crt0.o', for
use with profiling or other compilation options. Check your target's
definition of STARTFILE_SPEC to find out what start files it uses. Here's an
example of copying these files from a target machine:
ftp target-machine
lcd /usr/local/target/lib
prompt
cd /lib
mget *crt*.o
cd /usr/lib
mget *crt*.o
quit
ΓòÉΓòÉΓòÉ 5.3.4. 'libgcc.a' and Cross-Compilers ΓòÉΓòÉΓòÉ
Code compiled by GNU CC uses certain runtime support functions implicitly.
Some of these functions can be compiled successfully with GNU CC itself, but a
few cannot be. These problem functions are in the source file 'libgcc1.c';
the library made from them is called 'libgcc1.a'.
When you build a native compiler, these functions are compiled with some other
compiler--the one that you use for bootstrapping GNU CC. Presumably it knows
how to open code these operations, or else knows how to call the run-time
emulation facilities that the machine comes with. But this approach doesn't
work for building a cross-compiler. The compiler that you use for building
knows about the host system, not the target system.
So, when you build a cross-compiler you have to supply a suitable library
'libgcc1.a' that does the job it is expected to do.
To compile 'libgcc1.c' with the cross-compiler itself does not work. The
functions in this file are supposed to implement arithmetic operations that
GNU CC does not know how to open code for your target machine. If these
functions are compiled with GNU CC itself, they will compile into infinite
recursion.
On any given target, most of these functions are not needed. If GNU CC can
open code an arithmetic operation, it will not call these functions to perform
the operation. It is possible that on your target machine, none of these
functions is needed. If so, you can supply an empty library as 'libgcc1.a'.
Many targets need library support only for multiplication and division. If you
are linking with a library that contains functions for multiplication and
division, you can tell GNU CC to call them directly by defining the macros
MULSI3_LIBCALL, and the like. These macros need to be defined in the target
description macro file. For some targets, they are defined already. This may
be sufficient to avoid the need for libgcc1.a; if so, you can supply an empty
library.
Some targets do not have floating point instructions; they need other
functions in 'libgcc1.a', which do floating arithmetic. Recent versions of GNU
CC have a file which emulates floating point. With a certain amount of work,
you should be able to construct a floating point emulator that can be used as
'libgcc1.a'. Perhaps future versions will contain code to do this
automatically and conveniently. That depends on whether someone wants to
implement it.
Some embedded targets come with all the necessary 'libgcc1.a' routines written
in C or assembler. These targets build 'libgcc1.a' automatically and you do
not need to do anything special for them. Other embedded targets do not need
any 'libgcc1.a' routines since all the necessary operations are supported by
the hardware.
If your target system has another C compiler, you can configure GNU CC as a
native compiler on that machine, build just 'libgcc1.a' with 'make libgcc1.a'
on that machine, and use the resulting file with the cross-compiler. To do
this, execute the following on the target machine:
cd target-build-dir
┬╖/configure --host=sparc --target=sun3
make libgcc1.a
And then this on the host machine:
ftp target-machine
binary
cd target-build-dir
get libgcc1.a
quit
Another way to provide the functions you need in 'libgcc1.a' is to define the
appropriate perform_┬╖┬╖┬╖ macros for those functions. If these definitions do
not use the C arithmetic operators that they are meant to implement, you
should be able to compile them with the cross-compiler you are building. (If
these definitions already exist for your target file, then you are all set.)
To build 'libgcc1.a' using the perform macros, use 'LIBGCC1=libgcc1.a
OLDCC=./xgcc' when building the compiler. Otherwise, you should place your
replacement library under the name 'libgcc1.a' in the directory in which you
will build the cross-compiler, before you run make.
ΓòÉΓòÉΓòÉ 5.3.5. Cross-Compilers and Header Files ΓòÉΓòÉΓòÉ
If you are cross-compiling a standalone program or a program for an embedded
system, then you may not need any header files except the few that are part of
GNU CC (and those of your program). However, if you intend to link your
program with a standard C library such as 'libc.a', then you probably need to
compile with the header files that go with the library you use.
The GNU C compiler does not come with these files, because (1) they are
system-specific, and (2) they belong in a C library, not in a compiler.
If the GNU C library supports your target machine, then you can get the header
files from there (assuming you actually use the GNU library when you link your
program).
If your target machine comes with a C compiler, it probably comes with
suitable header files also. If you make these files accessible from the host
machine, the cross-compiler can use them also.
Otherwise, you're on your own in finding header files to use when
cross-compiling.
When you have found suitable header files, put them in the directory
'/usr/local/target/include', before building the cross compiler. Then
installation will run fixincludes properly and install the corrected versions
of the header files where the compiler will use them.
Provide the header files before you build the cross-compiler, because the
build stage actually runs the cross-compiler to produce parts of 'libgcc.a'.
(These are the parts that can be compiled with GNU CC.) Some of them need
suitable header files.
Here's an example showing how to copy the header files from a target machine.
On the target machine, do this:
(cd /usr/include; tar cf - .) > tarfile
Then, on the host machine, do this:
ftp target-machine
lcd /usr/local/target/include
get tarfile
quit
tar xf tarfile
ΓòÉΓòÉΓòÉ 5.3.6. Actually Building the Cross-Compiler ΓòÉΓòÉΓòÉ
Now you can proceed just as for compiling a single-machine compiler through
the step of building stage 1. If you have not provided some sort of
'libgcc1.a', then compilation will give up at the point where it needs that
file, printing a suitable error message. If you do provide 'libgcc1.a', then
building the compiler will automatically compile and link a test program
called 'libgcc1-test'; if you get errors in the linking, it means that not all
of the necessary routines in 'libgcc1.a' are available.
You must provide the header file 'float.h'. One way to do this is to compile
'enquire' and run it on your target machine. The job of 'enquire' is to run
on the target machine and figure out by experiment the nature of its floating
point representation. 'enquire' records its findings in the header file
'float.h'. If you can't produce this file by running 'enquire' on the target
machine, then you will need to come up with a suitable 'float.h' in some other
way (or else, avoid using it in your programs).
Do not try to build stage 2 for a cross-compiler. It doesn't work to rebuild
GNU CC as a cross-compiler using the cross-compiler, because that would
produce a program that runs on the target machine, not on the host. For
example, if you compile a 386-to-68030 cross-compiler with itself, the result
will not be right either for the 386 (because it was compiled into 68030 code)
or for the 68030 (because it was configured for a 386 as the host). If you
want to compile GNU CC into 68030 code, whether you compile it on a 68030 or
with a cross-compiler on a 386, you must specify a 68030 as the host when you
configure it.
To install the cross-compiler, use 'make install', as usual.
ΓòÉΓòÉΓòÉ 5.4. Installing GNU CC on the Sun ΓòÉΓòÉΓòÉ
On Solaris, do not use the linker or other tools in '/usr/ucb' to build GNU
CC. Use /usr/ccs/bin.
If the assembler reports 'Error: misaligned data' when bootstrapping, you are
probably using an obsolete version of the GNU assembler. Upgrade to the
latest version of GNU binutils, or use the Solaris assembler.
Make sure the environment variable FLOAT_OPTION is not set when you compile
'libgcc.a'. If this option were set to f68881 when 'libgcc.a' is compiled,
the resulting code would demand to be linked with a special startup file and
would not link properly without special pains.
There is a bug in alloca in certain versions of the Sun library. To avoid this
bug, install the binaries of GNU CC that were compiled by GNU CC. They use
alloca as a built-in function and never the one in the library.
Some versions of the Sun compiler crash when compiling GNU CC. The problem is
a segmentation fault in cpp. This problem seems to be due to the bulk of data
in the environment variables. You may be able to avoid it by using the
following command to compile GNU CC with Sun CC:
make CC="TERMCAP=x OBJS=x LIBFUNCS=x STAGESTUFF=x cc"
SunOS 4.1.3 and 4.1.3_U1 have bugs that can cause intermittent core dumps when
compiling GNU CC. A common symptom is an internal compiler error which does
not recur if you run it again. To fix the problem, install Sun recommended
patch 100726 (for SunOS 4.1.3) or 101508 (for SunOS 4.1.3_U1), or upgrade to a
later SunOS release.
ΓòÉΓòÉΓòÉ 5.5. Installing GNU CC on VMS ΓòÉΓòÉΓòÉ
The VMS version of GNU CC is distributed in a backup saveset containing both
source code and precompiled binaries.
To install the 'gcc' command so you can use the compiler easily, in the same
manner as you use the VMS C compiler, you must install the VMS CLD file for
GNU CC as follows:
1. Define the VMS logical names 'GNU_CC' and 'GNU_CC_INCLUDE' to point to
the directories where the GNU CC executables ('gcc-cpp.exe',
'gcc-cc1.exe', etc.) and the C include files are kept respectively. This
should be done with the commands:
$ assign /system /translation=concealed -
disk:[gcc.] gnu_cc
$ assign /system /translation=concealed -
disk:[gcc.include.] gnu_cc_include
with the appropriate disk and directory names. These commands can be placed
in your system startup file so they will be executed whenever the machine is
rebooted. You may, if you choose, do this via the 'GCC_INSTALL.COM' script in
the '[GCC]' directory.
2. Install the 'GCC' command with the command line:
$ set command /table=sys$common:[syslib]dcltables -
/output=sys$common:[syslib]dcltables gnu_cc:[000000]gcc
$ install replace sys$common:[syslib]dcltables
3. To install the help file, do the following:
$ library/help sys$library:helplib.hlb gcc.hlp
Now you can invoke the compiler with a command like 'gcc /verbose file.c',
which is equivalent to the command 'gcc -v -c file.c' in Unix.
If you wish to use GNU C++ you must first install GNU CC, and then perform the
following steps:
1. Define the VMS logical name 'GNU_GXX_INCLUDE' to point to the directory
where the preprocessor will search for the C++ header files. This can be
done with the command:
$ assign /system /translation=concealed -
disk:[gcc.gxx_include.] gnu_gxx_include
with the appropriate disk and directory name. If you are going to be using a
C++ runtime library, this is where its install procedure will install its
header files.
2. Obtain the file 'gcc-cc1plus.exe', and place this in the same directory
that 'gcc-cc1.exe' is kept.
The GNU C++ compiler can be invoked with a command like 'gcc /plus
/verbose file.cc', which is equivalent to the command 'g++ -v -c file.cc'
in Unix.
We try to put corresponding binaries and sources on the VMS distribution tape.
But sometimes the binaries will be from an older version than the sources,
because we don't always have time to update them. (Use the '/version' option
to determine the version number of the binaries and compare it with the source
file 'version.c' to tell whether this is so.) In this case, you should use
the binaries you get to recompile the sources. If you must recompile, here is
how:
1. Execute the command procedure 'vmsconfig.com' to set up the files 'tm.h',
'config.h', 'aux-output.c', and 'md.', and to create files 'tconfig.h'
and 'hconfig.h'. This procedure also creates several linker option files
used by 'make-cc1.com' and a data file used by 'make-l2.com'.
$ @vmsconfig.com
2. Setup the logical names and command tables as defined above. In
addition, define the VMS logical name 'GNU_BISON' to point at the to the
directories where the Bison executable is kept. This should be done with
the command:
$ assign /system /translation=concealed -
disk:[bison.] gnu_bison
You may, if you choose, use the 'INSTALL_BISON.COM' script in the '[BISON]'
directory.
3. Install the 'BISON' command with the command line:
$ set command /table=sys$common:[syslib]dcltables -
/output=sys$common:[syslib]dcltables -
gnu_bison:[000000]bison
$ install replace sys$common:[syslib]dcltables
4. Type '@make-gcc' to recompile everything (alternatively, submit the file
'make-gcc.com' to a batch queue). If you wish to build the GNU C++
compiler as well as the GNU CC compiler, you must first edit
'make-gcc.com' and follow the instructions that appear in the comments.
5. In order to use GCC, you need a library of functions which GCC compiled
code will call to perform certain tasks, and these functions are defined
in the file 'libgcc2.c'. To compile this you should use the command
procedure 'make-l2.com', which will generate the library 'libgcc2.olb'.
'libgcc2.olb' should be built using the compiler built from the same
distribution that 'libgcc2.c' came from, and 'make-gcc.com' will
automatically do all of this for you.
To install the library, use the following commands:
$ library gnu_cc:[000000]gcclib/delete=(new,eprintf)
$ library gnu_cc:[000000]gcclib/delete=L_*
$ library libgcc2/extract=*/output=libgcc2.obj
$ library gnu_cc:[000000]gcclib libgcc2.obj
The first command simply removes old modules that will be replaced with
modules from 'libgcc2' under different module names. The modules new and
eprintf may not actually be present in your 'gcclib.olb'---if the VMS
librarian complains about those modules not being present, simply ignore the
message and continue on with the next command. The second command removes the
modules that came from the previous version of the library 'libgcc2.c'.
Whenever you update the compiler on your system, you should also update the
library with the above procedure.
6. You may wish to build GCC in such a way that no files are written to the
directory where the source files reside. An example would be the when
the source files are on a read-only disk. In these cases, execute the
following DCL commands (substituting your actual path names):
$ assign dua0:[gcc.build_dir.]/translation=concealed, -
dua1:[gcc.source_dir.]/translation=concealed gcc_build
$ set default gcc_build:[000000]
where the directory 'dua1:[gcc.source_dir]' contains the source code, and the
directory 'dua0:[gcc.build_dir]' is meant to contain all of the generated
object files and executables. Once you have done this, you can proceed
building GCC as described above. (Keep in mind that 'gcc_build' is a rooted
logical name, and thus the device names in each element of the search list
must be an actual physical device name rather than another rooted logical
name).
7. If you are building GNU CC with a previous version of GNU CC, you also
should check to see that you have the newest version of the assembler.
In particular, GNU CC version 2 treats global constant variables slightly
differently from GNU CC version 1, and GAS version 1.38.1 does not have
the patches required to work with GCC version 2. If you use GAS 1.38.1,
then extern const variables will not have the read-only bit set, and the
linker will generate warning messages about mismatched psect attributes
for these variables. These warning messages are merely a nuisance, and
can safely be ignored.
If you are compiling with a version of GNU CC older than 1.33, specify
'/DEFINE=("inline=")' as an option in all the compilations. This
requires editing all the gcc commands in 'make-cc1.com'. (The older
versions had problems supporting inline.) Once you have a working 1.33
or newer GNU CC, you can change this file back.
8. If you want to build GNU CC with the VAX C compiler, you will need to
make minor changes in 'make-cccp.com' and 'make-cc1.com' to choose
alternate definitions of CC, CFLAGS, and LIBS. See comments in those
files. However, you must also have a working version of the GNU
assembler (GNU as, aka GAS) as it is used as the back-end for GNU CC to
produce binary object modules and is not included in the GNU CC sources.
GAS is also needed to compile 'libgcc2' in order to build 'gcclib' (see
above); 'make-l2.com' expects to be able to find it operational in
'gnu_cc:[000000]gnu-as.exe'.
To use GNU CC on VMS, you need the VMS driver programs 'gcc.exe',
'gcc.com', and 'gcc.cld'. They are distributed with the VMS binaries
('gcc-vms') rather than the GNU CC sources. GAS is also included in
'gcc-vms', as is Bison.
Once you have successfully built GNU CC with VAX C, you should use the
resulting compiler to rebuild itself. Before doing this, be sure to
restore the CC, CFLAGS, and LIBS definitions in 'make-cccp.com' and
'make-cc1.com'. The second generation compiler will be able to take
advantage of many optimizations that must be suppressed when building
with other compilers.
Under previous versions of GNU CC, the generated code would occasionally give
strange results when linked with the sharable 'VAXCRTL' library. Now this
should work.
Even with this version, however, GNU CC itself should not be linked with the
sharable 'VAXCRTL'. The version of qsort in 'VAXCRTL' has a bug (known to be
present in VMS versions V4.6 through V5.5) which causes the compiler to fail.
The executables are generated by 'make-cc1.com' and 'make-cccp.com' use the
object library version of 'VAXCRTL' in order to make use of the qsort routine
in 'gcclib.olb'. If you wish to link the compiler executables with the
shareable image version of 'VAXCRTL', you should edit the file 'tm.h' (created
by 'vmsconfig.com') to define the macro QSORT_WORKAROUND.
QSORT_WORKAROUND is always defined when GNU CC is compiled with VAX C, to
avoid a problem in case 'gcclib.olb' is not yet available.
ΓòÉΓòÉΓòÉ 5.6. collect2 ΓòÉΓòÉΓòÉ
Many target systems do not have support in the assembler and linker for
``constructors''---initialization functions to be called before the official
``start'' of main. On such systems, GNU CC uses a utility called collect2 to
arrange to call these functions at start time.
The program collect2 works by linking the program once and looking through the
linker output file for symbols with particular names indicating they are
constructor functions. If it finds any, it creates a new temporary '.c' file
containing a table of them, compiles it, and links the program a second time
including that file.
The actual calls to the constructors are carried out by a subroutine called
__main, which is called (automatically) at the beginning of the body of main
(provided main was compiled with GNU CC). Calling __main is necessary, even
when compiling C code, to allow linking C and C++ object code together. (If
you use '-nostdlib', you get an unresolved reference to __main, since it's
defined in the standard GCC library. Include '-lgcc' at the end of your
compiler command line to resolve this reference.)
The program collect2 is installed as ld in the directory where the passes of
the compiler are installed. When collect2 needs to find the real ld, it tries
the following file names:
'real-ld' in the directories listed in the compiler's search directories.
'real-ld' in the directories listed in the environment variable PATH.
The file specified in the REAL_LD_FILE_NAME configuration macro, if
specified.
'ld' in the compiler's search directories, except that collect2 will not
execute itself recursively.
'ld' in PATH.
``The compiler's search directories'' means all the directories where gcc
searches for passes of the compiler. This includes directories that you
specify with '-B'.
Cross-compilers search a little differently:
'real-ld' in the compiler's search directories.
'target-real-ld' in PATH.
The file specified in the REAL_LD_FILE_NAME configuration macro, if
specified.
'ld' in the compiler's search directories.
'target-ld' in PATH.
collect2 explicitly avoids running ld using the file name under which collect2
itself was invoked. In fact, it remembers up a list of such names---in case
one copy of collect2 finds another copy (or version) of collect2 installed as
ld in a second place in the search path.
collect2 searches for the utilities nm and strip using the same algorithm as
above for ld.
ΓòÉΓòÉΓòÉ 5.7. Standard Header File Directories ΓòÉΓòÉΓòÉ
GCC_INCLUDE_DIR means the same thing for native and cross. It is where GNU CC
stores its private include files, and also where GNU CC stores the fixed
include files. A cross compiled GNU CC runs fixincludes on the header files
in '$(tooldir)/include'. (If the cross compilation header files need to be
fixed, they must be installed before GNU CC is built. If the cross
compilation header files are already suitable for ANSI C and GNU CC, nothing
special need be done).
GPLUS_INCLUDE_DIR means the same thing for native and cross. It is where g++
looks first for header files. The C++ library installs only target
independent header files in that directory.
LOCAL_INCLUDE_DIR is used only for a native compiler. It is normally
'/usr/local/include'. GNU CC searches this directory so that users can
install header files in '/usr/local/include'.
CROSS_INCLUDE_DIR is used only for a cross compiler. GNU CC doesn't install
anything there.
TOOL_INCLUDE_DIR is used for both native and cross compilers. It is the place
for other packages to install header files that GNU CC will use. For a
cross-compiler, this is the equivalent of '/usr/include'. When you build a
cross-compiler, fixincludes processes any header files in this directory.
ΓòÉΓòÉΓòÉ 6. Extensions to the C Language Family ΓòÉΓòÉΓòÉ
GNU C provides several language features not found in ANSI standard C. (The
'-pedantic' option directs GNU CC to print a warning message if any of these
features is used.) To test for the availability of these features in
conditional compilation, check for a predefined macro __GNUC__, which is
always defined under GNU CC.
These extensions are available in C and Objective C. Most of them are also
available in C++. See Extensions to the C++ Language, for extensions that
apply only to C++.
Statement Exprs Putting statements and declarations inside
expressions.
Local Labels Labels local to a statement-expression.
Labels as Values Getting pointers to labels, and computed gotos.
Nested Functions As in Algol and Pascal, lexical scoping of
functions.
Constructing Calls Dispatching a call to another function.
Naming Types Giving a name to the type of some expression.
Typeof typeof: referring to the type of an expression.
Lvalues Using '?:', ',' and casts in lvalues.
Conditionals Omitting the middle operand of a '?:'
expression.
Long Long Double-word integers---long long int.
Complex Data types for complex numbers.
Zero Length Zero-length arrays.
Variable Length Arrays whose length is computed at run time.
Macro Varargs Macros with variable number of arguments.
Subscripting Any array can be subscripted, even if not an
lvalue.
Pointer Arith Arithmetic on void-pointers and function
pointers.
Initializers Non-constant initializers.
Constructors Constructor expressions give structures, unions
or arrays as values.
Labeled Elements Labeling elements of initializers.
Cast to Union Casting to union type from any member of the
union.
Case Ranges `case 1 ┬╖┬╖┬╖ 9' and such.
Function Attributes Declaring that functions have no side effects,
or that they can never return.
Function Prototypes Prototype declarations and old-style
definitions.
C++ Comments C++ comments are recognized.
Dollar Signs Dollar sign is allowed in identifiers.
Character Escapes '\e' stands for the character ESC.
Variable Attributes Specifying attributes of variables.
Type Attributes Specifying attributes of types.
Alignment Inquiring about the alignment of a type or
variable.
Inline Defining inline functions (as fast as macros).
Extended Asm Assembler instructions with C expressions as
operands. (With them you can define ``built-in''
functions.)
Constraints Constraints for asm operands
Asm Labels Specifying the assembler name to use for a C
symbol.
Explicit Reg Vars Defining variables residing in specified
registers.
Alternate Keywords __const__, __asm__, etc., for header files.
Incomplete Enums enum foo;, with details to follow.
Function Names Printable strings which are the name of the
current function.
Return Address Getting the return or frame address of a
function.
ΓòÉΓòÉΓòÉ 6.1. Statements and Declarations in Expressions ΓòÉΓòÉΓòÉ
A compound statement enclosed in parentheses may appear as an expression in
GNU C. This allows you to use loops, switches, and local variables within an
expression.
Recall that a compound statement is a sequence of statements surrounded by
braces; in this construct, parentheses go around the braces. For example:
({ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; })
is a valid (though slightly more complex than necessary) expression for the
absolute value of foo ().
The last thing in the compound statement should be an expression followed by a
semicolon; the value of this subexpression serves as the value of the entire
construct. (If you use some other kind of statement last within the braces,
the construct has type void, and thus effectively no value.)
This feature is especially useful in making macro definitions ``safe'' (so
that they evaluate each operand exactly once). For example, the ``maximum''
function is commonly defined as a macro in standard C as follows:
#define max(a,b) ((a) > (b) ? (a) : (b))
But this definition computes either a or b twice, with bad results if the
operand has side effects. In GNU C, if you know the type of the operands
(here let's assume int), you can define the macro safely as follows:
#define maxint(a,b) \
({int _a = (a), _b = (b); _a > _b ? _a : _b; })
Embedded statements are not allowed in constant expressions, such as the value
of an enumeration constant, the width of a bit field, or the initial value of
a static variable.
If you don't know the type of the operand, you can still do this, but you must
use typeof (see Typeof) or type naming (see Naming Types).
ΓòÉΓòÉΓòÉ 6.2. Locally Declared Labels ΓòÉΓòÉΓòÉ
Each statement expression is a scope in which local labels can be declared. A
local label is simply an identifier; you can jump to it with an ordinary goto
statement, but only from within the statement expression it belongs to.
A local label declaration looks like this:
__label__ label;
or
__label__ label1, label2, ┬╖┬╖┬╖;
Local label declarations must come at the beginning of the statement
expression, right after the '({', before any ordinary declarations.
The label declaration defines the label name, but does not define the label
itself. You must do this in the usual way, with label:, within the statements
of the statement expression.
The local label feature is useful because statement expressions are often used
in macros. If the macro contains nested loops, a goto can be useful for
breaking out of them. However, an ordinary label whose scope is the whole
function cannot be used: if the macro can be expanded several times in one
function, the label will be multiply defined in that function. A local label
avoids this problem. For example:
#define SEARCH(array, target) \
({ \
__label__ found; \
typeof (target) _SEARCH_target = (target); \
typeof (*(array)) *_SEARCH_array = (array); \
int i, j; \
int value; \
for (i = 0; i < max; i++) \
for (j = 0; j < max; j++) \
if (_SEARCH_array[i][j] == _SEARCH_target) \
{ value = i; goto found; } \
value = -1; \
found: \
value; \
})
ΓòÉΓòÉΓòÉ 6.3. Labels as Values ΓòÉΓòÉΓòÉ
You can get the address of a label defined in the current function (or a
containing function) with the unary operator '&&'. The value has type void *.
This value is a constant and can be used wherever a constant of that type is
valid. For example:
void *ptr;
┬╖┬╖┬╖
ptr = &&foo;
To use these values, you need to be able to jump to one. This is done with
the computed goto statement (2), goto *exp;. For example,
goto *ptr;
Any expression of type void * is allowed.
One way of using these constants is in initializing a static array that will
serve as a jump table:
static void *array[] = { &&foo, &&bar, &&hack };
Then you can select a label with indexing, like this:
goto *array[i];
Note that this does not check whether the subscript is in bounds---array
indexing in C never does that.
Such an array of label values serves a purpose much like that of the switch
statement. The switch statement is cleaner, so use that rather than an array
unless the problem does not fit a switch statement very well.
Another use of label values is in an interpreter for threaded code. The labels
within the interpreter function can be stored in the threaded code for
super-fast dispatching.
You can use this mechanism to jump to code in a different function. If you do
that, totally unpredictable things will happen. The best way to avoid this is
to store the label address only in automatic variables and never pass it as an
argument.
ΓòÉΓòÉΓòÉ 6.4. Nested Functions ΓòÉΓòÉΓòÉ
A nested function is a function defined inside another function. (Nested
functions are not supported for GNU C++.) The nested function's name is local
to the block where it is defined. For example, here we define a nested
function named square, and call it twice:
foo (double a, double b)
{
double square (double z) { return z * z; }
return square (a) + square (b);
}
The nested function can access all the variables of the containing function
that are visible at the point of its definition. This is called lexical
scoping. For example, here we show a nested function which uses an inherited
variable named offset:
bar (int *array, int offset, int size)
{
int access (int *array, int index)
{ return array[index + offset]; }
int i;
┬╖┬╖┬╖
for (i = 0; i < size; i++)
┬╖┬╖┬╖ access (array, i) ┬╖┬╖┬╖
}
Nested function definitions are permitted within functions in the places where
variable definitions are allowed; that is, in any block, before the first
statement in the block.
It is possible to call the nested function from outside the scope of its name
by storing its address or passing the address to another function:
hack (int *array, int size)
{
void store (int index, int value)
{ array[index] = value; }
intermediate (store, size);
}
Here, the function intermediate receives the address of store as an argument.
If intermediate calls store, the arguments given to store are used to store
into array. But this technique works only so long as the containing function
(hack, in this example) does not exit.
If you try to call the nested function through its address after the
containing function has exited, all hell will break loose. If you try to call
it after a containing scope level has exited, and if it refers to some of the
variables that are no longer in scope, you may be lucky, but it's not wise to
take the risk. If, however, the nested function does not refer to anything
that has gone out of scope, you should be safe.
GNU CC implements taking the address of a nested function using a technique
called trampolines.
A nested function can jump to a label inherited from a containing function,
provided the label was explicitly declared in the containing function (see
Local Labels). Such a jump returns instantly to the containing function,
exiting the nested function which did the goto and any intermediate functions
as well. Here is an example:
bar (int *array, int offset, int size)
{
__label__ failure;
int access (int *array, int index)
{
if (index > size)
goto failure;
return array[index + offset];
}
int i;
┬╖┬╖┬╖
for (i = 0; i < size; i++)
┬╖┬╖┬╖ access (array, i) ┬╖┬╖┬╖
┬╖┬╖┬╖
return 0;
/* Control comes here from access if it detects an error. */
failure:
return -1;
}
A nested function always has internal linkage. Declaring one with extern is
erroneous. If you need to declare the nested function before its definition,
use auto (which is otherwise meaningless for function declarations).
bar (int *array, int offset, int size)
{
__label__ failure;
auto int access (int *, int);
┬╖┬╖┬╖
int access (int *array, int index)
{
if (index > size)
goto failure;
return array[index + offset];
}
┬╖┬╖┬╖
}
ΓòÉΓòÉΓòÉ 6.5. Constructing Function Calls ΓòÉΓòÉΓòÉ
Using the built-in functions described below, you can record the arguments a
function received, and call another function with the same arguments, without
knowing the number or types of the arguments.
You can also record the return value of that function call, and later return
that value, without knowing what data type the function tried to return (as
long as your caller expects that data type).
__builtin_apply_args ()
This built-in function returns a pointer of type void * to data
describing how to perform a call with the same arguments as were
passed to the current function.
The function saves the arg pointer register, structure value
address, and all registers that might be used to pass arguments to a
function into a block of memory allocated on the stack. Then it
returns the address of that block.
__builtin_apply (function, arguments, size)
This built-in function invokes function (type void (*)()) with a
copy of the parameters described by arguments (type void *) and size
(type int).
The value of arguments should be the value returned by
__builtin_apply_args. The argument size specifies the size of the
stack argument data, in bytes.
This function returns a pointer of type void * to data describing
how to return whatever value was returned by function. The data is
saved in a block of memory allocated on the stack.
It is not always simple to compute the proper value for size. The
value is used by __builtin_apply to compute the amount of data that
should be pushed on the stack and copied from the incoming argument
area.
__builtin_return (result)
This built-in function returns the value described by result from
the containing function. You should specify, for result, a value
returned by __builtin_apply.
ΓòÉΓòÉΓòÉ 6.6. Naming an Expression's Type ΓòÉΓòÉΓòÉ
You can give a name to the type of an expression using a typedef declaration
with an initializer. Here is how to define name as a type name for the type
of exp:
typedef name = exp;
This is useful in conjunction with the statements-within-expressions feature.
Here is how the two together can be used to define a safe ``maximum'' macro
that operates on any arithmetic type:
#define max(a,b) \
({typedef _ta = (a), _tb = (b); \
_ta _a = (a); _tb _b = (b); \
_a > _b ? _a : _b; })
The reason for using names that start with underscores for the local variables
is to avoid conflicts with variable names that occur within the expressions
that are substituted for a and b. Eventually we hope to design a new form of
declaration syntax that allows you to declare variables whose scopes start
only after their initializers; this will be a more reliable way to prevent
such conflicts.
ΓòÉΓòÉΓòÉ 6.7. Referring to a Type with typeof ΓòÉΓòÉΓòÉ
Another way to refer to the type of an expression is with typeof. The syntax
of using of this keyword looks like sizeof, but the construct acts
semantically like a type name defined with typedef.
There are two ways of writing the argument to typeof: with an expression or
with a type. Here is an example with an expression:
typeof (x[0](1))
This assumes that x is an array of functions; the type described is that of
the values of the functions.
Here is an example with a typename as the argument:
typeof (int *)
Here the type described is that of pointers to int.
If you are writing a header file that must work when included in ANSI C
programs, write __typeof__ instead of typeof. See Alternate Keywords.
A typeof-construct can be used anywhere a typedef name could be used. For
example, you can use it in a declaration, in a cast, or inside of sizeof or
typeof.
This declares y with the type of what x points to.
typeof (*x) y;
This declares y as an array of such values.
typeof (*x) y[4];
This declares y as an array of pointers to characters:
typeof (typeof (char *)[4]) y;
It is equivalent to the following traditional C declaration:
char *y[4];
To see the meaning of the declaration using typeof, and why it might be a
useful way to write, let's rewrite it with these macros:
#define pointer(T) typeof(T *)
#define array(T, N) typeof(T [N])
Now the declaration can be rewritten this way:
array (pointer (char), 4) y;
Thus, array (pointer (char), 4) is the type of arrays of 4 pointers to char.
ΓòÉΓòÉΓòÉ 6.8. Generalized Lvalues ΓòÉΓòÉΓòÉ
Compound expressions, conditional expressions and casts are allowed as lvalues
provided their operands are lvalues. This means that you can take their
addresses or store values into them.
Standard C++ allows compound expressions and conditional expressions as
lvalues, and permits casts to reference type, so use of this extension is
deprecated for C++ code.
For example, a compound expression can be assigned, provided the last
expression in the sequence is an lvalue. These two expressions are
equivalent:
(a, b) += 5
a, (b += 5)
Similarly, the address of the compound expression can be taken. These two
expressions are equivalent:
&(a, b)
a, &b
A conditional expression is a valid lvalue if its type is not void and the
true and false branches are both valid lvalues. For example, these two
expressions are equivalent:
(a ? b : c) = 5
(a ? b = 5 : (c = 5))
A cast is a valid lvalue if its operand is an lvalue. A simple assignment
whose left-hand side is a cast works by converting the right-hand side first
to the specified type, then to the type of the inner left-hand side
expression. After this is stored, the value is converted back to the
specified type to become the value of the assignment. Thus, if a has type
char *, the following two expressions are equivalent:
(int)a = 5
(int)(a = (char *)(int)5)
An assignment-with-arithmetic operation such as '+=' applied to a cast
performs the arithmetic using the type resulting from the cast, and then
continues as in the previous case. Therefore, these two expressions are
equivalent:
(int)a += 5
(int)(a = (char *)(int) ((int)a + 5))
You cannot take the address of an lvalue cast, because the use of its address
would not work out coherently. Suppose that &(int)f were permitted, where f
has type float. Then the following statement would try to store an integer
bit-pattern where a floating point number belongs:
*&(int)f = 1;
This is quite different from what (int)f = 1 would do---that would convert 1
to floating point and store it. Rather than cause this inconsistency, we
think it is better to prohibit use of '&' on a cast.
If you really do want an int * pointer with the address of f, you can simply
write (int *)&f.
ΓòÉΓòÉΓòÉ 6.9. Conditionals with Omitted Operands ΓòÉΓòÉΓòÉ
The middle operand in a conditional expression may be omitted. Then if the
first operand is nonzero, its value is the value of the conditional
expression.
Therefore, the expression
x ? : y
has the value of x if that is nonzero; otherwise, the value of y.
This example is perfectly equivalent to
x ? x : y
In this simple case, the ability to omit the middle operand is not especially
useful. When it becomes useful is when the first operand does, or may (if it
is a macro argument), contain a side effect. Then repeating the operand in
the middle would perform the side effect twice. Omitting the middle operand
uses the value already computed without the undesirable effects of recomputing
it.
ΓòÉΓòÉΓòÉ 6.10. Double-Word Integers ΓòÉΓòÉΓòÉ
GNU C supports data types for integers that are twice as long as int. Simply
write long long int for a signed integer, or unsigned long long int for an
unsigned integer. To make an integer constant of type long long int, add the
suffix LL to the integer. To make an integer constant of type unsigned long
long int, add the suffix ULL to the integer.
You can use these types in arithmetic like any other integer types. Addition,
subtraction, and bitwise boolean operations on these types are open-coded on
all types of machines. Multiplication is open-coded if the machine supports
fullword-to-doubleword a widening multiply instruction. Division and shifts
are open-coded only on machines that provide special support. The operations
that are not open-coded use special library routines that come with GNU CC.
There may be pitfalls when you use long long types for function arguments,
unless you declare function prototypes. If a function expects type int for
its argument, and you pass a value of type long long int, confusion will
result because the caller and the subroutine will disagree about the number of
bytes for the argument. Likewise, if the function expects long long int and
you pass int. The best way to avoid such problems is to use prototypes.
ΓòÉΓòÉΓòÉ 6.11. Complex Numbers ΓòÉΓòÉΓòÉ
GNU C supports complex data types. You can declare both complex integer types
and complex floating types, using the keyword __complex__.
For example, '__complex__ double x;' declares x as a variable whose real part
and imaginary part are both of type double. '__complex__ short int y;'
declares y to have real and imaginary parts of type short int; this is not
likely to be useful, but it shows that the set of complex types is complete.
To write a constant with a complex data type, use the suffix 'i' or 'j'
(either one; they are equivalent). For example, 2.5fi has type __complex__
float and 3i has type __complex__ int. Such a constant always has a pure
imaginary value, but you can form any complex value you like by adding one to
a real constant.
To extract the real part of a complex-valued expression exp, write __real__
exp. Likewise, use __imag__ to extract the imaginary part.
The operator '~' performs complex conjugation when used on a value with a
complex type.
GNU CC can allocate complex automatic variables in a noncontiguous fashion;
it's even possible for the real part to be in a register while the imaginary
part is on the stack (or vice-versa). None of the supported debugging info
formats has a way to represent noncontiguous allocation like this, so GNU CC
describes a noncontiguous complex variable as if it were two separate
variables of noncomplex type. If the variable's actual name is foo, the two
fictitious variables are named foo$real and foo$imag. You can examine and set
these two fictitious variables with your debugger.
A future version of GDB will know how to recognize such pairs and treat them
as a single variable with a complex type.
ΓòÉΓòÉΓòÉ 6.12. Arrays of Length Zero ΓòÉΓòÉΓòÉ
Zero-length arrays are allowed in GNU C. They are very useful as the last
element of a structure which is really a header for a variable-length object:
struct line {
int length;
char contents[0];
};
{
struct line *thisline = (struct line *)
malloc (sizeof (struct line) + this_length);
thisline->length = this_length;
}
In standard C, you would have to give contents a length of 1, which means
either you waste space or complicate the argument to malloc.
ΓòÉΓòÉΓòÉ 6.13. Arrays of Variable Length ΓòÉΓòÉΓòÉ
Variable-length automatic arrays are allowed in GNU C. These arrays are
declared like any other automatic arrays, but with a length that is not a
constant expression. The storage is allocated at the point of declaration and
deallocated when the brace-level is exited. For example:
FILE *
concat_fopen (char *s1, char *s2, char *mode)
{
char str[strlen (s1) + strlen (s2) + 1];
strcpy (str, s1);
strcat (str, s2);
return fopen (str, mode);
}
Jumping or breaking out of the scope of the array name deallocates the
storage. Jumping into the scope is not allowed; you get an error message for
it.
You can use the function alloca to get an effect much like variable-length
arrays. The function alloca is available in many other C implementations (but
not in all). On the other hand, variable-length arrays are more elegant.
There are other differences between these two methods. Space allocated with
alloca exists until the containing function returns. The space for a
variable-length array is deallocated as soon as the array name's scope ends.
(If you use both variable-length arrays and alloca in the same function,
deallocation of a variable-length array will also deallocate anything more
recently allocated with alloca.)
You can also use variable-length arrays as arguments to functions:
struct entry
tester (int len, char data[len][len])
{
┬╖┬╖┬╖
}
The length of an array is computed once when the storage is allocated and is
remembered for the scope of the array in case you access it with sizeof.
If you want to pass the array first and the length afterward, you can use a
forward declaration in the parameter list---another GNU extension.
struct entry
tester (int len; char data[len][len], int len)
{
┬╖┬╖┬╖
}
The 'int len' before the semicolon is a parameter forward declaration, and it
serves the purpose of making the name len known when the declaration of data
is parsed.
You can write any number of such parameter forward declarations in the
parameter list. They can be separated by commas or semicolons, but the last
one must end with a semicolon, which is followed by the ``real'' parameter
declarations. Each forward declaration must match a ``real'' declaration in
parameter name and data type.
ΓòÉΓòÉΓòÉ 6.14. Macros with Variable Numbers of Arguments ΓòÉΓòÉΓòÉ
In GNU C, a macro can accept a variable number of arguments, much as a
function can. The syntax for defining the macro looks much like that used for
a function. Here is an example:
#define eprintf(format, args┬╖┬╖┬╖) \
fprintf (stderr, format , ## args)
Here args is a rest argument: it takes in zero or more arguments, as many as
the call contains. All of them plus the commas between them form the value of
args, which is substituted into the macro body where args is used. Thus, we
have this expansion:
eprintf ("%s:%d: ", input_file_name, line_number)
==>
fprintf (stderr, "%s:%d: " , input_file_name, line_number)
Note that the comma after the string constant comes from the definition of
eprintf, whereas the last comma comes from the value of args.
The reason for using '##' is to handle the case when args matches no arguments
at all. In this case, args has an empty value. In this case, the second
comma in the definition becomes an embarrassment: if it got through to the
expansion of the macro, we would get something like this:
fprintf (stderr, "success!\n" , )
which is invalid C syntax. '##' gets rid of the comma, so we get the
following instead:
fprintf (stderr, "success!\n")
This is a special feature of the GNU C preprocessor: '##' before a rest
argument that is empty discards the preceding sequence of non-whitespace
characters from the macro definition. (If another macro argument precedes,
none of it is discarded.)
It might be better to discard the last preprocessor token instead of the last
preceding sequence of non-whitespace characters; in fact, we may someday
change this feature to do so. We advise you to write the macro definition so
that the preceding sequence of non-whitespace characters is just a single
token, so that the meaning will not change if we change the definition of this
feature.
ΓòÉΓòÉΓòÉ 6.15. Non-Lvalue Arrays May Have Subscripts ΓòÉΓòÉΓòÉ
Subscripting is allowed on arrays that are not lvalues, even though the unary
'&' operator is not. For example, this is valid in GNU C though not valid in
other C dialects:
struct foo {int a[4];};
struct foo f();
bar (int index)
{
return f().a[index];
}
ΓòÉΓòÉΓòÉ 6.16. Arithmetic on void- and Function-Pointers ΓòÉΓòÉΓòÉ
In GNU C, addition and subtraction operations are supported on pointers to
void and on pointers to functions. This is done by treating the size of a
void or of a function as 1.
A consequence of this is that sizeof is also allowed on void and on function
types, and returns 1.
The option '-Wpointer-arith' requests a warning if these extensions are used.
ΓòÉΓòÉΓòÉ 6.17. Non-Constant Initializers ΓòÉΓòÉΓòÉ
As in standard C++, the elements of an aggregate initializer for an automatic
variable are not required to be constant expressions in GNU C. Here is an
example of an initializer with run-time varying elements:
foo (float f, float g)
{
float beat_freqs[2] = { f-g, f+g };
┬╖┬╖┬╖
}
ΓòÉΓòÉΓòÉ 6.18. Constructor Expressions ΓòÉΓòÉΓòÉ
GNU C supports constructor expressions. A constructor looks like a cast
containing an initializer. Its value is an object of the type specified in
the cast, containing the elements specified in the initializer.
Usually, the specified type is a structure. Assume that struct foo and
structure are declared as shown:
struct foo {int a; char b[2];} structure;
Here is an example of constructing a struct foo with a constructor:
structure = ((struct foo) {x + y, 'a', 0});
This is equivalent to writing the following:
{
struct foo temp = {x + y, 'a', 0};
structure = temp;
}
You can also construct an array. If all the elements of the constructor are
(made up of) simple constant expressions, suitable for use in initializers,
then the constructor is an lvalue and can be coerced to a pointer to its first
element, as shown here:
char **foo = (char *[]) { "x", "y", "z" };
Array constructors whose elements are not simple constants are not very
useful, because the constructor is not an lvalue. There are only two valid
ways to use it: to subscript it, or initialize an array variable with it. The
former is probably slower than a switch statement, while the latter does the
same thing an ordinary C initializer would do. Here is an example of
subscripting an array constructor:
output = ((int[]) { 2, x, 28 }) [input];
Constructor expressions for scalar types and union types are is also allowed,
but then the constructor expression is equivalent to a cast.
ΓòÉΓòÉΓòÉ 6.19. Labeled Elements in Initializers ΓòÉΓòÉΓòÉ
Standard C requires the elements of an initializer to appear in a fixed order,
the same as the order of the elements in the array or structure being
initialized.
In GNU C you can give the elements in any order, specifying the array indices
or structure field names they apply to. This extension is not implemented in
GNU C++.
To specify an array index, write '[index]' or '[index] =' before the element
value. For example,
int a[6] = { [4] 29, [2] = 15 };
is equivalent to
int a[6] = { 0, 0, 15, 0, 29, 0 };
The index values must be constant expressions, even if the array being
initialized is automatic.
To initialize a range of elements to the same value, write '[first ┬╖┬╖┬╖ last] =
value'. For example,
int widths[] = { [0 ┬╖┬╖┬╖ 9] = 1, [10 ┬╖┬╖┬╖ 99] = 2, [100] = 3 };
Note that the length of the array is the highest value specified plus one.
In a structure initializer, specify the name of a field to initialize with
'fieldname:' before the element value. For example, given the following
structure,
struct point { int x, y; };
the following initialization
struct point p = { y: yvalue, x: xvalue };
is equivalent to
struct point p = { xvalue, yvalue };
Another syntax which has the same meaning is '.fieldname ='., as shown here:
struct point p = { .y = yvalue, .x = xvalue };
You can also use an element label (with either the colon syntax or the
period-equal syntax) when initializing a union, to specify which element of
the union should be used. For example,
union foo { int i; double d; };
union foo f = { d: 4 };
will convert 4 to a double to store it in the union using the second element.
By contrast, casting 4 to type union foo would store it into the union as the
integer i, since it is an integer. (See Cast to Union.)
You can combine this technique of naming elements with ordinary C
initialization of successive elements. Each initializer element that does not
have a label applies to the next consecutive element of the array or
structure. For example,
int a[6] = { [1] = v1, v2, [4] = v4 };
is equivalent to
int a[6] = { 0, v1, v2, 0, v4, 0 };
Labeling the elements of an array initializer is especially useful when the
indices are characters or belong to an enum type. For example:
int whitespace[256]
= { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };
ΓòÉΓòÉΓòÉ 6.20. Case Ranges ΓòÉΓòÉΓòÉ
You can specify a range of consecutive values in a single case label, like
this:
case low ┬╖┬╖┬╖ high:
This has the same effect as the proper number of individual case labels, one
for each integer value from low to high, inclusive.
This feature is especially useful for ranges of ASCII character codes:
case 'A' ┬╖┬╖┬╖ 'Z':
Be careful: Write spaces around the ┬╖┬╖┬╖, for otherwise it may be parsed wrong
when you use it with integer values. For example, write this:
case 1 ┬╖┬╖┬╖ 5:
rather than this:
case 1┬╖┬╖┬╖5:
ΓòÉΓòÉΓòÉ 6.21. Cast to a Union Type ΓòÉΓòÉΓòÉ
A cast to union type is similar to other casts, except that the type specified
is a union type. You can specify the type either with union tag or with a
typedef name. A cast to union is actually a constructor though, not a cast,
and hence does not yield an lvalue like normal casts. (See Constructors.)
The types that may be cast to the union type are those of the members of the
union. Thus, given the following union and variables:
union foo { int i; double d; };
int x;
double y;
both x and y can be cast to type union foo.
Using the cast as the right-hand side of an assignment to a variable of union
type is equivalent to storing in a member of the union:
union foo u;
┬╖┬╖┬╖
u = (union foo) x == u.i = x
u = (union foo) y == u.d = y
You can also use the union cast as a function argument:
void hack (union foo);
┬╖┬╖┬╖
hack ((union foo) x);
ΓòÉΓòÉΓòÉ 6.22. Declaring Attributes of Functions ΓòÉΓòÉΓòÉ
In GNU C, you declare certain things about functions called in your program
which help the compiler optimize function calls and check your code more
carefully.
The keyword __attribute__ allows you to specify special attributes when making
a declaration. This keyword is followed by an attribute specification inside
double parentheses. Eight attributes, noreturn, const, format, section,
constructor, destructor, unused and weak are currently defined for functions.
Other attributes, including section are supported for variables declarations
(see Variable Attributes) and for types (see Type Attributes).
You may also specify attributes with '__' preceding and following each
keyword. This allows you to use them in header files without being concerned
about a possible macro of the same name. For example, you may use
__noreturn__ instead of noreturn.
noreturn
A few standard library functions, such as abort and exit, cannot
return. GNU CC knows this automatically. Some programs define
their own functions that never return. You can declare them
noreturn to tell the compiler this fact. For example,
void fatal () __attribute__ ((noreturn));
void
fatal (┬╖┬╖┬╖)
{
┬╖┬╖┬╖ /* Print error message. */ ┬╖┬╖┬╖
exit (1);
}
The noreturn keyword tells the compiler to assume that fatal cannot return.
It can then optimize without regard to what would happen if fatal ever did
return. This makes slightly better code. More importantly, it helps avoid
spurious warnings of uninitialized variables.
Do not assume that registers saved by the calling function are restored before
calling the noreturn function.
It does not make sense for a noreturn function to have a return type other
than void.
The attribute noreturn is not implemented in GNU C versions earlier than 2.5.
An alternative way to declare that a function does not return, which works in
the current version and in some older versions, is as follows:
typedef void voidfn ();
volatile voidfn fatal;
const
Many functions do not examine any values except their arguments, and
have no effects except the return value. Such a function can be
subject to common subexpression elimination and loop optimization
just as an arithmetic operator would be. These functions should be
declared with the attribute const. For example,
int square (int) __attribute__ ((const));
says that the hypothetical function square is safe to call fewer times than
the program says.
The attribute const is not implemented in GNU C versions earlier than 2.5. An
alternative way to declare that a function has no side effects, which works in
the current version and in some older versions, is as follows:
typedef int intfn ();
extern const intfn square;
This approach does not work in GNU C++ from 2.6.0 on, since the language
specifies that the 'const' must be attached to the return value.
Note that a function that has pointer arguments and examines the data pointed
to must not be declared const. Likewise, a function that calls a non-const
function usually must not be const. It does not make sense for a const
function to return void.
format (archetype, string-index, first-to-check)
The format attribute specifies that a function takes printf or scanf
style arguments which should be type-checked against a format
string. For example, the declaration:
extern int
my_printf (void *my_object, const char *my_format, ┬╖┬╖┬╖)
__attribute__ ((format (printf, 2, 3)));
causes the compiler to check the arguments in calls to my_printf for
consistency with the printf style format string argument my_format.
The parameter archetype determines how the format string is interpreted, and
should be either printf or scanf. The parameter string-index specifies which
argument is the format string argument (starting from 1), while first-to-check
is the number of the first argument to check against the format string. For
functions where the arguments are not available to be checked (such as
vprintf), specify the third parameter as zero. In this case the compiler only
checks the format string for consistency.
In the example above, the format string (my_format) is the second argument of
the function my_print, and the arguments to check start with the third
argument, so the correct parameters for the format attribute are 2 and 3.
The format attribute allows you to identify your own functions which take
format strings as arguments, so that GNU CC can check the calls to these
functions for errors. The compiler always checks formats for the ANSI library
functions printf, fprintf, sprintf, scanf, fscanf, sscanf, vprintf, vfprintf
and vsprintf whenever such warnings are requested (using '-Wformat'), so there
is no need to modify the header file 'stdio.h'.
format_arg (string-index)
The format_arg attribute specifies that a function takes printf or
scanf style arguments, modifies it (for example, to translate it
into another language), and passes it to a printf or scanf style
function. For example, the declaration:
extern char *
my_dgettext (char *my_domain, const char *my_format)
__attribute__ ((format_arg (2)));
causes the compiler to check the arguments in calls to my_dgettext whose
result is passed to a printf or scanf type function for consistency with the
printf style format string argument my_format.
The parameter string-index specifies which argument is the format string
argument (starting from 1).
The format-arg attribute allows you to identify your own functions which
modify format strings, so that GNU CC can check the calls to printf and scanf
function whose operands are a call to one of your own function. The compiler
always treats gettext, dgettext, and dcgettext in this manner.
section ("section-name")
Normally, the compiler places the code it generates in the text
section. Sometimes, however, you need additional sections, or you
need certain particular functions to appear in special sections.
The section attribute specifies that a function lives in a
particular section. For example, the declaration:
extern void foobar (void) __attribute__ ((section ("bar")));
puts the function foobar in the bar section.
Some file formats do not support arbitrary sections so the section attribute
is not available on all platforms. If you need to map the entire contents of a
module to a particular section, consider using the facilities of the linker
instead.
constructor
destructor
The constructor attribute causes the function to be called
automatically before execution enters main (). Similarly, the
destructor attribute causes the function to be called automatically
after main () has completed or exit () has been called. Functions
with these attributes are useful for initializing data that will be
used implicitly during the execution of the program.
These attributes are not currently implemented for Objective C.
unused
This attribute, attached to a function, means that the function is
meant to be possibly unused. GNU CC will not produce a warning for
this function. GNU C++ does not currently support this attribute as
definitions without parameters are valid in C++.
weak
The weak attribute causes the declaration to be emitted as a weak
symbol rather than a global. This is primarily useful in defining
library functions which can be overridden in user code, though it
can also be used with non-function declarations. Weak symbols are
supported for ELF targets, and also for a.out targets when using the
GNU assembler and linker.
alias ("target")
The alias attribute causes the declaration to be emitted as an alias
for another symbol, which must be specified. For instance,
void __f () { /* do something */; }
void f () __attribute__ ((weak, alias ("__f")));
declares 'f' to be a weak alias for '__f'. In C++, the mangled name for the
target must be used.
Not all target machines support this attribute.
regparm (number)
On the Intel 386, the regparm attribute causes the compiler to pass
up to number integer arguments in registers EAX, EDX, and ECX
instead of on the stack. Functions that take a variable number of
arguments will continue to be passed all of their arguments on the
stack.
stdcall
On the Intel 386, the stdcall attribute causes the compiler to
assume that the called function will pop off the stack space used to
pass arguments, unless it takes a variable number of arguments.
The PowerPC compiler for Windows NT currently ignores the stdcall
attribute.
cdecl
On the Intel 386, the cdecl attribute causes the compiler to assume
that the calling function will pop off the stack space used to pass
arguments. This is useful to override the effects of the '-mrtd'
switch.
The PowerPC compiler for Windows NT currently ignores the cdecl
attribute.
longcall
On the RS/6000 and PowerPC, the longcall attribute causes the
compiler to always call the function via a pointer, so that
functions which reside further than 32 megabytes from the current
location can be called.
shortcall
On the RS/6000 and PowerPC, the shortcall attribute causes the
compiler to always generate a direct call if it can, overriding the
attribute longcall and the command line flag '-mlongcall'.
dllimport
On the PowerPC running Windows NT, the dllimport attribute causes
the compiler to call the function via a global pointer to the
function pointer that is set up by the Windows NT dll library. The
pointer name is formed by combining __imp_ and the function name.
dllexport
On the PowerPC running Windows NT, the dllexport attribute causes
the compiler to provide a global pointer to the function pointer, so
that it can be called with the dllimport attribute. The pointer
name is formed by combining __imp_ and the function name.
exception (except-func [, except-arg])
On the PowerPC running Windows NT, the exception attribute causes
the compiler to modify the structured exception table entry it emits
for the declared function. The string or identifier except-func is
placed in the third entry of the structured exception table. It
represents a function, which is called by the exception handling
mechanism if an exception occurs. If it was specified, the string
or identifier except-arg is placed in the fourth entry of the
structured exception table.
function_vector
Use this option on the H8/300 and H8/300H to indicate that the
specified function should be called through the function vector.
Calling a function through the function vector will reduce code
size, however; the function vector has a limited size (maximum 128
entries on the H8/300 and 64 entries on the H8/300H) and shares
space with the interrupt vector.
You must use GAS and GLD from GNU binutils version 2.7 or later for
this option to work correctly.
interrupt_handler
Use this option on the H8/300 and H8/300H to indicate that the
specified function is an interrupt handler. The compiler will
generate function entry and exit sequences suitable for use in an
interrupt handler when this attribute is present.
eightbit_data
Use this option on the H8/300 and H8/300H to indicate that the
specified variable should be placed into the eight bit data section.
The compiler will generate more efficient code for certain
operations on data in the eight bit data area. Note the eight bit
data area is limited to 256 bytes of data.
You must use GAS and GLD from GNU binutils version 2.7 or later for
this option to work correctly.
tiny_data
Use this option on the H8/300H to indicate that the specified
variable should be placed into the tiny data section. The compiler
will generate more efficient code for loads and stores on data in
the tiny data section. Note the tiny data area is limited to
slightly under 32kbytes of data.
interrupt
Use this option on the M32R/D to indicate that the specified
function is an interrupt handler. The compiler will generate
function entry and exit sequences suitable for use in an interrupt
handler when this attribute is present.
model (model-name)
Use this attribute on the M32R/D to set the addressability of an
object, and the code generated for a function. The identifier
model-name is one of small, medium, or large, representing each of
the code models.
Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the ld24 instruction), and are callable
with the bl instruction.
Medium model objects may live anywhere in the 32 bit address space
(the compiler will generate seth/add3 instructions to load their
addresses), and are callable with the bl instruction.
Large model objects may live anywhere in the 32 bit address space
(the compiler will generate seth/add3 instructions to load their
addresses), and may not be reachable with the bl instruction (the
compiler will generate the much slower seth/add3/jl instruction
sequence).
You can specify multiple attributes in a declaration by separating them by
commas within the double parentheses or by immediately following an attribute
declaration with another attribute declaration.
Some people object to the __attribute__ feature, suggesting that ANSI C's
#pragma should be used instead. There are two reasons for not doing this.
1. It is impossible to generate #pragma commands from a macro.
2. There is no telling what the same #pragma might mean in another compiler.
These two reasons apply to almost any application that might be proposed for
#pragma. It is basically a mistake to use #pragma for anything.
ΓòÉΓòÉΓòÉ 6.23. Prototypes and Old-Style Function Definitions ΓòÉΓòÉΓòÉ
GNU C extends ANSI C to allow a function prototype to override a later
old-style non-prototype definition. Consider the following example:
/* Use prototypes unless the compiler is old-fashioned. */
#ifdef __STDC__
#define P(x) x
#else
#define P(x) ()
#endif
/* Prototype function declaration. */
int isroot P((uid_t));
/* Old-style function definition. */
int
isroot (x) /* ??? lossage here ??? */
uid_t x;
{
return x == 0;
}
Suppose the type uid_t happens to be short. ANSI C does not allow this
example, because subword arguments in old-style non-prototype definitions are
promoted. Therefore in this example the function definition's argument is
really an int, which does not match the prototype argument type of short.
This restriction of ANSI C makes it hard to write code that is portable to
traditional C compilers, because the programmer does not know whether the
uid_t type is short, int, or long. Therefore, in cases like these GNU C
allows a prototype to override a later old-style definition. More precisely,
in GNU C, a function prototype argument type overrides the argument type
specified by a later old-style definition if the former type is the same as
the latter type before promotion. Thus in GNU C the above example is
equivalent to the following:
int isroot (uid_t);
int
isroot (uid_t x)
{
return x == 0;
}
GNU C++ does not support old-style function definitions, so this extension is
irrelevant.
ΓòÉΓòÉΓòÉ 6.24. C++ Style Comments ΓòÉΓòÉΓòÉ
In GNU C, you may use C++ style comments, which start with '//' and continue
until the end of the line. Many other C implementations allow such comments,
and they are likely to be in a future C standard. However, C++ style comments
are not recognized if you specify '-ansi' or '-traditional', since they are
incompatible with traditional constructs like dividend//*comment*/divisor.
ΓòÉΓòÉΓòÉ 6.25. Dollar Signs in Identifier Names ΓòÉΓòÉΓòÉ
In GNU C, you may normally use dollar signs in identifier names. This is
because many traditional C implementations allow such identifiers. However,
dollar signs in identifiers are not supported on a few target machines,
typically because the target assembler does not allow them.
ΓòÉΓòÉΓòÉ 6.26. The Character ESC in Constants ΓòÉΓòÉΓòÉ
You can use the sequence '\e' in a string or character constant to stand for
the ASCII character ESC.
ΓòÉΓòÉΓòÉ 6.27. Inquiring on Alignment of Types or Variables ΓòÉΓòÉΓòÉ
The keyword __alignof__ allows you to inquire about how an object is aligned,
or the minimum alignment usually required by a type. Its syntax is just like
sizeof.
For example, if the target machine requires a double value to be aligned on an
8-byte boundary, then __alignof__ (double) is 8. This is true on many RISC
machines. On more traditional machine designs, __alignof__ (double) is 4 or
even 2.
Some machines never actually require alignment; they allow reference to any
data type even at an odd addresses. For these machines, __alignof__ reports
the recommended alignment of a type.
When the operand of __alignof__ is an lvalue rather than a type, the value is
the largest alignment that the lvalue is known to have. It may have this
alignment as a result of its data type, or because it is part of a structure
and inherits alignment from that structure. For example, after this
declaration:
struct foo { int x; char y; } foo1;
the value of __alignof__ (foo1.y) is probably 2 or 4, the same as __alignof__
(int), even though the data type of foo1.y does not itself demand any
alignment.
A related feature which lets you specify the alignment of an object is
__attribute__ ((aligned (alignment))); see the following section.
ΓòÉΓòÉΓòÉ 6.28. Specifying Attributes of Variables ΓòÉΓòÉΓòÉ
The keyword __attribute__ allows you to specify special attributes of
variables or structure fields. This keyword is followed by an attribute
specification inside double parentheses. Eight attributes are currently
defined for variables: aligned, mode, nocommon, packed, section,
transparent_union, unused, and weak. Other attributes are available for
functions (see Function Attributes) and for types (see Type Attributes).
You may also specify attributes with '__' preceding and following each
keyword. This allows you to use them in header files without being concerned
about a possible macro of the same name. For example, you may use __aligned__
instead of aligned.
aligned (alignment)
This attribute specifies a minimum alignment for the variable or
structure field, measured in bytes. For example, the declaration:
int x __attribute__ ((aligned (16))) = 0;
causes the compiler to allocate the global variable x on a 16-byte boundary.
On a 68040, this could be used in conjunction with an asm expression to access
the move16 instruction which requires 16-byte aligned operands.
You can also specify the alignment of structure fields. For example, to
create a double-word aligned int pair, you could write:
struct foo { int x[2] __attribute__ ((aligned (8))); };
This is an alternative to creating a union with a double member that forces
the union to be double-word aligned.
It is not possible to specify the alignment of functions; the alignment of
functions is determined by the machine's requirements and cannot be changed.
You cannot specify alignment for a typedef name because such a name is just an
alias, not a distinct type.
As in the preceding examples, you can explicitly specify the alignment (in
bytes) that you wish the compiler to use for a given variable or structure
field. Alternatively, you can leave out the alignment factor and just ask the
compiler to align a variable or field to the maximum useful alignment for the
target machine you are compiling for. For example, you could write:
short array[3] __attribute__ ((aligned));
Whenever you leave out the alignment factor in an aligned attribute
specification, the compiler automatically sets the alignment for the declared
variable or field to the largest alignment which is ever used for any data
type on the target machine you are compiling for. Doing this can often make
copy operations more efficient, because the compiler can use whatever
instructions copy the biggest chunks of memory when performing copies to or
from the variables or fields that you have aligned this way.
The aligned attribute can only increase the alignment; but you can decrease it
by specifying packed as well. See below.
Note that the effectiveness of aligned attributes may be limited by inherent
limitations in your linker. On many systems, the linker is only able to
arrange for variables to be aligned up to a certain maximum alignment. (For
some linkers, the maximum supported alignment may be very very small.) If
your linker is only able to align variables up to a maximum of 8 byte
alignment, then specifying aligned(16) in an __attribute__ will still only
provide you with 8 byte alignment. See your linker documentation for further
information.
mode (mode)
This attribute specifies the data type for the
declaration---whichever type corresponds to the mode mode. This in
effect lets you request an integer or floating point type according
to its width.
You may also specify a mode of 'byte' or '__byte__' to indicate the
mode corresponding to a one-byte integer, 'word' or '__word__' for
the mode of a one-word integer, and 'pointer' or '__pointer__' for
the mode used to represent pointers.
nocommon
This attribute specifies requests GNU CC not to place a variable
``common'' but instead to allocate space for it directly. If you
specify the '-fno-common' flag, GNU CC will do this for all
variables.
Specifying the nocommon attribute for a variable provides an
initialization of zeros. A variable may only be initialized in one
source file.
packed
The packed attribute specifies that a variable or structure field
should have the smallest possible alignment---one byte for a
variable, and one bit for a field, unless you specify a larger value
with the aligned attribute.
Here is a structure in which the field x is packed, so that it
immediately follows a:
struct foo
{
char a;
int x[2] __attribute__ ((packed));
};
section ("section-name")
Normally, the compiler places the objects it generates in sections
like data and bss. Sometimes, however, you need additional
sections, or you need certain particular variables to appear in
special sections, for example to map to special hardware. The
section attribute specifies that a variable (or function) lives in a
particular section. For example, this small program uses several
specific section names:
struct duart a __attribute__ ((section ("DUART_A"))) = { 0 };
struct duart b __attribute__ ((section ("DUART_B"))) = { 0 };
char stack[10000] __attribute__ ((section ("STACK"))) = { 0 };
int init_data __attribute__ ((section ("INITDATA"))) = 0;
main()
{
/* Initialize stack pointer */
init_sp (stack + sizeof (stack));
/* Initialize initialized data */
memcpy (&init_data, &data, &edata - &data);
/* Turn on the serial ports */
init_duart (&a);
init_duart (&b);
}
Use the section attribute with an initialized definition of a global variable,
as shown in the example. GNU CC issues a warning and otherwise ignores the
section attribute in uninitialized variable declarations.
You may only use the section attribute with a fully initialized global
definition because of the way linkers work. The linker requires each object
be defined once, with the exception that uninitialized variables tentatively
go in the common (or bss) section and can be multiply "defined". You can
force a variable to be initialized with the '-fno-common' flag or the nocommon
attribute.
Some file formats do not support arbitrary sections so the section attribute
is not available on all platforms. If you need to map the entire contents of a
module to a particular section, consider using the facilities of the linker
instead.
transparent_union
This attribute, attached to a function parameter which is a union,
means that the corresponding argument may have the type of any union
member, but the argument is passed as if its type were that of the
first union member. For more details see See Type Attributes. You
can also use this attribute on a typedef for a union data type; then
it applies to all function parameters with that type.
unused
This attribute, attached to a variable, means that the variable is
meant to be possibly unused. GNU CC will not produce a warning for
this variable.
weak
The weak attribute is described in See Function Attributes.
model (model-name)
Use this attribute on the M32R/D to set the addressability of an
object. The identifier model-name is one of small, medium, or large,
representing each of the code models.
Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the ld24 instruction).
Medium and large model objects may live anywhere in the 32 bit
address space (the compiler will generate seth/add3 instructions to
load their addresses).
To specify multiple attributes, separate them by commas within the double
parentheses: for example, '__attribute__ ((aligned (16), packed))'.
ΓòÉΓòÉΓòÉ 6.29. Specifying Attributes of Types ΓòÉΓòÉΓòÉ
The keyword __attribute__ allows you to specify special attributes of struct
and union types when you define such types. This keyword is followed by an
attribute specification inside double parentheses. Three attributes are
currently defined for types: aligned, packed, and transparent_union. Other
attributes are defined for functions (see Function Attributes) and for
variables (see Variable Attributes).
You may also specify any one of these attributes with '__' preceding and
following its keyword. This allows you to use these attributes in header
files without being concerned about a possible macro of the same name. For
example, you may use __aligned__ instead of aligned.
You may specify the aligned and transparent_union attributes either in a
typedef declaration or just past the closing curly brace of a complete enum,
struct or union type definition and the packed attribute only past the closing
brace of a definition.
aligned (alignment)
This attribute specifies a minimum alignment (in bytes) for
variables of the specified type. For example, the declarations:
struct S { short f[3]; } __attribute__ ((aligned (8)));
typedef int more_aligned_int __attribute__ ((aligned (8)));
force the compiler to insure (as far as it can) that each variable whose type
is struct S or more_aligned_int will be allocated and aligned at least on a
8-byte boundary. On a Sparc, having all variables of type struct S aligned to
8-byte boundaries allows the compiler to use the ldd and std (doubleword load
and store) instructions when copying one variable of type struct S to another,
thus improving run-time efficiency.
Note that the alignment of any given struct or union type is required by the
ANSI C standard to be at least a perfect multiple of the lowest common
multiple of the alignments of all of the members of the struct or union in
question. This means that you can effectively adjust the alignment of a
struct or union type by attaching an aligned attribute to any one of the
members of such a type, but the notation illustrated in the example above is a
more obvious, intuitive, and readable way to request the compiler to adjust
the alignment of an entire struct or union type.
As in the preceding example, you can explicitly specify the alignment (in
bytes) that you wish the compiler to use for a given struct or union type.
Alternatively, you can leave out the alignment factor and just ask the
compiler to align a type to the maximum useful alignment for the target
machine you are compiling for. For example, you could write:
struct S { short f[3]; } __attribute__ ((aligned));
Whenever you leave out the alignment factor in an aligned attribute
specification, the compiler automatically sets the alignment for the type to
the largest alignment which is ever used for any data type on the target
machine you are compiling for. Doing this can often make copy operations more
efficient, because the compiler can use whatever instructions copy the biggest
chunks of memory when performing copies to or from the variables which have
types that you have aligned this way.
In the example above, if the size of each short is 2 bytes, then the size of
the entire struct S type is 6 bytes. The smallest power of two which is
greater than or equal to that is 8, so the compiler sets the alignment for the
entire struct S type to 8 bytes.
Note that although you can ask the compiler to select a time-efficient
alignment for a given type and then declare only individual stand-alone
objects of that type, the compiler's ability to select a time-efficient
alignment is primarily useful only when you plan to create arrays of variables
having the relevant (efficiently aligned) type. If you declare or use arrays
of variables of an efficiently-aligned type, then it is likely that your
program will also be doing pointer arithmetic (or subscripting, which amounts
to the same thing) on pointers to the relevant type, and the code that the
compiler generates for these pointer arithmetic operations will often be more
efficient for efficiently-aligned types than for other types.
The aligned attribute can only increase the alignment; but you can decrease it
by specifying packed as well. See below.
Note that the effectiveness of aligned attributes may be limited by inherent
limitations in your linker. On many systems, the linker is only able to
arrange for variables to be aligned up to a certain maximum alignment. (For
some linkers, the maximum supported alignment may be very very small.) If
your linker is only able to align variables up to a maximum of 8 byte
alignment, then specifying aligned(16) in an __attribute__ will still only
provide you with 8 byte alignment. See your linker documentation for further
information.
packed
This attribute, attached to an enum, struct, or union type
definition, specified that the minimum required memory be used to
represent the type.
Specifying this attribute for struct and union types is equivalent
to specifying the packed attribute on each of the structure or union
members. Specifying the '-fshort-enums' flag on the line is
equivalent to specifying the packed attribute on all enum
definitions.
You may only specify this attribute after a closing curly brace on
an enum definition, not in a typedef declaration, unless that
declaration also contains the definition of the enum.
transparent_union
This attribute, attached to a union type definition, indicates that
any function parameter having that union type causes calls to that
function to be treated in a special way.
First, the argument corresponding to a transparent union type can be
of any type in the union; no cast is required. Also, if the union
contains a pointer type, the corresponding argument can be a null
pointer constant or a void pointer expression; and if the union
contains a void pointer type, the corresponding argument can be any
pointer expression. If the union member type is a pointer,
qualifiers like const on the referenced type must be respected, just
as with normal pointer conversions.
Second, the argument is passed to the function using the calling
conventions of first member of the transparent union, not the
calling conventions of the union itself. All members of the union
must have the same machine representation; this is necessary for
this argument passing to work properly.
Transparent unions are designed for library functions that have
multiple interfaces for compatibility reasons. For example, suppose
the wait function must accept either a value of type int * to comply
with Posix, or a value of type union wait * to comply with the
4.1BSD interface. If wait's parameter were void *, wait would
accept both kinds of arguments, but it would also accept any other
pointer type and this would make argument type checking less useful.
Instead, <sys/wait.h> might define the interface as follows:
typedef union
{
int *__ip;
union wait *__up;
} wait_status_ptr_t __attribute__ ((__transparent_union__));
pid_t wait (wait_status_ptr_t);
This interface allows either int * or union wait * arguments to be passed,
using the int * calling convention. The program can call wait with arguments
of either type:
int w1 () { int w; return wait (&w); }
int w2 () { union wait w; return wait (&w); }
With this interface, wait's implementation might look like this:
pid_t wait (wait_status_ptr_t p)
{
return waitpid (-1, p.__ip, 0);
}
unused
When attached to a type (including a union or a struct), this
attribute means that variables of that type are meant to appear
possibly unused. GNU CC will not produce a warning for any
variables of that type, even if the variable appears to do nothing.
This is often the case with lock or thread classes, which are
usually defined and then not referenced, but contain constructors
and destructors that have nontrivial bookkeeping functions.
To specify multiple attributes, separate them by commas within the double
parentheses: for example, '__attribute__ ((aligned (16), packed))'.
ΓòÉΓòÉΓòÉ 6.30. An Inline Function is As Fast As a Macro ΓòÉΓòÉΓòÉ
By declaring a function inline, you can direct GNU CC to integrate that
function's code into the code for its callers. This makes execution faster by
eliminating the function-call overhead; in addition, if any of the actual
argument values are constant, their known values may permit simplifications at
compile time so that not all of the inline function's code needs to be
included. The effect on code size is less predictable; object code may be
larger or smaller with function inlining, depending on the particular case.
Inlining of functions is an optimization and it really ``works'' only in
optimizing compilation. If you don't use '-O', no function is really inline.
To declare a function inline, use the inline keyword in its declaration, like
this:
inline int
inc (int *a)
{
(*a)++;
}
(If you are writing a header file to be included in ANSI C programs, write
__inline__ instead of inline. See Alternate Keywords.)
You can also make all ``simple enough'' functions inline with the option
'-finline-functions'. Note that certain usages in a function definition can
make it unsuitable for inline substitution.
Note that in C and Objective C, unlike C++, the inline keyword does not affect
the linkage of the function.
GNU CC automatically inlines member functions defined within the class body of
C++ programs even if they are not explicitly declared inline. (You can
override this with '-fno-default-inline'; see Options Controlling C++
Dialect.)
When a function is both inline and static, if all calls to the function are
integrated into the caller, and the function's address is never used, then the
function's own assembler code is never referenced. In this case, GNU CC does
not actually output assembler code for the function, unless you specify the
option '-fkeep-inline-functions'. Some calls cannot be integrated for various
reasons (in particular, calls that precede the function's definition cannot be
integrated, and neither can recursive calls within the definition). If there
is a nonintegrated call, then the function is compiled to assembler code as
usual. The function must also be compiled as usual if the program refers to
its address, because that can't be inlined.
When an inline function is not static, then the compiler must assume that
there may be calls from other source files; since a global symbol can be
defined only once in any program, the function must not be defined in the
other source files, so the calls therein cannot be integrated. Therefore, a
non-static inline function is always compiled on its own in the usual fashion.
If you specify both inline and extern in the function definition, then the
definition is used only for inlining. In no case is the function compiled on
its own, not even if you refer to its address explicitly. Such an address
becomes an external reference, as if you had only declared the function, and
had not defined it.
This combination of inline and extern has almost the effect of a macro. The
way to use it is to put a function definition in a header file with these
keywords, and put another copy of the definition (lacking inline and extern)
in a library file. The definition in the header file will cause most calls to
the function to be inlined. If any uses of the function remain, they will
refer to the single copy in the library.
GNU C does not inline any functions when not optimizing. It is not clear
whether it is better to inline or not, in this case, but we found that a
correct implementation when not optimizing was difficult. So we did the easy
thing, and turned it off.
ΓòÉΓòÉΓòÉ 6.31. Assembler Instructions with C Expression Operands ΓòÉΓòÉΓòÉ
In an assembler instruction using asm, you can specify the operands of the
instruction using C expressions. This means you need not guess which
registers or memory locations will contain the data you want to use.
You must specify an assembler instruction template much like what appears in a
machine description, plus an operand constraint string for each operand.
For example, here is how to use the 68881's fsinx instruction:
asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));
Here angle is the C expression for the input operand while result is that of
the output operand. Each has '"f"' as its operand constraint, saying that a
floating point register is required. The '=' in '=f' indicates that the
operand is an output; all output operands' constraints must use '='. The
constraints use the same language used in the machine description (see
Constraints).
Each operand is described by an operand-constraint string followed by the C
expression in parentheses. A colon separates the assembler template from the
first output operand and another separates the last output operand from the
first input, if any. Commas separate the operands within each group. The
total number of operands is limited to ten or to the maximum number of
operands in any instruction pattern in the machine description, whichever is
greater.
If there are no output operands but there are input operands, you must place
two consecutive colons surrounding the place where the output operands would
go.
Output operand expressions must be lvalues; the compiler can check this. The
input operands need not be lvalues. The compiler cannot check whether the
operands have data types that are reasonable for the instruction being
executed. It does not parse the assembler instruction template and does not
know what it means or even whether it is valid assembler input. The extended
asm feature is most often used for machine instructions the compiler itself
does not know exist. If the output expression cannot be directly addressed
(for example, it is a bit field), your constraint must allow a register. In
that case, GNU CC will use the register as the output of the asm, and then
store that register into the output.
The ordinary output operands must be write-only; GNU CC will assume that the
values in these operands before the instruction are dead and need not be
generated. Extended asm supports input-output or read-write operands. Use
the constraint character '+' to indicate such an operand and list it with the
output operands.
When the constraints for the read-write operand (or the operand in which only
some of the bits are to be changed) allows a register, you may, as an
alternative, logically split its function into two separate operands, one
input operand and one write-only output operand. The connection between them
is expressed by constraints which say they need to be in the same location
when the instruction executes. You can use the same C expression for both
operands, or different expressions. For example, here we write the
(fictitious) 'combine' instruction with bar as its read-only source operand
and foo as its read-write destination:
asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));
The constraint '"0"' for operand 1 says that it must occupy the same location
as operand 0. A digit in constraint is allowed only in an input operand and
it must refer to an output operand.
Only a digit in the constraint can guarantee that one operand will be in the
same place as another. The mere fact that foo is the value of both operands
is not enough to guarantee that they will be in the same place in the
generated assembler code. The following would not work reliably:
asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));
Various optimizations or reloading could cause operands 0 and 1 to be in
different registers; GNU CC knows no reason not to do so. For example, the
compiler might find a copy of the value of foo in one register and use it for
operand 1, but generate the output operand 0 in a different register (copying
it afterward to foo's own address). Of course, since the register for operand
1 is not even mentioned in the assembler code, the result will not work, but
GNU CC can't tell that.
Some instructions clobber specific hard registers. To describe this, write a
third colon after the input operands, followed by the names of the clobbered
hard registers (given as strings). Here is a realistic example for the VAX:
asm volatile ("movc3 %0,%1,%2"
: /* no outputs */
: "g" (from), "g" (to), "g" (count)
: "r0", "r1", "r2", "r3", "r4", "r5");
If you refer to a particular hardware register from the assembler code, you
will probably have to list the register after the third colon to tell the
compiler the register's value is modified. In some assemblers, the register
names begin with '%'; to produce one '%' in the assembler code, you must write
'%%' in the input.
If your assembler instruction can alter the condition code register, add 'cc'
to the list of clobbered registers. GNU CC on some machines represents the
condition codes as a specific hardware register; 'cc' serves to name this
register. On other machines, the condition code is handled differently, and
specifying 'cc' has no effect. But it is valid no matter what the machine.
If your assembler instruction modifies memory in an unpredictable fashion, add
'memory' to the list of clobbered registers. This will cause GNU CC to not
keep memory values cached in registers across the assembler instruction.
You can put multiple assembler instructions together in a single asm template,
separated either with newlines (written as '\n') or with semicolons if the
assembler allows such semicolons. The GNU assembler allows semicolons and most
Unix assemblers seem to do so. The input operands are guaranteed not to use
any of the clobbered registers, and neither will the output operands'
addresses, so you can read and write the clobbered registers as many times as
you like. Here is an example of multiple instructions in a template; it
assumes the subroutine _foo accepts arguments in registers 9 and 10:
asm ("movl %0,r9;movl %1,r10;call _foo"
: /* no outputs */
: "g" (from), "g" (to)
: "r9", "r10");
Unless an output operand has the '&' constraint modifier, GNU CC may allocate
it in the same register as an unrelated input operand, on the assumption the
inputs are consumed before the outputs are produced. This assumption may be
false if the assembler code actually consists of more than one instruction.
In such a case, use '&' for each output operand that may not overlap an input.
See Modifiers.
If you want to test the condition code produced by an assembler instruction,
you must include a branch and a label in the asm construct, as follows:
asm ("clr %0;frob %1;beq 0f;mov #1,%0;0:"
: "g" (result)
: "g" (input));
This assumes your assembler supports local labels, as the GNU assembler and
most Unix assemblers do.
Speaking of labels, jumps from one asm to another are not supported. The
compiler's optimizers do not know about these jumps, and therefore they cannot
take account of them when deciding how to optimize.
Usually the most convenient way to use these asm instructions is to
encapsulate them in macros that look like functions. For example,
#define sin(x) \
({ double __value, __arg = (x); \
asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \
__value; })
Here the variable __arg is used to make sure that the instruction operates on
a proper double value, and to accept only those arguments x which can convert
automatically to a double.
Another way to make sure the instruction operates on the correct data type is
to use a cast in the asm. This is different from using a variable __arg in
that it converts more different types. For example, if the desired type were
int, casting the argument to int would accept a pointer with no complaint,
while assigning the argument to an int variable named __arg would warn about
using a pointer unless the caller explicitly casts it.
If an asm has output operands, GNU CC assumes for optimization purposes the
instruction has no side effects except to change the output operands. This
does not mean instructions with a side effect cannot be used, but you must be
careful, because the compiler may eliminate them if the output operands aren't
used, or move them out of loops, or replace two with one if they constitute a
common subexpression. Also, if your instruction does have a side effect on a
variable that otherwise appears not to change, the old value of the variable
may be reused later if it happens to be found in a register.
You can prevent an asm instruction from being deleted, moved significantly, or
combined, by writing the keyword volatile after the asm. For example:
#define get_and_set_priority(new) \
({ int __old; \
asm volatile ("get_and_set_priority %0, %1": "=g" (__old) : "g" (new)); \
__old; })
b
If you write an asm instruction with no outputs, GNU CC will know the
instruction has side-effects and will not delete the instruction or move it
outside of loops. If the side-effects of your instruction are not purely
external, but will affect variables in your program in ways other than reading
the inputs and clobbering the specified registers or memory, you should write
the volatile keyword to prevent future versions of GNU CC from moving the
instruction around within a core region.
An asm instruction without any operands or clobbers (and ``old style'' asm)
will not be deleted or moved significantly, regardless, unless it is
unreachable, the same wasy as if you had written a volatile keyword.
Note that even a volatile asm instruction can be moved in ways that appear
insignificant to the compiler, such as across jump instructions. You can't
expect a sequence of volatile asm instructions to remain perfectly
consecutive. If you want consecutive output, use a single asm.
It is a natural idea to look for a way to give access to the condition code
left by the assembler instruction. However, when we attempted to implement
this, we found no way to make it work reliably. The problem is that output
operands might need reloading, which would result in additional following
``store'' instructions. On most machines, these instructions would alter the
condition code before there was time to test it. This problem doesn't arise
for ordinary ``test'' and ``compare'' instructions because they don't have any
output operands.
If you are writing a header file that should be includable in ANSI C programs,
write __asm__ instead of asm. See Alternate Keywords.
ΓòÉΓòÉΓòÉ 6.32. Constraints for asm Operands ΓòÉΓòÉΓòÉ
Here are specific details on what constraint letters you can use with asm
operands. Constraints can say whether an operand may be in a register, and
which kinds of register; whether the operand can be a memory reference, and
which kinds of address; whether the operand may be an immediate constant, and
which possible values it may have. Constraints can also require two operands
to match.
Simple Constraints Basic use of constraints.
Multi-Alternative When an insn has two alternative
constraint-patterns.
Modifiers More precise control over effects of
constraints.
Machine Constraints Special constraints for some particular
machines.
ΓòÉΓòÉΓòÉ 6.32.1. Simple Constraints ΓòÉΓòÉΓòÉ
The simplest kind of constraint is a string full of letters, each of which
describes one kind of operand that is permitted. Here are the letters that
are allowed:
'm'
A memory operand is allowed, with any kind of address that the
machine supports in general.
'o'
A memory operand is allowed, but only if the address is offsettable.
This means that adding a small integer (actually, the width in bytes
of the operand, as determined by its machine mode) may be added to
the address and the result is also a valid memory address.
For example, an address which is constant is offsettable; so is an
address that is the sum of a register and a constant (as long as a
slightly larger constant is also within the range of address-offsets
supported by the machine); but an autoincrement or autodecrement
address is not offsettable. More complicated indirect/indexed
addresses may or may not be offsettable depending on the other
addressing modes that the machine supports.
Note that in an output operand which can be matched by another
operand, the constraint letter 'o' is valid only when accompanied by
both '<' (if the target machine has predecrement addressing) and '>'
(if the target machine has preincrement addressing).
'V'
A memory operand that is not offsettable. In other words, anything
that would fit the 'm' constraint but not the 'o' constraint.
'<'
A memory operand with autodecrement addressing (either predecrement
or postdecrement) is allowed.
'>'
A memory operand with autoincrement addressing (either preincrement
or postincrement) is allowed.
'r'
A register operand is allowed provided that it is in a general
register.
'd', 'a', 'f', ┬╖┬╖┬╖
Other letters can be defined in machine-dependent fashion to stand
for particular classes of registers. 'd', 'a' and 'f' are defined
on the 68000/68020 to stand for data, address and floating point
registers.
'i'
An immediate integer operand (one with constant value) is allowed.
This includes symbolic constants whose values will be known only at
assembly time.
'n'
An immediate integer operand with a known numeric value is allowed.
Many systems cannot support assembly-time constants for operands
less than a word wide. Constraints for these operands should use
'n' rather than 'i'.
'I', 'J', 'K', ┬╖┬╖┬╖ 'P'
Other letters in the range 'I' through 'P' may be defined in a
machine-dependent fashion to permit immediate integer operands with
explicit integer values in specified ranges. For example, on the
68000, 'I' is defined to stand for the range of values 1 to 8. This
is the range permitted as a shift count in the shift instructions.
'E'
An immediate floating operand (expression code const_double) is
allowed, but only if the target floating point format is the same as
that of the host machine (on which the compiler is running).
'F'
An immediate floating operand (expression code const_double) is
allowed.
'G', 'H'
'G' and 'H' may be defined in a machine-dependent fashion to permit
immediate floating operands in particular ranges of values.
's'
An immediate integer operand whose value is not an explicit integer
is allowed.
This might appear strange; if an insn allows a constant operand with
a value not known at compile time, it certainly must allow any known
value. So why use 's' instead of 'i'? Sometimes it allows better
code to be generated.
For example, on the 68000 in a fullword instruction it is possible
to use an immediate operand; but if the immediate value is between
-128 and 127, better code results from loading the value into a
register and using the register. This is because the load into the
register can be done with a 'moveq' instruction. We arrange for
this to happen by defining the letter 'K' to mean ``any integer
outside the range -128 to 127'', and then specifying 'Ks' in the
operand constraints.
'g'
Any register, memory or immediate integer operand is allowed, except
for registers that are not general registers.
'X'
Any operand whatsoever is allowed.
' 0', '1', '2', ┬╖┬╖┬╖ '9'
An operand that matches the specified operand number is allowed. If
a digit is used together with letters within the same alternative,
the digit should come last.
This is called a matching constraint and what it really means is
that the assembler has only a single operand that fills two roles
which asm distinguishes. For example, an add instruction uses two
input operands and an output operand, but on most CISC machines an
add instruction really has only two operands, one of them an
input-output operand:
addl #35,r12
Matching constraints are used in these circumstances. More precisely, the two
operands that match must include one input-only operand and one output-only
operand. Moreover, the digit must be a smaller number than the number of the
operand that uses it in the constraint.
'p'
An operand that is a valid memory address is allowed. This is for
``load address'' and ``push address'' instructions.
'p' in the constraint must be accompanied by address_operand as the
predicate in the match_operand. This predicate interprets the mode
specified in the match_operand as the mode of the memory reference
for which the address would be valid.
'Q', 'R', 'S', ┬╖┬╖┬╖ 'U'
Letters in the range 'Q' through 'U' may be defined in a
machine-dependent fashion to stand for arbitrary operand types.
ΓòÉΓòÉΓòÉ 6.32.2. Multiple Alternative Constraints ΓòÉΓòÉΓòÉ
Sometimes a single instruction has multiple alternative sets of possible
operands. For example, on the 68000, a logical-or instruction can combine
register or an immediate value into memory, or it can combine any kind of
operand into a register; but it cannot combine one memory location into
another.
These constraints are represented as multiple alternatives. An alternative
can be described by a series of letters for each operand. The overall
constraint for an operand is made from the letters for this operand from the
first alternative, a comma, the letters for this operand from the second
alternative, a comma, and so on until the last alternative.
If all the operands fit any one alternative, the instruction is valid.
Otherwise, for each alternative, the compiler counts how many instructions
must be added to copy the operands so that that alternative applies. The
alternative requiring the least copying is chosen. If two alternatives need
the same amount of copying, the one that comes first is chosen. These choices
can be altered with the '?' and '!' characters:
?
Disparage slightly the alternative that the '?' appears in, as a
choice when no alternative applies exactly. The compiler regards
this alternative as one unit more costly for each '?' that appears
in it.
!
Disparage severely the alternative that the '!' appears in. This
alternative can still be used if it fits without reloading, but if
reloading is needed, some other alternative will be used.
ΓòÉΓòÉΓòÉ 6.32.3. Constraint Modifier Characters ΓòÉΓòÉΓòÉ
Here are constraint modifier characters.
'='
Means that this operand is write-only for this instruction: the
previous value is discarded and replaced by output data.
'+'
Means that this operand is both read and written by the instruction.
When the compiler fixes up the operands to satisfy the constraints,
it needs to know which operands are inputs to the instruction and
which are outputs from it. '=' identifies an output; '+' identifies
an operand that is both input and output; all other operands are
assumed to be input only.
'&'
Means (in a particular alternative) that this operand is an
earlyclobber operand, which is modified before the instruction is
finished using the input operands. Therefore, this operand may not
lie in a register that is used as an input operand or as part of any
memory address.
'&' applies only to the alternative in which it is written. In
constraints with multiple alternatives, sometimes one alternative
requires '&' while others do not. See, for example, the 'movdf'
insn of the 68000.
An input operand can be tied to an earlyclobber operand if its only
use as an input occurs before the early result is written. Adding
alternatives of this form often allows GCC to produce better code
when only some of the inputs can be affected by the earlyclobber.
See, for example, the 'mulsi3' insn of the ARM.
'&' does not obviate the need to write '='.
'%'
Declares the instruction to be commutative for this operand and the
following operand. This means that the compiler may interchange the
two operands if that is the cheapest way to make all operands fit
the constraints.
'#'
Says that all following characters, up to the next comma, are to be
ignored as a constraint. They are significant only for choosing
register preferences.
ΓòÉΓòÉΓòÉ 6.32.4. Constraints for Particular Machines ΓòÉΓòÉΓòÉ
Whenever possible, you should use the general-purpose constraint letters in
asm arguments, since they will convey meaning more readily to people reading
your code. Failing that, use the constraint letters that usually have very
similar meanings across architectures. The most commonly used constraints are
'm' and 'r' (for memory and general-purpose registers respectively; see Simple
Constraints), and 'I', usually the letter indicating the most common
immediate-constant format.
For each machine architecture, the 'config/machine.h' file defines additional
constraints. These constraints are used by the compiler itself for
instruction generation, as well as for asm statements; therefore, some of the
constraints are not particularly interesting for asm. The constraints are
defined through these macros:
REG_CLASS_FROM_LETTER
Register class constraints (usually lower case).
CONST_OK_FOR_LETTER_P
Immediate constant constraints, for non-floating point constants of
word size or smaller precision (usually upper case).
CONST_DOUBLE_OK_FOR_LETTER_P
Immediate constant constraints, for all floating point constants and
for constants of greater than word size precision (usually upper
case).
EXTRA_CONSTRAINT
Special cases of registers or memory. This macro is not required,
and is only defined for some machines.
Inspecting these macro definitions in the compiler source for your machine is
the best way to be certain you have the right constraints. However, here is a
summary of the machine-dependent constraints available on some particular
machines.
ARM family---'arm.h'
f
Floating-point register
F
One of the floating-point constants 0.0, 0.5, 1.0,
2.0, 3.0, 4.0, 5.0 or 10.0
G
Floating-point constant that would satisfy the
constraint 'F' if it were negated
I
Integer that is valid as an immediate operand in a
data processing instruction. That is, an integer in
the range 0 to 255 rotated by a multiple of 2
J
Integer in the range -4095 to 4095
K
Integer that satisfies constraint 'I' when inverted
(ones complement)
L
Integer that satisfies constraint 'I' when negated
(twos complement)
M
Integer in the range 0 to 32
Q
A memory reference where the exact address is in a
single register (`'m'' is preferable for asm
statements)
R
An item in the constant pool
S
A symbol in the text segment of the current file
AMD 29000 family---'a29k.h'
l
Local register 0
b
Byte Pointer ('BP') register
q
'Q' register
h
Special purpose register
A
First accumulator register
a
Other accumulator register
f
Floating point register
I
Constant greater than 0, less than 0x100
J
Constant greater than 0, less than 0x10000
K
Constant whose high 24 bits are on (1)
L
16 bit constant whose high 8 bits are on (1)
M
32 bit constant whose high 16 bits are on (1)
N
32 bit negative constant that fits in 8 bits
O
The constant 0x80000000 or, on the 29050, any 32 bit
constant whose low 16 bits are 0.
P
16 bit negative constant that fits in 8 bits
G
H
A floating point constant (in asm statements, use the
machine independent 'E' or 'F' instead)
IBM RS6000---'rs6000.h'
b
Address base register
f
Floating point register
h
'MQ', 'CTR', or 'LINK' register
q
'MQ' register
c
'CTR' register
l
'LINK' register
x
'CR' register (condition register) number 0
y
'CR' register (condition register)
I
Signed 16 bit constant
J
Constant whose low 16 bits are 0
K
Constant whose high 16 bits are 0
L
Constant suitable as a mask operand
M
Constant larger than 31
N
Exact power of 2
O
Zero
P
Constant whose negation is a signed 16 bit constant
G
Floating point constant that can be loaded into a
register with one instruction per word
Q
Memory operand that is an offset from a register ('m'
is preferable for asm statements)
R
AIX TOC entry
S
Windows NT SYMBOL_REF
T
Windows NT LABEL_REF
U
System V Release 4 small data area reference
Intel 386---'i386.h'
q
'a', b, c, or d register
A
'a', or d register (for 64-bit ints)
f
Floating point register
t
First (top of stack) floating point register
u
Second floating point register
a
'a' register
b
'b' register
c
'c' register
d
'd' register
D
'di' register
S
'si' register
I
Constant in range 0 to 31 (for 32 bit shifts)
J
Constant in range 0 to 63 (for 64 bit shifts)
K
'0xff'
L
'0xffff'
M
0, 1, 2, or 3 (shifts for lea instruction)
N
Constant in range 0 to 255 (for out instruction)
G
Standard 80387 floating point constant
Intel 960---'i960.h'
f
Floating point register (fp0 to fp3)
l
Local register (r0 to r15)
b
Global register (g0 to g15)
d
Any local or global register
I
Integers from 0 to 31
J
0
K
Integers from -31 to 0
G
Floating point 0
H
Floating point 1
MIPS---'mips.h'
d
General-purpose integer register
f
Floating-point register (if available)
h
'Hi' register
l
'Lo' register
x
'Hi' or 'Lo' register
y
General-purpose integer register
z
Floating-point status register
I
Signed 16 bit constant (for arithmetic instructions)
J
Zero
K
Zero-extended 16-bit constant (for logic instructions)
L
Constant with low 16 bits zero (can be loaded with
lui)
M
32 bit constant which requires two instructions to
load (a constant which is not 'I', 'K', or 'L')
N
Negative 16 bit constant
O
Exact power of two
P
Positive 16 bit constant
G
Floating point zero
Q
Memory reference that can be loaded with more than one
instruction ('m' is preferable for asm statements)
R
Memory reference that can be loaded with one
instruction ('m' is preferable for asm statements)
S
Memory reference in external OSF/rose PIC format ('m'
is preferable for asm statements)
Motorola 680x0---'m68k.h'
a
Address register
d
Data register
f
68881 floating-point register, if available
x
Sun FPA (floating-point) register, if available
y
First 16 Sun FPA registers, if available
I
Integer in the range 1 to 8
J
16 bit signed number
K
Signed number whose magnitude is greater than 0x80
L
Integer in the range -8 to -1
M
Signed number whose magnitude is greater than 0x100
G
Floating point constant that is not a 68881 constant
H
Floating point constant that can be used by Sun FPA
SPARC---'sparc.h'
f
Floating-point register that can hold 32 or 64 bit
values.
e
Floating-point register that can hold 64 or 128 bit
values.
I
Signed 13 bit constant
J
Zero
K
32 bit constant with the low 12 bits clear (a constant
that can be loaded with the sethi instruction)
G
Floating-point zero
H
Signed 13 bit constant, sign-extended to 32 or 64 bits
Q
Memory reference that can be loaded with one
instruction ('m' is more appropriate for asm
statements)
S
Constant, or memory address
T
Memory address aligned to an 8-byte boundary
U
Even register
ΓòÉΓòÉΓòÉ 6.33. Controlling Names Used in Assembler Code ΓòÉΓòÉΓòÉ
You can specify the name to be used in the assembler code for a C function or
variable by writing the asm (or __asm__) keyword after the declarator as
follows:
int foo asm ("myfoo") = 2;
This specifies that the name to be used for the variable foo in the assembler
code should be 'myfoo' rather than the usual '_foo'.
On systems where an underscore is normally prepended to the name of a C
function or variable, this feature allows you to define names for the linker
that do not start with an underscore.
You cannot use asm in this way in a function definition; but you can get the
same effect by writing a declaration for the function before its definition
and putting asm there, like this:
extern func () asm ("FUNC");
func (x, y)
int x, y;
┬╖┬╖┬╖
It is up to you to make sure that the assembler names you choose do not
conflict with any other assembler symbols. Also, you must not use a register
name; that would produce completely invalid assembler code. GNU CC does not
as yet have the ability to store static variables in registers. Perhaps that
will be added.
ΓòÉΓòÉΓòÉ 6.34. Variables in Specified Registers ΓòÉΓòÉΓòÉ
GNU C allows you to put a few global variables into specified hardware
registers. You can also specify the register in which an ordinary register
variable should be allocated.
Global register variables reserve registers throughout the program. This
may be useful in programs such as programming language interpreters which
have a couple of global variables that are accessed very often.
Local register variables in specific registers do not reserve the
registers. The compiler's data flow analysis is capable of determining
where the specified registers contain live values, and where they are
available for other uses.
These local variables are sometimes convenient for use with the extended
asm feature (see Extended Asm), if you want to write one output of the
assembler instruction directly into a particular register. (This will
work provided the register you specify fits the constraints specified for
that operand in the asm.)
Global Reg Vars Global Reg Vars
Local Reg Vars Local Reg Vars
ΓòÉΓòÉΓòÉ 6.34.1. Defining Global Register Variables ΓòÉΓòÉΓòÉ
You can define a global register variable in GNU C like this:
register int *foo asm ("a5");
Here a5 is the name of the register which should be used. Choose a register
which is normally saved and restored by function calls on your machine, so
that library routines will not clobber it.
Naturally the register name is cpu-dependent, so you would need to
conditionalize your program according to cpu type. The register a5 would be a
good choice on a 68000 for a variable of pointer type. On machines with
register windows, be sure to choose a ``global'' register that is not affected
magically by the function call mechanism.
In addition, operating systems on one type of cpu may differ in how they name
the registers; then you would need additional conditionals. For example, some
68000 operating systems call this register %a5.
Eventually there may be a way of asking the compiler to choose a register
automatically, but first we need to figure out how it should choose and how to
enable you to guide the choice. No solution is evident.
Defining a global register variable in a certain register reserves that
register entirely for this use, at least within the current compilation. The
register will not be allocated for any other purpose in the functions in the
current compilation. The register will not be saved and restored by these
functions. Stores into this register are never deleted even if they would
appear to be dead, but references may be deleted or moved or simplified.
It is not safe to access the global register variables from signal handlers,
or from more than one thread of control, because the system library routines
may temporarily use the register for other things (unless you recompile them
specially for the task at hand).
It is not safe for one function that uses a global register variable to call
another such function foo by way of a third function lose that was compiled
without knowledge of this variable (i.e. in a different source file in which
the variable wasn't declared). This is because lose might save the register
and put some other value there. For example, you can't expect a global
register variable to be available in the comparison-function that you pass to
qsort, since qsort might have put something else in that register. (If you
are prepared to recompile qsort with the same global register variable, you
can solve this problem.)
If you want to recompile qsort or other source files which do not actually use
your global register variable, so that they will not use that register for any
other purpose, then it suffices to specify the compiler option '-ffixed-reg'.
You need not actually add a global register declaration to their source code.
A function which can alter the value of a global register variable cannot
safely be called from a function compiled without this variable, because it
could clobber the value the caller expects to find there on return. Therefore,
the function which is the entry point into the part of the program that uses
the global register variable must explicitly save and restore the value which
belongs to its caller.
On most machines, longjmp will restore to each global register variable the
value it had at the time of the setjmp. On some machines, however, longjmp
will not change the value of global register variables. To be portable, the
function that called setjmp should make other arrangements to save the values
of the global register variables, and to restore them in a longjmp. This way,
the same thing will happen regardless of what longjmp does.
All global register variable declarations must precede all function
definitions. If such a declaration could appear after function definitions,
the declaration would be too late to prevent the register from being used for
other purposes in the preceding functions.
Global register variables may not have initial values, because an executable
file has no means to supply initial contents for a register.
On the Sparc, there are reports that g3 ┬╖┬╖┬╖ g7 are suitable registers, but
certain library functions, such as getwd, as well as the subroutines for
division and remainder, modify g3 and g4. g1 and g2 are local temporaries.
On the 68000, a2 ┬╖┬╖┬╖ a5 should be suitable, as should d2 ┬╖┬╖┬╖ d7. Of course, it
will not do to use more than a few of those.
ΓòÉΓòÉΓòÉ 6.34.2. Specifying Registers for Local Variables ΓòÉΓòÉΓòÉ
You can define a local register variable with a specified register like this:
register int *foo asm ("a5");
Here a5 is the name of the register which should be used. Note that this is
the same syntax used for defining global register variables, but for a local
variable it would appear within a function.
Naturally the register name is cpu-dependent, but this is not a problem, since
specific registers are most often useful with explicit assembler instructions
(see Extended Asm). Both of these things generally require that you
conditionalize your program according to cpu type.
In addition, operating systems on one type of cpu may differ in how they name
the registers; then you would need additional conditionals. For example, some
68000 operating systems call this register %a5.
Defining such a register variable does not reserve the register; it remains
available for other uses in places where flow control determines the
variable's value is not live. However, these registers are made unavailable
for use in the reload pass; excessive use of this feature leaves the compiler
too few available registers to compile certain functions.
This option does not guarantee that GNU CC will generate code that has this
variable in the register you specify at all times. You may not code an
explicit reference to this register in an asm statement and assume it will
always refer to this variable.
ΓòÉΓòÉΓòÉ 6.35. Alternate Keywords ΓòÉΓòÉΓòÉ
The option '-traditional' disables certain keywords; '-ansi' disables certain
others. This causes trouble when you want to use GNU C extensions, or ANSI C
features, in a general-purpose header file that should be usable by all
programs, including ANSI C programs and traditional ones. The keywords asm,
typeof and inline cannot be used since they won't work in a program compiled
with '-ansi', while the keywords const, volatile, signed, typeof and inline
won't work in a program compiled with '-traditional'.
The way to solve these problems is to put '__' at the beginning and end of
each problematical keyword. For example, use __asm__ instead of asm,
__const__ instead of const, and __inline__ instead of inline.
Other C compilers won't accept these alternative keywords; if you want to
compile with another compiler, you can define the alternate keywords as macros
to replace them with the customary keywords. It looks like this:
#ifndef __GNUC__
#define __asm__ asm
#endif
'-pedantic' causes warnings for many GNU C extensions. You can prevent such
warnings within one expression by writing __extension__ before the expression.
__extension__ has no effect aside from this.
ΓòÉΓòÉΓòÉ 6.36. Incomplete enum Types ΓòÉΓòÉΓòÉ
You can define an enum tag without specifying its possible values. This
results in an incomplete type, much like what you get if you write struct foo
without describing the elements. A later declaration which does specify the
possible values completes the type.
You can't allocate variables or storage using the type while it is incomplete.
However, you can work with pointers to that type.
This extension may not be very useful, but it makes the handling of enum more
consistent with the way struct and union are handled.
This extension is not supported by GNU C++.
ΓòÉΓòÉΓòÉ 6.37. Function Names as Strings ΓòÉΓòÉΓòÉ
GNU CC predefines two string variables to be the name of the current function.
The variable __FUNCTION__ is the name of the function as it appears in the
source. The variable __PRETTY_FUNCTION__ is the name of the function pretty
printed in a language specific fashion.
These names are always the same in a C function, but in a C++ function they
may be different. For example, this program:
extern "C" {
extern int printf (char *, ┬╖┬╖┬╖);
}
class a {
public:
sub (int i)
{
printf ("__FUNCTION__ = %s\n", __FUNCTION__);
printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
}
};
int
main (void)
{
a ax;
ax.sub (0);
return 0;
}
gives this output:
__FUNCTION__ = sub
__PRETTY_FUNCTION__ = int a::sub (int)
These names are not macros: they are predefined string variables. For example,
'#ifdef __FUNCTION__' does not have any special meaning inside a function,
since the preprocessor does not do anything special with the identifier
__FUNCTION__.
ΓòÉΓòÉΓòÉ 6.38. Getting the Return or Frame Address of a Function ΓòÉΓòÉΓòÉ
These functions may be used to get information about the callers of a
function.
__builtin_return_address (level)
This function returns the return address of the current function, or
of one of its callers. The level argument is number of frames to
scan up the call stack. A value of 0 yields the return address of
the current function, a value of 1 yields the return address of the
caller of the current function, and so forth.
The level argument must be a constant integer.
On some machines it may be impossible to determine the return
address of any function other than the current one; in such cases,
or when the top of the stack has been reached, this function will
return 0.
This function should only be used with a non-zero argument for
debugging purposes.
__builtin_frame_address (level)
This function is similar to __builtin_return_address, but it returns
the address of the function frame rather than the return address of
the function. Calling __builtin_frame_address with a value of 0
yields the frame address of the current function, a value of 1
yields the frame address of the caller of the current function, and
so forth.
The frame is the area on the stack which holds local variables and
saved registers. The frame address is normally the address of the
first word pushed on to the stack by the function. However, the
exact definition depends upon the processor and the calling
convention. If the processor has a dedicated frame pointer
register, and the function has a frame, then __builtin_frame_address
will return the value of the frame pointer register.
The caveats that apply to __builtin_return_address apply to this
function as well.
ΓòÉΓòÉΓòÉ 7. Extensions to the C++ Language ΓòÉΓòÉΓòÉ
The GNU compiler provides these extensions to the C++ language (and you can
also use most of the C language extensions in your C++ programs). If you want
to write code that checks whether these features are available, you can test
for the GNU compiler the same way as for C programs: check for a predefined
macro __GNUC__. You can also use __GNUG__ to test specifically for GNU C++
(see Section Standard Predefined Macros of The C Preprocessor).
Naming Results Giving a name to C++ function return values.
Min and Max C++ Minimum and maximum operators.
Destructors and Goto Goto is safe to use in C++ even when destructors
are needed.
C++ Interface You can use a single C++ header file for both
declarations and definitions.
Template Instantiation Methods for ensuring that exactly one copy of
each needed template instantiation is emitted.
C++ Signatures You can specify abstract types to get subtype
polymorphism independent from inheritance.
ΓòÉΓòÉΓòÉ 7.1. Named Return Values in C++ ΓòÉΓòÉΓòÉ
GNU C++ extends the function-definition syntax to allow you to specify a name
for the result of a function outside the body of the definition, in C++
programs:
type
functionname (args) return resultname;
{
┬╖┬╖┬╖
body
┬╖┬╖┬╖
}
You can use this feature to avoid an extra constructor call when a function
result has a class type. For example, consider a function m, declared as 'X v
= m ();', whose result is of class X:
X
m ()
{
X b;
b.a = 23;
return b;
}
Although m appears to have no arguments, in fact it has one implicit argument:
the address of the return value. At invocation, the address of enough space
to hold v is sent in as the implicit argument. Then b is constructed and its a
field is set to the value 23. Finally, a copy constructor (a constructor of
the form 'X(X&)') is applied to b, with the (implicit) return value location
as the target, so that v is now bound to the return value.
But this is wasteful. The local b is declared just to hold something that
will be copied right out. While a compiler that combined an ``elision''
algorithm with interprocedural data flow analysis could conceivably eliminate
all of this, it is much more practical to allow you to assist the compiler in
generating efficient code by manipulating the return value explicitly, thus
avoiding the local variable and copy constructor altogether.
Using the extended GNU C++ function-definition syntax, you can avoid the
temporary allocation and copying by naming r as your return value at the
outset, and assigning to its a field directly:
X
m () return r;
{
r.a = 23;
}
The declaration of r is a standard, proper declaration, whose effects are
executed before any of the body of m.
Functions of this type impose no additional restrictions; in particular, you
can execute return statements, or return implicitly by reaching the end of the
function body (``falling off the edge''). Cases like
X
m () return r (23);
{
return;
}
(or even 'X m () return r (23); { }') are unambiguous, since the return value
r has been initialized in either case. The following code may be hard to
read, but also works predictably:
X
m () return r;
{
X b;
return b;
}
The return value slot denoted by r is initialized at the outset, but the
statement 'return b;' overrides this value. The compiler deals with this by
destroying r (calling the destructor if there is one, or doing nothing if
there is not), and then reinitializing r with b.
This extension is provided primarily to help people who use overloaded
operators, where there is a great need to control not just the arguments, but
the return values of functions. For classes where the copy constructor incurs
a heavy performance penalty (especially in the common case where there is a
quick default constructor), this is a major savings. The disadvantage of this
extension is that you do not control when the default constructor for the
return value is called: it is always called at the beginning.
ΓòÉΓòÉΓòÉ 7.2. Minimum and Maximum Operators in C++ ΓòÉΓòÉΓòÉ
It is very convenient to have operators which return the ``minimum'' or the
``maximum'' of two arguments. In GNU C++ (but not in GNU C),
a <? b
is the minimum, returning the smaller of the numeric values a and b;
a >? b
is the maximum, returning the larger of the numeric values a and b.
These operations are not primitive in ordinary C++, since you can use a macro
to return the minimum of two things in C++, as in the following example.
#define MIN(X,Y) ((X) < (Y) ? : (X) : (Y))
You might then use 'int min = MIN (i, j);' to set min to the minimum value of
variables i and j.
However, side effects in X or Y may cause unintended behavior. For example,
MIN (i++, j++) will fail, incrementing the smaller counter twice. A GNU C
extension allows you to write safe macros that avoid this kind of problem (see
Naming an Expression's Type). However, writing MIN and MAX as macros also
forces you to use function-call notation for a fundamental arithmetic
operation. Using GNU C++ extensions, you can write 'int min = i <? j;'
instead.
Since <? and >? are built into the compiler, they properly handle expressions
with side-effects; 'int min = i++ <? j++;' works correctly.
ΓòÉΓòÉΓòÉ 7.3. goto and Destructors in GNU C++ ΓòÉΓòÉΓòÉ
In C++ programs, you can safely use the goto statement. When you use it to
exit a block which contains aggregates requiring destructors, the destructors
will run before the goto transfers control.
The compiler still forbids using goto to enter a scope that requires
constructors.
ΓòÉΓòÉΓòÉ 7.4. Declarations and Definitions in One Header ΓòÉΓòÉΓòÉ
C++ object definitions can be quite complex. In principle, your source code
will need two kinds of things for each object that you use across more than
one source file. First, you need an interface specification, describing its
structure with type declarations and function prototypes. Second, you need
the implementation itself. It can be tedious to maintain a separate interface
description in a header file, in parallel to the actual implementation. It is
also dangerous, since separate interface and implementation definitions may
not remain parallel.
With GNU C++, you can use a single header file for both purposes.
Warning: The mechanism to specify this is in transition. For the
nonce, you must use one of two #pragma commands; in a future release
of GNU C++, an alternative mechanism will make these #pragma
commands unnecessary.
The header file contains the full definitions, but is marked with '#pragma
interface' in the source code. This allows the compiler to use the header
file only as an interface specification when ordinary source files incorporate
it with #include. In the single source file where the full implementation
belongs, you can use either a naming convention or '#pragma implementation' to
indicate this alternate use of the header file.
#pragma interface
#pragma interface "subdir/objects.h"
Use this directive in header files that define object classes, to
save space in most of the object files that use those classes.
Normally, local copies of certain information (backup copies of
inline member functions, debugging information, and the internal
tables that implement virtual functions) must be kept in each object
file that includes class definitions. You can use this pragma to
avoid such duplication. When a header file containing '#pragma
interface' is included in a compilation, this auxiliary information
will not be generated (unless the main input source file itself uses
'#pragma implementation'). Instead, the object files will contain
references to be resolved at link time.
The second form of this directive is useful for the case where you
have multiple headers with the same name in different directories.
If you use this form, you must specify the same string to '#pragma
implementation'.
#pragma implementation
#pragma implementation "objects.h"
Use this pragma in a main input file, when you want full output from
included header files to be generated (and made globally visible).
The included header file, in turn, should use '#pragma interface'.
Backup copies of inline member functions, debugging information, and
the internal tables used to implement virtual functions are all
generated in implementation files.
If you use '#pragma implementation' with no argument, it applies to
an include file with the same basename (3) as your source file. For
example, in 'allclass.cc', giving just '#pragma implementation' by
itself is equivalent to '#pragma implementation "allclass.h"'.
In versions of GNU C++ prior to 2.6.0 'allclass.h' was treated as an
implementation file whenever you would include it from 'allclass.cc'
even if you never specified '#pragma implementation'. This was
deemed to be more trouble than it was worth, however, and disabled.
If you use an explicit '#pragma implementation', it must appear in
your source file before you include the affected header files.
Use the string argument if you want a single implementation file to
include code from multiple header files. (You must also use
'#include' to include the header file; '#pragma implementation' only
specifies how to use the file---it doesn't actually include it.)
There is no way to split up the contents of a single header file
into multiple implementation files.
'#pragma implementation' and '#pragma interface' also have an effect on
function inlining.
If you define a class in a header file marked with '#pragma interface', the
effect on a function defined in that class is similar to an explicit extern
declaration---the compiler emits no code at all to define an independent
version of the function. Its definition is used only for inlining with its
callers.
Conversely, when you include the same header file in a main source file that
declares it as '#pragma implementation', the compiler emits code for the
function itself; this defines a version of the function that can be found via
pointers (or by callers compiled without inlining). If all calls to the
function can be inlined, you can avoid emitting the function by compiling with
'-fno-implement-inlines'. If any calls were not inlined, you will get linker
errors.
ΓòÉΓòÉΓòÉ 7.5. Where's the Template? ΓòÉΓòÉΓòÉ
C++ templates are the first language feature to require more intelligence from
the environment than one usually finds on a UNIX system. Somehow the compiler
and linker have to make sure that each template instance occurs exactly once
in the executable if it is needed, and not at all otherwise. There are two
basic approaches to this problem, which I will refer to as the Borland model
and the Cfront model.
Borland model
Borland C++ solved the template instantiation problem by adding the
code equivalent of common blocks to their linker; the compiler emits
template instances in each translation unit that uses them, and the
linker collapses them together. The advantage of this model is that
the linker only has to consider the object files themselves; there
is no external complexity to worry about. This disadvantage is that
compilation time is increased because the template code is being
compiled repeatedly. Code written for this model tends to include
definitions of all templates in the header file, since they must be
seen to be instantiated.
Cfront model
The AT&T C++ translator, Cfront, solved the template instantiation
problem by creating the notion of a template repository, an
automatically maintained place where template instances are stored.
A more modern version of the repository works as follows: As
individual object files are built, the compiler places any template
definitions and instantiations encountered in the repository. At
link time, the link wrapper adds in the objects in the repository
and compiles any needed instances that were not previously emitted.
The advantages of this model are more optimal compilation speed and
the ability to use the system linker; to implement the Borland model
a compiler vendor also needs to replace the linker. The
disadvantages are vastly increased complexity, and thus potential
for error; for some code this can be just as transparent, but in
practice it can been very difficult to build multiple programs in
one directory and one program in multiple directories. Code written
for this model tends to separate definitions of non-inline member
templates into a separate file, which should be compiled separately.
When used with GNU ld version 2.8 or later on an ELF system such as Linux/GNU
or Solaris 2, or on Microsoft Windows, g++ supports the Borland model. On
other systems, g++ implements neither automatic model.
A future version of g++ will support a hybrid model whereby the compiler will
emit any instantiations for which the template definition is included in the
compile, and store template definitions and instantiation context information
into the object file for the rest. The link wrapper will extract that
information as necessary and invoke the compiler to produce the remaining
instantiations. The linker will then combine duplicate instantiations.
In the mean time, you have the following options for dealing with template
instantiations:
1. Compile your code with '-fno-implicit-templates' to disable the implicit
generation of template instances, and explicitly instantiate all the ones
you use. This approach requires more knowledge of exactly which
instances you need than do the others, but it's less mysterious and
allows greater control. You can scatter the explicit instantiations
throughout your program, perhaps putting them in the translation units
where the instances are used or the translation units that define the
templates themselves; you can put all of the explicit instantiations you
need into one big file; or you can create small files like
#include "Foo.h"
#include "Foo.cc"
template class Foo<int>;
template ostream& operator <<
(ostream&, const Foo<int>&);
for each of the instances you need, and create a template instantiation
library from those.
If you are using Cfront-model code, you can probably get away with not using
'-fno-implicit-templates' when compiling files that don't '#include' the
member template definitions.
If you use one big file to do the instantiations, you may want to compile it
without '-fno-implicit-templates' so you get all of the instances required by
your explicit instantiations (but not by any other files) without having to
specify them as well.
g++ has extended the template instantiation syntax outlined in the Working
Paper to allow forward declaration of explicit instantiations, explicit
instantiation of members of template classes and instantiation of the compiler
support data for a template class (i.e. the vtable) without instantiating any
of its members:
extern template int max (int, int);
template void Foo<int>::f ();
inline template class Foo<int>;
2. Do nothing. Pretend g++ does implement automatic instantiation
management. Code written for the Borland model will work fine, but each
translation unit will contain instances of each of the templates it uses.
In a large program, this can lead to an unacceptable amount of code
duplication.
3. Add '#pragma interface' to all files containing template definitions.
For each of these files, add '#pragma implementation "filename"' to the
top of some '.C' file which '#include's it. Then compile everything with
'-fexternal-templates'. The templates will then only be expanded in the
translation unit which implements them (i.e. has a '#pragma
implementation' line for the file where they live); all other files will
use external references. If you're lucky, everything should work
properly. If you get undefined symbol errors, you need to make sure that
each template instance which is used in the program is used in the file
which implements that template. If you don't have any use for a
particular instance in that file, you can just instantiate it explicitly,
using the syntax from the latest C++ working paper:
template class A<int>;
template ostream& operator << (ostream&, const A<int>&);
This strategy will work with code written for either model. If you are using
code written for the Cfront model, the file containing a class template and
the file containing its member templates should be implemented in the same
translation unit.
A slight variation on this approach is to instead use the flag
'-falt-external-templates'; this flag causes template instances to be emitted
in the translation unit that implements the header where they are first
instantiated, rather than the one which implements the file where the
templates are defined. This header must be the same in all translation units,
or things are likely to break.
See Declarations and Definitions in One Header, for more discussion of these
pragmas.
ΓòÉΓòÉΓòÉ 7.6. Type Abstraction using Signatures ΓòÉΓòÉΓòÉ
In GNU C++, you can use the keyword signature to define a completely abstract
class interface as a datatype. You can connect this abstraction with actual
classes using signature pointers. If you want to use signatures, run the GNU
compiler with the '-fhandle-signatures' command-line option. (With this
option, the compiler reserves a second keyword sigof as well, for a future
extension.)
Roughly, signatures are type abstractions or interfaces of classes. Some other
languages have similar facilities. C++ signatures are related to ML's
signatures, Haskell's type classes, definition modules in Modula-2, interface
modules in Modula-3, abstract types in Emerald, type modules in Trellis/Owl,
categories in Scratchpad II, and types in POOL-I. For a more detailed
discussion of signatures, see Signatures: A Language Extension for Improving
Type Abstraction and Subtype Polymorphism in C++ by Gerald Baumgartner and
Vincent F. Russo (Tech report CSD--TR--95--051, Dept. of Computer Sciences,
Purdue University, August 1995, a slightly improved version appeared in
Software---Practice & Experience, 25(8), pp. 863--889, August 1995). You can
get the tech report by anonymous FTP from ftp.cs.purdue.edu in
'pub/gb/Signature-design.ps.gz'.
Syntactically, a signature declaration is a collection of member function
declarations and nested type declarations. For example, this signature
declaration defines a new abstract type S with member functions 'int foo ()'
and 'int bar (int)':
signature S
{
int foo ();
int bar (int);
};
Since signature types do not include implementation definitions, you cannot
write an instance of a signature directly. Instead, you can define a pointer
to any class that contains the required interfaces as a signature pointer.
Such a class implements the signature type. To use a class as an
implementation of S, you must ensure that the class has public member
functions 'int foo ()' and 'int bar (int)'. The class can have other member
functions as well, public or not; as long as it offers what's declared in the
signature, it is suitable as an implementation of that signature type.
For example, suppose that C is a class that meets the requirements of
signature S (C conforms to S). Then
C obj;
S * p = &obj;
defines a signature pointer p and initializes it to point to an object of type
C. The member function call 'int i = p->foo ();' executes 'obj.foo ()'.
Abstract virtual classes provide somewhat similar facilities in standard C++.
There are two main advantages to using signatures instead:
1. Subtyping becomes independent from inheritance. A class or signature
type T is a subtype of a signature type S independent of any inheritance
hierarchy as long as all the member functions declared in S are also
found in T. So you can define a subtype hierarchy that is completely
independent from any inheritance (implementation) hierarchy, instead of
being forced to use types that mirror the class inheritance hierarchy.
2. Signatures allow you to work with existing class hierarchies as
implementations of a signature type. If those class hierarchies are only
available in compiled form, you're out of luck with abstract virtual
classes, since an abstract virtual class cannot be retrofitted on top of
existing class hierarchies. So you would be required to write interface
classes as subtypes of the abstract virtual class.
There is one more detail about signatures. A signature declaration can
contain member function definitions as well as member function declarations.
A signature member function with a full definition is called a default
implementation; classes need not contain that particular interface in order to
conform. For example, a class C can conform to the signature
signature T
{
int f (int);
int f0 () { return f (0); };
};
whether or not C implements the member function 'int f0 ()'. If you define
C::f0, that definition takes precedence; otherwise, the default implementation
S::f0 applies.
ΓòÉΓòÉΓòÉ 8. gcov: a Test Coverage Program ΓòÉΓòÉΓòÉ
gcov is a tool you can use in conjunction with GNU CC to test code coverage in
your programs.
This chapter describes version 1.5 of gcov.
Gcov Intro Introduction to gcov.
Invoking Gcov How to use gcov.
Gcov and Optimization Using gcov with GCC optimization.
Gcov Data Files The files used by gcov.
ΓòÉΓòÉΓòÉ 8.1. Introduction to gcov ΓòÉΓòÉΓòÉ
gcov is a test coverage program. Use it in concert with GNU CC to analyze
your programs to help create more efficient, faster running code. You can use
gcov as a profiling tool to help discover where your optimization efforts will
best affect your code. You can also use gcov along with the other profiling
tool, gprof, to assess which parts of your code use the greatest amount of
computing time.
Profiling tools help you analyze your code's performance. Using a profiler
such as gcov or gprof, you can find out some basic performance statistics,
such as:
how often each line of code executes
what lines of code are actually executed
how much computing time each section of code uses
Once you know these things about how your code works when compiled, you can
look at each module to see which modules should be optimized. gcov helps you
determine where to work on optimization.
Software developers also use coverage testing in concert with testsuites, to
make sure software is actually good enough for a release. Testsuites can
verify that a program works as expected; a coverage program tests to see how
much of the program is exercised by the testsuite. Developers can then
determine what kinds of test cases need to be added to the testsuites to
create both better testing and a better final product.
You should compile your code without optimization if you plan to use gcov
because the optimization, by combining some lines of code into one function,
may not give you as much information as you need to look for `hot spots' where
the code is using a great deal of computer time. Likewise, because gcov
accumulates statistics by line (at the lowest resolution), it works best with
a programming style that places only one statement on each line. If you use
complicated macros that expand to loops or to other control structures, the
statistics are less helpful---they only report on the line where the macro
call appears. If your complex macros behave like functions, you can replace
them with inline functions to solve this problem.
gcov creates a logfile called 'sourcefile.gcov' which indicates how many times
each line of a source file 'sourcefile.c' has executed. You can use these
logfiles along with gprof to aid in fine-tuning the performance of your
programs. gprof gives timing information you can use along with the
information you get from gcov.
gcov works only on code compiled with GNU CC. It is not compatible with any
other profiling or test coverage mechanism.
ΓòÉΓòÉΓòÉ 8.2. Invoking gcov ΓòÉΓòÉΓòÉ
gcov [-b] [-v] [-n] [-l] [-f] [-o directory] sourcefile
-b
Write branch frequencies to the output file, and write branch
summary info to the standard output. This option allows you to see
how often each branch in your program was taken.
-v
Display the gcov version number (on the standard error stream).
-n
Do not create the gcov output file.
-l
Create long file names for included source files. For example, if
the header file 'x.h' contains code, and was included in the file
'a.c', then running gcov on the file 'a.c' will produce an output
file called 'a.c.x.h.gcov' instead of 'x.h.gcov'. This can be useful
if 'x.h' is included in multiple source files.
-f
Output summaries for each function in addition to the file level
summary.
-o
The directory where the object files live. Gcov will search for
.bb, .bbg, and .da files in this directory.
When using gcov, you must first compile your program with two special GNU CC
options: '-fprofile-arcs -ftest-coverage'. This tells the compiler to generate
additional information needed by gcov (basically a flow graph of the program)
and also includes additional code in the object files for generating the extra
profiling information needed by gcov. These additional files are placed in
the directory where the source code is located.
Running the program will cause profile output to be generated. For each
source file compiled with -fprofile-arcs, an accompanying .da file will be
placed in the source directory.
Running gcov with your program's source file names as arguments will now
produce a listing of the code along with frequency of execution for each line.
For example, if your program is called 'tmp.c', this is what you see when you
use the basic gcov facility:
$ gcc -fprofile-arcs -ftest-coverage tmp.c
$ a.out
$ gcov tmp.c
87.50% of 8 source lines executed in file tmp.c
Creating tmp.c.gcov.
The file 'tmp.c.gcov' contains output from gcov. Here is a sample:
main()
{
1 int i, total;
1 total = 0;
11 for (i = 0; i < 10; i++)
10 total += i;
1 if (total != 45)
###### printf ("Failure\n");
else
1 printf ("Success\n");
1 }
When you use the '-b' option, your output looks like this:
$ gcov -b tmp.c
87.50% of 8 source lines executed in file tmp.c
80.00% of 5 branches executed in file tmp.c
80.00% of 5 branches taken at least once in file tmp.c
50.00% of 2 calls executed in file tmp.c
Creating tmp.c.gcov.
Here is a sample of a resulting 'tmp.c.gcov' file:
main()
{
1 int i, total;
1 total = 0;
11 for (i = 0; i < 10; i++)
branch 0 taken = 91%
branch 1 taken = 100%
branch 2 taken = 100%
10 total += i;
1 if (total != 45)
branch 0 taken = 100%
###### printf ("Failure\n");
call 0 never executed
branch 1 never executed
else
1 printf ("Success\n");
call 0 returns = 100%
1 }
For each basic block, a line is printed after the last line of the basic block
describing the branch or call that ends the basic block. There can be
multiple branches and calls listed for a single source line if there are
multiple basic blocks that end on that line. In this case, the branches and
calls are each given a number. There is no simple way to map these branches
and calls back to source constructs. In general, though, the lowest numbered
branch or call will correspond to the leftmost construct on the source line.
For a branch, if it was executed at least once, then a percentage indicating
the number of times the branch was taken divided by the number of times the
branch was executed will be printed. Otherwise, the message ``never
executed'' is printed.
For a call, if it was executed at least once, then a percentage indicating the
number of times the call returned divided by the number of times the call was
executed will be printed. This will usually be 100%, but may be less for
functions call exit or longjmp, and thus may not return everytime they are
called.
The execution counts are cumulative. If the example program were executed
again without removing the .da file, the count for the number of times each
line in the source was executed would be added to the results of the previous
run(s). This is potentially useful in several ways. For example, it could be
used to accumulate data over a number of program runs as part of a test
verification suite, or to provide more accurate long-term information over a
large number of program runs.
The data in the .da files is saved immediately before the program exits. For
each source file compiled with -fprofile-arcs, the profiling code first
attempts to read in an existing .da file; if the file doesn't match the
executable (differing number of basic block counts) it will ignore the
contents of the file. It then adds in the new execution counts and finally
writes the data to the file.
ΓòÉΓòÉΓòÉ 8.3. Using gcov with GCC Optimization ΓòÉΓòÉΓòÉ
If you plan to use gcov to help optimize your code, you must first compile
your program with two special GNU CC options: '-fprofile-arcs
-ftest-coverage'. Aside from that, you can use any other GNU CC options; but
if you want to prove that every single line in your program was executed, you
should not compile with optimization at the same time. On some machines the
optimizer can eliminate some simple code lines by combining them with other
lines. For example, code like this:
if (a != b)
c = 1;
else
c = 0;
can be compiled into one instruction on some machines. In this case, there is
no way for gcov to calculate separate execution counts for each line because
there isn't separate code for each line. Hence the gcov output looks like
this if you compiled the program with optimization:
100 if (a != b)
100 c = 1;
100 else
100 c = 0;
The output shows that this block of code, combined by optimization, executed
100 times. In one sense this result is correct, because there was only one
instruction representing all four of these lines. However, the output does
not indicate how many times the result was 0 and how many times the result was
1.
ΓòÉΓòÉΓòÉ 8.4. Brief description of gcov data files ΓòÉΓòÉΓòÉ
gcov uses three files for doing profiling. The names of these files are
derived from the original source file by substituting the file suffix with
either .bb, .bbg, or .da. All of these files are placed in the same directory
as the source file, and contain data stored in a platform-independent method.
The .bb and .bbg files are generated when the source file is compiled with the
GNU CC '-ftest-coverage' option. The .bb file contains a list of source files
(including headers), functions within those files, and line numbers
corresponding to each basic block in the source file.
The .bb file format consists of several lists of 4-byte integers which
correspond to the line numbers of each basic block in the file. Each list is
terminated by a line number of 0. A line number of -1 is used to designate
that the source file name (padded to a 4-byte boundary and followed by another
-1) follows. In addition, a line number of -2 is used to designate that the
name of a function (also padded to a 4-byte boundary and followed by a -2)
follows.
The .bbg file is used to reconstruct the program flow graph for the source
file. It contains a list of the program flow arcs (possible branches taken
from one basic block to another) for each function which, in combination with
the .bb file, enables gcov to reconstruct the program flow.
In the .bbg file, the format is:
number of basic blocks for function #0 (4-byte number)
total number of arcs for function #0 (4-byte number)
count of arcs in basic block #0 (4-byte number)
destination basic block of arc #0 (4-byte number)
flag bits (4-byte number)
destination basic block of arc #1 (4-byte number)
flag bits (4-byte number)
┬╖┬╖┬╖
destination basic block of arc #N (4-byte number)
flag bits (4-byte number)
count of arcs in basic block #1 (4-byte number)
destination basic block of arc #0 (4-byte number)
flag bits (4-byte number)
┬╖┬╖┬╖
A -1 (stored as a 4-byte number) is used to separate each function's list of
basic blocks, and to verify that the file has been read correctly.
The .da file is generated when a program containing object files built with
the GNU CC '-fprofile-arcs' option is executed. A separate .da file is
created for each source file compiled with this option, and the name of the
.da file is stored as an absolute pathname in the resulting object file. This
path name is derived from the source file name by substituting a .da suffix.
The format of the .da file is fairly simple. The first 8-byte number is the
number of counts in the file, followed by the counts (stored as 8-byte
numbers). Each count corresponds to the number of times each arc in the
program is executed. The counts are cumulative; each time the program is
executed, it attemps to combine the existing .da files with the new counts for
this invocation of the program. It ignores the contents of any .da files
whose number of arcs doesn't correspond to the current program, and merely
overwrites them instead.
All three of these files use the functions in gcov-io.h to store integers; the
functions in this header provide a machine-independent mechanism for storing
and retrieving data from a stream.
ΓòÉΓòÉΓòÉ 9. Known Causes of Trouble with GNU CC ΓòÉΓòÉΓòÉ
This section describes known problems that affect users of GNU CC. Most of
these are not GNU CC bugs per se---if they were, we would fix them. But the
result for a user may be like the result of a bug.
Some of these problems are due to bugs in other software, some are missing
features that are too much work to add, and some are places where people's
opinions differ as to what is best.
Actual Bugs Bugs we will fix later.
Installation Problems Problems that manifest when you install GNU CC.
Cross-Compiler Problems Common problems of cross compiling with GNU CC.
Interoperation Problems using GNU CC with other compilers, and
with certain linkers, assemblers and debuggers.
External Bugs Problems compiling certain programs.
Incompatibilities GNU CC is incompatible with traditional C.
Fixed Headers GNU C uses corrected versions of system header
files. This is necessary, but doesn't always
work smoothly.
Standard Libraries GNU C uses the system C library, which might not
be compliant with the ISO/ANSI C standard.
Disappointments Regrettable things we can't change, but not
quite bugs.
C++ Misunderstandings Common misunderstandings with GNU C++.
Protoize Caveats Things to watch out for when using protoize.
Non-bugs Things we think are right, but some others
disagree.
Warnings and Errors Which problems in your code get warnings, and
which get errors.
ΓòÉΓòÉΓòÉ 9.1. Actual Bugs We Haven't Fixed Yet ΓòÉΓòÉΓòÉ
The fixincludes script interacts badly with automounters; if the
directory of system header files is automounted, it tends to be unmounted
while fixincludes is running. This would seem to be a bug in the
automounter. We don't know any good way to work around it.
The fixproto script will sometimes add prototypes for the sigsetjmp and
siglongjmp functions that reference the jmp_buf type before that type is
defined. To work around this, edit the offending file and place the
typedef in front of the prototypes.
There are several obscure case of mis-using struct, union, and enum tags
that are not detected as errors by the compiler.
When '-pedantic-errors' is specified, GNU C will incorrectly give an
error message when a function name is specified in an expression
involving the comma operator.
Loop unrolling doesn't work properly for certain C++ programs. This is a
bug in the C++ front end. It sometimes emits incorrect debug info, and
the loop unrolling code is unable to recover from this error.
ΓòÉΓòÉΓòÉ 9.2. Installation Problems ΓòÉΓòÉΓòÉ
This is a list of problems (and some apparent problems which don't really mean
anything is wrong) that show up during installation of GNU CC.
On certain systems, defining certain environment variables such as CC can
interfere with the functioning of make.
If you encounter seemingly strange errors when trying to build the
compiler in a directory other than the source directory, it could be
because you have previously configured the compiler in the source
directory. Make sure you have done all the necessary preparations. See
Other Dir.
If you build GNU CC on a BSD system using a directory stored in a System
V file system, problems may occur in running fixincludes if the System V
file system doesn't support symbolic links. These problems result in a
failure to fix the declaration of size_t in 'sys/types.h'. If you find
that size_t is a signed type and that type mismatches occur, this could
be the cause.
The solution is not to use such a directory for building GNU CC.
In previous versions of GNU CC, the gcc driver program looked for as and
ld in various places; for example, in files beginning with
'/usr/local/lib/gcc-'. GNU CC version 2 looks for them in the directory
'/usr/local/lib/gcc-lib/target/version'.
Thus, to use a version of as or ld that is not the system default, for
example gas or GNU ld, you must put them in that directory (or make links
to them from that directory).
Some commands executed when making the compiler may fail (return a
non-zero status) and be ignored by make. These failures, which are often
due to files that were not found, are expected, and can safely be
ignored.
It is normal to have warnings in compiling certain files about
unreachable code and about enumeration type clashes. These files' names
begin with 'insn-'. Also, 'real.c' may get some warnings that you can
ignore.
Sometimes make recompiles parts of the compiler when installing the
compiler. In one case, this was traced down to a bug in make. Either
ignore the problem or switch to GNU Make.
If you have installed a program known as purify, you may find that it
causes errors while linking enquire, which is part of building GNU CC.
The fix is to get rid of the file real-ld which purify installs---so that
GNU CC won't try to use it.
On GNU/Linux SLS 1.01, there is a problem with 'libc.a': it does not
contain the obstack functions. However, GNU CC assumes that the obstack
functions are in 'libc.a' when it is the GNU C library. To work around
this problem, change the __GNU_LIBRARY__ conditional around line 31 to
'#if 1'.
On some 386 systems, building the compiler never finishes because enquire
hangs due to a hardware problem in the motherboard---it reports floating
point exceptions to the kernel incorrectly. You can install GNU CC
except for 'float.h' by patching out the command to run enquire. You may
also be able to fix the problem for real by getting a replacement
motherboard. This problem was observed in Revision E of the Micronics
motherboard, and is fixed in Revision F. It has also been observed in the
MYLEX MXA-33 motherboard.
If you encounter this problem, you may also want to consider removing the
FPU from the socket during the compilation. Alternatively, if you are
running SCO Unix, you can reboot and force the FPU to be ignored. To do
this, type 'hd(40)unix auto ignorefpu'.
On some 386 systems, GNU CC crashes trying to compile 'enquire.c'. This
happens on machines that don't have a 387 FPU chip. On 386 machines, the
system kernel is supposed to emulate the 387 when you don't have one.
The crash is due to a bug in the emulator.
One of these systems is the Unix from Interactive Systems: 386/ix. On
this system, an alternate emulator is provided, and it does work. To use
it, execute this command as super-user:
ln /etc/emulator.rel1 /etc/emulator
and then reboot the system. (The default emulator file remains present under
the name 'emulator.dflt'.)
Try using '/etc/emulator.att', if you have such a problem on the SCO system.
Another system which has this problem is Esix. We don't know whether it has
an alternate emulator that works.
On NetBSD 0.8, a similar problem manifests itself as these error messages:
enquire.c: In function `fprop':
enquire.c:2328: floating overflow
On SCO systems, when compiling GNU CC with the system's compiler, do not
use '-O'. Some versions of the system's compiler miscompile GNU CC with
'-O'.
Sometimes on a Sun 4 you may observe a crash in the program genflags or
genoutput while building GNU CC. This is said to be due to a bug in sh.
You can probably get around it by running genflags or genoutput manually
and then retrying the make.
On Solaris 2, executables of GNU CC version 2.0.2 are commonly available,
but they have a bug that shows up when compiling current versions of GNU
CC: undefined symbol errors occur during assembly if you use '-g'.
The solution is to compile the current version of GNU CC without '-g'.
That makes a working compiler which you can use to recompile with '-g'.
Solaris 2 comes with a number of optional OS packages. Some of these
packages are needed to use GNU CC fully. If you did not install all
optional packages when installing Solaris, you will need to verify that
the packages that GNU CC needs are installed.
To check whether an optional package is installed, use the pkginfo
command. To add an optional package, use the pkgadd command. For
further details, see the Solaris documentation.
For Solaris 2.0 and 2.1, GNU CC needs six packages: 'SUNWarc',
'SUNWbtool', 'SUNWesu', ' SUNWhea', 'SUNWlibm', and 'SUNWtoo'.
For Solaris 2.2, GNU CC needs an additional seventh package: 'SUNWsprot'.
On Solaris 2, trying to use the linker and other tools in '/usr/ucb' to
install GNU CC has been observed to cause trouble. For example, the
linker may hang indefinitely. The fix is to remove '/usr/ucb' from your
PATH.
If you use the 1.31 version of the MIPS assembler (such as was shipped
with Ultrix 3.1), you will need to use the -fno-delayed-branch switch
when optimizing floating point code. Otherwise, the assembler will
complain when the GCC compiler fills a branch delay slot with a floating
point instruction, such as add.d.
If on a MIPS system you get an error message saying ``does not have gp
sections for all it's [sic] sectons [sic]'', don't worry about it. This
happens whenever you use GAS with the MIPS linker, but there is not
really anything wrong, and it is okay to use the output file. You can
stop such warnings by installing the GNU linker.
It would be nice to extend GAS to produce the gp tables, but they are
optional, and there should not be a warning about their absence.
In Ultrix 4.0 on the MIPS machine, 'stdio.h' does not work with GNU CC at
all unless it has been fixed with fixincludes. This causes problems in
building GNU CC. Once GNU CC is installed, the problems go away.
To work around this problem, when making the stage 1 compiler, specify
this option to Make:
GCC_FOR_TARGET="./xgcc -B./ -I./include"
When making stage 2 and stage 3, specify this option:
CFLAGS="-g -I./include"
Users have reported some problems with version 2.0 of the MIPS compiler
tools that were shipped with Ultrix 4.1. Version 2.10 which came with
Ultrix 4.2 seems to work fine.
Users have also reported some problems with version 2.20 of the MIPS
compiler tools that were shipped with RISC/os 4.x. The earlier version
2.11 seems to work fine.
Some versions of the MIPS linker will issue an assertion failure when
linking code that uses alloca against shared libraries on RISC-OS 5.0,
and DEC's OSF/1 systems. This is a bug in the linker, that is supposed
to be fixed in future revisions. To protect against this, GNU CC passes
'-non_shared' to the linker unless you pass an explicit '-shared' or
'-call_shared' switch.
On System V release 3, you may get this error message while linking:
ld fatal: failed to write symbol name something
in strings table for file whatever
This probably indicates that the disk is full or your ULIMIT won't allow the
file to be as large as it needs to be.
This problem can also result because the kernel parameter MAXUMEM is too
small. If so, you must regenerate the kernel and make the value much larger.
The default value is reported to be 1024; a value of 32768 is said to work.
Smaller values may also work.
On System V, if you get an error like this,
/usr/local/lib/bison.simple: In function `yyparse':
/usr/local/lib/bison.simple:625: virtual memory exhausted
that too indicates a problem with disk space, ULIMIT, or MAXUMEM.
Current GNU CC versions probably do not work on version 2 of the NeXT
operating system.
On NeXTStep 3.0, the Objective C compiler does not work, due, apparently,
to a kernel bug that it happens to trigger. This problem does not happen
on 3.1.
On the Tower models 4n0 and 6n0, by default a process is not allowed to
have more than one megabyte of memory. GNU CC cannot compile itself (or
many other programs) with '-O' in that much memory.
To solve this problem, reconfigure the kernel adding the following line
to the configuration file:
MAXUMEM = 4096
On HP 9000 series 300 or 400 running HP-UX release 8.0, there is a bug in
the assembler that must be fixed before GNU CC can be built. This bug
manifests itself during the first stage of compilation, while building
'libgcc2.a':
_floatdisf
cc1: warning: `-g' option not supported on this version of GCC
cc1: warning: `-g1' option not supported on this version of GCC
┬╖/xgcc: Internal compiler error: program as got fatal signal 11
A patched version of the assembler is available by anonymous ftp from
altdorf.ai.mit.edu as the file 'archive/cph/hpux-8.0-assembler'. If you have
HP software support, the patch can also be obtained directly from HP, as
described in the following note:
This is the patched assembler, to patch SR#1653-010439, where the
assembler aborts on floating point constants.
The bug is not really in the assembler, but in the shared library
version of the function ``cvtnum(3c)''. The bug on ``cvtnum(3c)''
is SR#4701-078451. Anyway, the attached assembler uses the archive
library version of ``cvtnum(3c)'' and thus does not exhibit the bug.
This patch is also known as PHCO_4484.
On HP-UX version 8.05, but not on 8.07 or more recent versions, the
fixproto shell script triggers a bug in the system shell. If you
encounter this problem, upgrade your operating system or use BASH (the
GNU shell) to run fixproto.
Some versions of the Pyramid C compiler are reported to be unable to
compile GNU CC. You must use an older version of GNU CC for
bootstrapping. One indication of this problem is if you get a crash when
GNU CC compiles the function muldi3 in file 'libgcc2.c'.
You may be able to succeed by getting GNU CC version 1, installing it,
and using it to compile GNU CC version 2. The bug in the Pyramid C
compiler does not seem to affect GNU CC version 1.
There may be similar problems on System V Release 3.1 on 386 systems.
On the Intel Paragon (an i860 machine), if you are using operating system
version 1.0, you will get warnings or errors about redefinition of va_arg
when you build GNU CC.
If this happens, then you need to link most programs with the library
'iclib.a'. You must also modify 'stdio.h' as follows: before the lines
#if defined(__i860__) && !defined(_VA_LIST)
#include <va_list.h>
insert the line
#if __PGC__
and after the lines
extern int vprintf(const char *, va_list );
extern int vsprintf(char *, const char *, va_list );
#endif
insert the line
#endif /* __PGC__ */
These problems don't exist in operating system version 1.1.
On the Altos 3068, programs compiled with GNU CC won't work unless you
fix a kernel bug. This happens using system versions V.2.2 1.0gT1 and
V.2.2 1.0e and perhaps later versions as well. See the file
'README.ALTOS'.
You will get several sorts of compilation and linking errors on the we32k
if you don't follow the special instructions. See Configurations.
A bug in the HP-UX 8.05 (and earlier) shell will cause the fixproto
program to report an error of the form:
┬╖/fixproto: sh internal 1K buffer overflow
To fix this, change the first line of the fixproto script to look like:
#!/bin/ksh
ΓòÉΓòÉΓòÉ 9.3. Cross-Compiler Problems ΓòÉΓòÉΓòÉ
You may run into problems with cross compilation on certain machines, for
several reasons.
Cross compilation can run into trouble for certain machines because some
target machines' assemblers require floating point numbers to be written
as integer constants in certain contexts.
The compiler writes these integer constants by examining the floating
point value as an integer and printing that integer, because this is
simple to write and independent of the details of the floating point
representation. But this does not work if the compiler is running on a
different machine with an incompatible floating point format, or even a
different byte-ordering.
In addition, correct constant folding of floating point values requires
representing them in the target machine's format. (The C standard does
not quite require this, but in practice it is the only way to win.)
It is now possible to overcome these problems by defining macros such as
REAL_VALUE_TYPE. But doing so is a substantial amount of work for each
target machine. See Section Cross Compilation and Floating Point Format
of Using and Porting GCC.
At present, the program 'mips-tfile' which adds debug support to object
files on MIPS systems does not work in a cross compile environment.
ΓòÉΓòÉΓòÉ 9.4. Interoperation ΓòÉΓòÉΓòÉ
This section lists various difficulties encountered in using GNU C or GNU C++
together with other compilers or with the assemblers, linkers, libraries and
debuggers on certain systems.
Objective C does not work on the RS/6000.
GNU C++ does not do name mangling in the same way as other C++ compilers.
This means that object files compiled with one compiler cannot be used
with another.
This effect is intentional, to protect you from more subtle problems.
Compilers differ as to many internal details of C++ implementation,
including: how class instances are laid out, how multiple inheritance is
implemented, and how virtual function calls are handled. If the name
encoding were made the same, your programs would link against libraries
provided from other compilers---but the programs would then crash when
run. Incompatible libraries are then detected at link time, rather than
at run time.
Older GDB versions sometimes fail to read the output of GNU CC version 2.
If you have trouble, get GDB version 4.4 or later.
DBX rejects some files produced by GNU CC, though it accepts similar
constructs in output from PCC. Until someone can supply a coherent
description of what is valid DBX input and what is not, there is nothing
I can do about these problems. You are on your own.
The GNU assembler (GAS) does not support PIC. To generate PIC code, you
must use some other assembler, such as '/bin/as'.
On some BSD systems, including some versions of Ultrix, use of profiling
causes static variable destructors (currently used only in C++) not to be
run.
Use of '-I/usr/include' may cause trouble.
Many systems come with header files that won't work with GNU CC unless
corrected by fixincludes. The corrected header files go in a new
directory; GNU CC searches this directory before '/usr/include'. If you
use '-I/usr/include', this tells GNU CC to search '/usr/include' earlier
on, before the corrected headers. The result is that you get the
uncorrected header files.
Instead, you should use these options (when compiling C programs):
-I/usr/local/lib/gcc-lib/target/version/include -I/usr/include
For C++ programs, GNU CC also uses a special directory that defines C++
interfaces to standard C subroutines. This directory is meant to be searched
before other standard include directories, so that it takes precedence. If
you are compiling C++ programs and specifying include directories explicitly,
use this option first, then the two options above:
-I/usr/local/lib/g++-include
On some SGI systems, when you use '-lgl_s' as an option, it gets
translated magically to '-lgl_s -lX11_s -lc_s'. Naturally, this does not
happen when you use GNU CC. You must specify all three options
explicitly.
On a Sparc, GNU CC aligns all values of type double on an 8-byte
boundary, and it expects every double to be so aligned. The Sun compiler
usually gives double values 8-byte alignment, with one exception:
function arguments of type double may not be aligned.
As a result, if a function compiled with Sun CC takes the address of an
argument of type double and passes this pointer of type double * to a
function compiled with GNU CC, dereferencing the pointer may cause a
fatal signal.
One way to solve this problem is to compile your entire program with GNU
CC. Another solution is to modify the function that is compiled with Sun
CC to copy the argument into a local variable; local variables are always
properly aligned. A third solution is to modify the function that uses
the pointer to dereference it via the following function access_double
instead of directly with '*':
inline double
access_double (double *unaligned_ptr)
{
union d2i { double d; int i[2]; };
union d2i *p = (union d2i *) unaligned_ptr;
union d2i u;
u.i[0] = p->i[0];
u.i[1] = p->i[1];
return u.d;
}
Storing into the pointer can be done likewise with the same union.
On Solaris, the malloc function in the 'libmalloc.a' library may allocate
memory that is only 4 byte aligned. Since GNU CC on the Sparc assumes
that doubles are 8 byte aligned, this may result in a fatal signal if
doubles are stored in memory allocated by the 'libmalloc.a' library.
The solution is to not use the 'libmalloc.a' library. Use instead malloc
and related functions from 'libc.a'; they do not have this problem.
Sun forgot to include a static version of 'libdl.a' with some versions of
SunOS (mainly 4.1). This results in undefined symbols when linking
static binaries (that is, if you use '-static'). If you see undefined
symbols _dlclose, _dlsym or _dlopen when linking, compile and link
against the file 'mit/util/misc/dlsym.c' from the MIT version of X
windows.
The 128-bit long double format that the Sparc port supports currently
works by using the architecturally defined quad-word floating point
instructions. Since there is no hardware that supports these
instructions they must be emulated by the operating system. Long doubles
do not work in Sun OS versions 4.0.3 and earlier, because the kernel
emulator uses an obsolete and incompatible format. Long doubles do not
work in Sun OS version 4.1.1 due to a problem in a Sun library. Long
doubles do work on Sun OS versions 4.1.2 and higher, but GNU CC does not
enable them by default. Long doubles appear to work in Sun OS 5.x
(Solaris 2.x).
On HP-UX version 9.01 on the HP PA, the HP compiler cc does not compile
GNU CC correctly. We do not yet know why. However, GNU CC compiled on
earlier HP-UX versions works properly on HP-UX 9.01 and can compile
itself properly on 9.01.
On the HP PA machine, ADB sometimes fails to work on functions compiled
with GNU CC. Specifically, it fails to work on functions that use alloca
or variable-size arrays. This is because GNU CC doesn't generate HP-UX
unwind descriptors for such functions. It may even be impossible to
generate them.
Debugging ('-g') is not supported on the HP PA machine, unless you use
the preliminary GNU tools (see Installation).
Taking the address of a label may generate errors from the HP-UX PA
assembler. GAS for the PA does not have this problem.
Using floating point parameters for indirect calls to static functions
will not work when using the HP assembler. There simply is no way for
GCC to specify what registers hold arguments for static functions when
using the HP assembler. GAS for the PA does not have this problem.
In extremely rare cases involving some very large functions you may
receive errors from the HP linker complaining about an out of bounds
unconditional branch offset. This used to occur more often in previous
versions of GNU CC, but is now exceptionally rare. If you should run
into it, you can work around by making your function smaller.
GNU CC compiled code sometimes emits warnings from the HP-UX assembler of
the form:
(warning) Use of GR3 when
frame >= 8192 may cause conflict.
These warnings are harmless and can be safely ignored.
The current version of the assembler ('/bin/as') for the RS/6000 has
certain problems that prevent the '-g' option in GCC from working. Note
that 'Makefile.in' uses '-g' by default when compiling 'libgcc2.c'.
IBM has produced a fixed version of the assembler. The upgraded
assembler unfortunately was not included in any of the AIX 3.2 update PTF
releases (3.2.2, 3.2.3, or 3.2.3e). Users of AIX 3.1 should request PTF
U403044 from IBM and users of AIX 3.2 should request PTF U416277. See the
file 'README.RS6000' for more details on these updates.
You can test for the presense of a fixed assembler by using the command
as -u < /dev/null
If the command exits normally, the assembler fix already is installed. If the
assembler complains that "-u" is an unknown flag, you need to order the fix.
On the IBM RS/6000, compiling code of the form
extern int foo;
┬╖┬╖┬╖ foo ┬╖┬╖┬╖
static int foo;
will cause the linker to report an undefined symbol foo. Although this
behavior differs from most other systems, it is not a bug because redefining
an extern variable as static is undefined in ANSI C.
AIX on the RS/6000 provides support (NLS) for environments outside of the
United States. Compilers and assemblers use NLS to support
locale-specific representations of various objects including
floating-point numbers ("." vs "," for separating decimal fractions).
There have been problems reported where the library linked with GCC does
not produce the same floating-point formats that the assembler accepts.
If you have this problem, set the LANG environment variable to "C" or
"En_US".
Even if you specify '-fdollars-in-identifiers', you cannot successfully
use '$' in identifiers on the RS/6000 due to a restriction in the IBM
assembler. GAS supports these identifiers.
On the RS/6000, XLC version 1.3.0.0 will miscompile 'jump.c'. XLC
version 1.3.0.1 or later fixes this problem. You can obtain XLC-1.3.0.2
by requesting PTF 421749 from IBM.
There is an assembler bug in versions of DG/UX prior to 5.4.2.01 that
occurs when the 'fldcr' instruction is used. GNU CC uses 'fldcr' on the
88100 to serialize volatile memory references. Use the option
'-mno-serialize-volatile' if your version of the assembler has this bug.
On VMS, GAS versions 1.38.1 and earlier may cause spurious warning
messages from the linker. These warning messages complain of mismatched
psect attributes. You can ignore them. See VMS Install.
On NewsOS version 3, if you include both of the files 'stddef.h' and
'sys/types.h', you get an error because there are two typedefs of size_t.
You should change 'sys/types.h' by adding these lines around the
definition of size_t:
#ifndef _SIZE_T
#define _SIZE_T
actual typedef here
#endif
On the Alliant, the system's own convention for returning structures and
unions is unusual, and is not compatible with GNU CC no matter what
options are used.
On the IBM RT PC, the MetaWare HighC compiler (hc) uses a different
convention for structure and union returning. Use the option
'-mhc-struct-return' to tell GNU CC to use a convention compatible with
it.
On Ultrix, the Fortran compiler expects registers 2 through 5 to be saved
by function calls. However, the C compiler uses conventions compatible
with BSD Unix: registers 2 through 5 may be clobbered by function calls.
GNU CC uses the same convention as the Ultrix C compiler. You can use
these options to produce code compatible with the Fortran compiler:
-fcall-saved-r2 -fcall-saved-r3 -fcall-saved-r4 -fcall-saved-r5
On the WE32k, you may find that programs compiled with GNU CC do not work
with the standard shared C library. You may need to link with the
ordinary C compiler. If you do so, you must specify the following
options:
-L/usr/local/lib/gcc-lib/we32k-att-sysv/2.8.1 -lgcc -lc_s
The first specifies where to find the library 'libgcc.a' specified with the
'-lgcc' option.
GNU CC does linking by invoking ld, just as cc does, and there is no reason
why it should matter which compilation program you use to invoke ld. If
someone tracks this problem down, it can probably be fixed easily.
On the Alpha, you may get assembler errors about invalid syntax as a
result of floating point constants. This is due to a bug in the C
library functions ecvt, fcvt and gcvt. Given valid floating point
numbers, they sometimes print 'NaN'.
On Irix 4.0.5F (and perhaps in some other versions), an assembler bug
sometimes reorders instructions incorrectly when optimization is turned
on. If you think this may be happening to you, try using the GNU
assembler; GAS version 2.1 supports ECOFF on Irix.
Or use the '-noasmopt' option when you compile GNU CC with itself, and
then again when you compile your program. (This is a temporary kludge to
turn off assembler optimization on Irix.) If this proves to be what you
need, edit the assembler spec in the file 'specs' so that it
unconditionally passes '-O0' to the assembler, and never passes '-O2' or
'-O3'.
ΓòÉΓòÉΓòÉ 9.5. Problems Compiling Certain Programs ΓòÉΓòÉΓòÉ
Certain programs have problems compiling.
Parse errors may occur compiling X11 on a Decstation running Ultrix 4.2
because of problems in DEC's versions of the X11 header files
'X11/Xlib.h' and 'X11/Xutil.h'. People recommend adding
'-I/usr/include/mit' to use the MIT versions of the header files, using
the '-traditional' switch to turn off ANSI C, or fixing the header files
by adding this:
#ifdef __STDC__
#define NeedFunctionPrototypes 0
#endif
If you have trouble compiling Perl on a SunOS 4 system, it may be because
Perl specifies '-I/usr/ucbinclude'. This accesses the unfixed header
files. Perl specifies the options
-traditional -Dvolatile=__volatile__
-I/usr/include/sun -I/usr/ucbinclude
-fpcc-struct-return
most of which are unnecessary with GCC 2.4.5 and newer versions. You can make
a properly working Perl by setting ccflags to '-fwritable-strings' (implied by
the '-traditional' in the original options) and cppflags to empty in
'config.sh', then typing './doSH; make depend; make'.
On various 386 Unix systems derived from System V, including SCO, ISC,
and ESIX, you may get error messages about running out of virtual memory
while compiling certain programs.
You can prevent this problem by linking GNU CC with the GNU malloc (which
thus replaces the malloc that comes with the system). GNU malloc is
available as a separate package, and also in the file 'src/gmalloc.c' in
the GNU Emacs 19 distribution.
If you have installed GNU malloc as a separate library package, use this
option when you relink GNU CC:
MALLOC=/usr/local/lib/libgmalloc.a
Alternatively, if you have compiled 'gmalloc.c' from Emacs 19, copy the object
file to 'gmalloc.o' and use this option when you relink GNU CC:
MALLOC=gmalloc.o
ΓòÉΓòÉΓòÉ 9.6. Incompatibilities of GNU CC ΓòÉΓòÉΓòÉ
There are several noteworthy incompatibilities between GNU C and most existing
(non-ANSI) versions of C. The '-traditional' option eliminates many of these
incompatibilities, but not all, by telling GNU C to behave like the other C
compilers.
GNU CC normally makes string constants read-only. If several
identical-looking string constants are used, GNU CC stores only one copy
of the string.
One consequence is that you cannot call mktemp with a string constant
argument. The function mktemp always alters the string its argument
points to.
Another consequence is that sscanf does not work on some systems when
passed a string constant as its format control string or input. This is
because sscanf incorrectly tries to write into the string constant.
Likewise fscanf and scanf.
The best solution to these problems is to change the program to use
char-array variables with initialization strings for these purposes
instead of string constants. But if this is not possible, you can use
the '-fwritable-strings' flag, which directs GNU CC to handle string
constants the same way most C compilers do. '-traditional' also has this
effect, among others.
-2147483648 is positive.
This is because 2147483648 cannot fit in the type int, so (following the
ANSI C rules) its data type is unsigned long int. Negating this value
yields 2147483648 again.
GNU CC does not substitute macro arguments when they appear inside of
string constants. For example, the following macro in GNU CC
#define foo(a) "a"
will produce output "a" regardless of what the argument a is.
The '-traditional' option directs GNU CC to handle such cases (among others)
in the old-fashioned (non-ANSI) fashion.
When you use setjmp and longjmp, the only automatic variables guaranteed
to remain valid are those declared volatile. This is a consequence of
automatic register allocation. Consider this function:
jmp_buf j;
foo ()
{
int a, b;
a = fun1 ();
if (setjmp (j))
return a;
a = fun2 ();
/* longjmp (j) may occur in fun3. */
return a + fun3 ();
}
Here a may or may not be restored to its first value when the longjmp occurs.
If a is allocated in a register, then its first value is restored; otherwise,
it keeps the last value stored in it.
If you use the '-W' option with the '-O' option, you will get a warning when
GNU CC thinks such a problem might be possible.
The '-traditional' option directs GNU C to put variables in the stack by
default, rather than in registers, in functions that call setjmp. This
results in the behavior found in traditional C compilers.
Programs that use preprocessing directives in the middle of macro
arguments do not work with GNU CC. For example, a program like this will
not work:
foobar (
#define luser
hack)
ANSI C does not permit such a construct. It would make sense to support it
when '-traditional' is used, but it is too much work to implement.
Declarations of external variables and functions within a block apply
only to the block containing the declaration. In other words, they have
the same scope as any other declaration in the same place.
In some other C compilers, a extern declaration affects all the rest of
the file even if it happens within a block.
The '-traditional' option directs GNU C to treat all extern declarations
as global, like traditional compilers.
In traditional C, you can combine long, etc., with a typedef name, as
shown here:
typedef int foo;
typedef long foo bar;
In ANSI C, this is not allowed: long and other type modifiers require an
explicit int. Because this criterion is expressed by Bison grammar rules
rather than C code, the '-traditional' flag cannot alter it.
PCC allows typedef names to be used as function parameters. The
difficulty described immediately above applies here too.
PCC allows whitespace in the middle of compound assignment operators such
as '+='. GNU CC, following the ANSI standard, does not allow this. The
difficulty described immediately above applies here too.
GNU CC complains about unterminated character constants inside of
preprocessing conditionals that fail. Some programs have English
comments enclosed in conditionals that are guaranteed to fail; if these
comments contain apostrophes, GNU CC will probably report an error. For
example, this code would produce an error:
#if 0
You can't expect this to work.
#endif
The best solution to such a problem is to put the text into an actual C
comment delimited by '/*┬╖┬╖┬╖*/'. However, '-traditional' suppresses these
error messages.
Many user programs contain the declaration 'long time ();'. In the past,
the system header files on many systems did not actually declare time, so
it did not matter what type your program declared it to return. But in
systems with ANSI C headers, time is declared to return time_t, and if
that is not the same as long, then 'long time ();' is erroneous.
The solution is to change your program to use time_t as the return type
of time.
When compiling functions that return float, PCC converts it to a double.
GNU CC actually returns a float. If you are concerned with PCC
compatibility, you should declare your functions to return double; you
might as well say what you mean.
When compiling functions that return structures or unions, GNU CC output
code normally uses a method different from that used on most versions of
Unix. As a result, code compiled with GNU CC cannot call a
structure-returning function compiled with PCC, and vice versa.
The method used by GNU CC is as follows: a structure or union which is 1,
2, 4 or 8 bytes long is returned like a scalar. A structure or union
with any other size is stored into an address supplied by the caller
(usually in a special, fixed register, but on some machines it is passed
on the stack). The machine-description macros STRUCT_VALUE and
STRUCT_INCOMING_VALUE tell GNU CC where to pass this address.
By contrast, PCC on most target machines returns structures and unions of
any size by copying the data into an area of static storage, and then
returning the address of that storage as if it were a pointer value. The
caller must copy the data from that memory area to the place where the
value is wanted. GNU CC does not use this method because it is slower
and nonreentrant.
On some newer machines, PCC uses a reentrant convention for all structure
and union returning. GNU CC on most of these machines uses a compatible
convention when returning structures and unions in memory, but still
returns small structures and unions in registers.
You can tell GNU CC to use a compatible convention for all structure and
union returning with the option '-fpcc-struct-return'.
GNU C complains about program fragments such as '0x74ae-0x4000' which
appear to be two hexadecimal constants separated by the minus operator.
Actually, this string is a single preprocessing token. Each such token
must correspond to one token in C. Since this does not, GNU C prints an
error message. Although it may appear obvious that what is meant is an
operator and two values, the ANSI C standard specifically requires that
this be treated as erroneous.
A preprocessing token is a preprocessing number if it begins with a digit
and is followed by letters, underscores, digits, periods and 'e+', 'e-',
'E+', or 'E-' character sequences.
To make the above program fragment valid, place whitespace in front of
the minus sign. This whitespace will end the preprocessing number.
ΓòÉΓòÉΓòÉ 9.7. Fixed Header Files ΓòÉΓòÉΓòÉ
GNU CC needs to install corrected versions of some system header files. This
is because most target systems have some header files that won't work with GNU
CC unless they are changed. Some have bugs, some are incompatible with ANSI
C, and some depend on special features of other compilers.
Installing GNU CC automatically creates and installs the fixed header files,
by running a program called fixincludes (or for certain targets an alternative
such as fixinc.svr4). Normally, you don't need to pay attention to this. But
there are cases where it doesn't do the right thing automatically.
If you update the system's header files, such as by installing a new
system version, the fixed header files of GNU CC are not automatically
updated. The easiest way to update them is to reinstall GNU CC. (If you
want to be clever, look in the makefile and you can find a shortcut.)
On some systems, in particular SunOS 4, header file directories contain
machine-specific symbolic links in certain places. This makes it
possible to share most of the header files among hosts running the same
version of SunOS 4 on different machine models.
The programs that fix the header files do not understand this special way
of using symbolic links; therefore, the directory of fixed header files
is good only for the machine model used to build it.
In SunOS 4, only programs that look inside the kernel will notice the
difference between machine models. Therefore, for most purposes, you
need not be concerned about this.
It is possible to make separate sets of fixed header files for the
different machine models, and arrange a structure of symbolic links so as
to use the proper set, but you'll have to do this by hand.
On Lynxos, GNU CC by default does not fix the header files. This is
because bugs in the shell cause the fixincludes script to fail.
This means you will encounter problems due to bugs in the system header
files. It may be no comfort that they aren't GNU CC's fault, but it does
mean that there's nothing for us to do about them.
ΓòÉΓòÉΓòÉ 9.8. Standard Libraries ΓòÉΓòÉΓòÉ
GNU CC by itself attempts to be what the ISO/ANSI C standard calls a
conforming freestanding implementation. This means all ANSI C language
features are available, as well as the contents of 'float.h', 'limits.h',
'stdarg.h', and 'stddef.h'. The rest of the C library is supplied by the
vendor of the operating system. If that C library doesn't conform to the C
standards, then your programs might get warnings (especially when using
'-Wall') that you don't expect.
For example, the sprintf function on SunOS 4.1.3 returns char * while the C
standard says that sprintf returns an int. The fixincludes program could make
the prototype for this function match the Standard, but that would be wrong,
since the function will still return char *.
If you need a Standard compliant library, then you need to find one, as GNU CC
does not provide one. The GNU C library (called glibc) has been ported to a
number of operating systems, and provides ANSI/ISO, POSIX, BSD and SystemV
compatibility. You could also ask your operating system vendor if newer
libraries are available.
ΓòÉΓòÉΓòÉ 9.9. Disappointments and Misunderstandings ΓòÉΓòÉΓòÉ
These problems are perhaps regrettable, but we don't know any practical way
around them.
Certain local variables aren't recognized by debuggers when you compile
with optimization.
This occurs because sometimes GNU CC optimizes the variable out of
existence. There is no way to tell the debugger how to compute the value
such a variable ``would have had'', and it is not clear that would be
desirable anyway. So GNU CC simply does not mention the eliminated
variable when it writes debugging information.
You have to expect a certain amount of disagreement between the
executable and your source code, when you use optimization.
Users often think it is a bug when GNU CC reports an error for code like
this:
int foo (struct mumble *);
struct mumble { ┬╖┬╖┬╖ };
int foo (struct mumble *x)
{ ┬╖┬╖┬╖ }
This code really is erroneous, because the scope of struct mumble in the
prototype is limited to the argument list containing it. It does not refer to
the struct mumble defined with file scope immediately below---they are two
unrelated types with similar names in different scopes.
But in the definition of foo, the file-scope type is used because that is
available to be inherited. Thus, the definition and the prototype do not
match, and you get an error.
This behavior may seem silly, but it's what the ANSI standard specifies. It is
easy enough for you to make your code work by moving the definition of struct
mumble above the prototype. It's not worth being incompatible with ANSI C
just to avoid an error for the example shown above.
Accesses to bitfields even in volatile objects works by accessing larger
objects, such as a byte or a word. You cannot rely on what size of
object is accessed in order to read or write the bitfield; it may even
vary for a given bitfield according to the precise usage.
If you care about controlling the amount of memory that is accessed, use
volatile but do not use bitfields.
GNU CC comes with shell scripts to fix certain known problems in system
header files. They install corrected copies of various header files in a
special directory where only GNU CC will normally look for them. The
scripts adapt to various systems by searching all the system header files
for the problem cases that we know about.
If new system header files are installed, nothing automatically arranges
to update the corrected header files. You will have to reinstall GNU CC
to fix the new header files. More specifically, go to the build
directory and delete the files 'stmp-fixinc' and 'stmp-headers', and the
subdirectory include; then do 'make install' again.
On 68000 and x86 systems, for instance, you can get paradoxical results
if you test the precise values of floating point numbers. For example,
you can find that a floating point value which is not a NaN is not equal
to itself. This results from the fact that the floating point registers
hold a few more bits of precision than fit in a double in memory.
Compiled code moves values between memory and floating point registers at
its convenience, and moving them into memory truncates them.
You can partially avoid this problem by using the '-ffloat-store' option
(see Optimize Options).
On the MIPS, variable argument functions using 'varargs.h' cannot have a
floating point value for the first argument. The reason for this is that
in the absence of a prototype in scope, if the first argument is a
floating point, it is passed in a floating point register, rather than an
integer register.
If the code is rewritten to use the ANSI standard 'stdarg.h' method of
variable arguments, and the prototype is in scope at the time of the
call, everything will work fine.
On the H8/300 and H8/300H, variable argument functions must be
implemented using the ANSI standard 'stdarg.h' method of variable
arguments. Furthermore, calls to functions using 'stdarg.h' variable
arguments must have a prototype for the called function in scope at the
time of the call.
ΓòÉΓòÉΓòÉ 9.10. Common Misunderstandings with GNU C++ ΓòÉΓòÉΓòÉ
C++ is a complex language and an evolving one, and its standard definition
(the ANSI C++ draft standard) is also evolving. As a result, your C++
compiler may occasionally surprise you, even when its behavior is correct.
This section discusses some areas that frequently give rise to questions of
this sort.
Static Definitions Static member declarations are not definitions
Temporaries Temporaries may vanish before you expect
ΓòÉΓòÉΓòÉ 9.10.1. Declare and Define Static Members ΓòÉΓòÉΓòÉ
When a class has static data members, it is not enough to declare the static
member; you must also define it. For example:
class Foo
{
┬╖┬╖┬╖
void method();
static int bar;
};
This declaration only establishes that the class Foo has an int named
Foo::bar, and a member function named Foo::method. But you still need to
define both method and bar elsewhere. According to the draft ANSI standard,
you must supply an initializer in one (and only one) source file, such as:
int Foo::bar = 0;
Other C++ compilers may not correctly implement the standard behavior. As a
result, when you switch to g++ from one of these compilers, you may discover
that a program that appeared to work correctly in fact does not conform to the
standard: g++ reports as undefined symbols any static data members that lack
definitions.
ΓòÉΓòÉΓòÉ 9.10.2. Temporaries May Vanish Before You Expect ΓòÉΓòÉΓòÉ
It is dangerous to use pointers or references to portions of a temporary
object. The compiler may very well delete the object before you expect it to,
leaving a pointer to garbage. The most common place where this problem crops
up is in classes like the libg++ String class, that define a conversion
function to type char * or const char *. However, any class that returns a
pointer to some internal structure is potentially subject to this problem.
For example, a program may use a function strfunc that returns String objects,
and another function charfunc that operates on pointers to char:
String strfunc ();
void charfunc (const char *);
In this situation, it may seem natural to write 'charfunc (strfunc ());' based
on the knowledge that class String has an explicit conversion to char
pointers. However, what really happens is akin to 'charfunc (strfunc
().convert ());', where the convert method is a function to do the same data
conversion normally performed by a cast. Since the last use of the temporary
String object is the call to the conversion function, the compiler may delete
that object before actually calling charfunc. The compiler has no way of
knowing that deleting the String object will invalidate the pointer. The
pointer then points to garbage, so that by the time charfunc is called, it
gets an invalid argument.
Code like this may run successfully under some other compilers, especially
those that delete temporaries relatively late. However, the GNU C++ behavior
is also standard-conforming, so if your program depends on late destruction of
temporaries it is not portable.
If you think this is surprising, you should be aware that the ANSI C++
committee continues to debate the lifetime-of-temporaries problem.
For now, at least, the safe way to write such code is to give the temporary a
name, which forces it to remain until the end of the scope of the name. For
example:
String& tmp = strfunc ();
charfunc (tmp);
ΓòÉΓòÉΓòÉ 9.11. Caveats of using protoize ΓòÉΓòÉΓòÉ
The conversion programs protoize and unprotoize can sometimes change a source
file in a way that won't work unless you rearrange it.
protoize can insert references to a type name or type tag before the
definition, or in a file where they are not defined.
If this happens, compiler error messages should show you where the new
references are, so fixing the file by hand is straightforward.
There are some C constructs which protoize cannot figure out. For
example, it can't determine argument types for declaring a
pointer-to-function variable; this you must do by hand. protoize inserts
a comment containing '???' each time it finds such a variable; so you can
find all such variables by searching for this string. ANSI C does not
require declaring the argument types of pointer-to-function types.
Using unprotoize can easily introduce bugs. If the program relied on
prototypes to bring about conversion of arguments, these conversions will
not take place in the program without prototypes. One case in which you
can be sure unprotoize is safe is when you are removing prototypes that
were made with protoize; if the program worked before without any
prototypes, it will work again without them.
You can find all the places where this problem might occur by compiling
the program with the '-Wconversion' option. It prints a warning whenever
an argument is converted.
Both conversion programs can be confused if there are macro calls in and
around the text to be converted. In other words, the standard syntax for
a declaration or definition must not result from expanding a macro. This
problem is inherent in the design of C and cannot be fixed. If only a
few functions have confusing macro calls, you can easily convert them
manually.
protoize cannot get the argument types for a function whose definition
was not actually compiled due to preprocessing conditionals. When this
happens, protoize changes nothing in regard to such a function. protoize
tries to detect such instances and warn about them.
You can generally work around this problem by using protoize step by
step, each time specifying a different set of '-D' options for
compilation, until all of the functions have been converted. There is no
automatic way to verify that you have got them all, however.
Confusion may result if there is an occasion to convert a function
declaration or definition in a region of source code where there is more
than one formal parameter list present. Thus, attempts to convert code
containing multiple (conditionally compiled) versions of a single
function header (in the same vicinity) may not produce the desired (or
expected) results.
If you plan on converting source files which contain such code, it is
recommended that you first make sure that each conditionally compiled
region of source code which contains an alternative function header also
contains at least one additional follower token (past the final right
parenthesis of the function header). This should circumvent the problem.
unprotoize can become confused when trying to convert a function
definition or declaration which contains a declaration for a
pointer-to-function formal argument which has the same name as the
function being defined or declared. We recommand you avoid such choices
of formal parameter names.
You might also want to correct some of the indentation by hand and break
long lines. (The conversion programs don't write lines longer than
eighty characters in any case.)
ΓòÉΓòÉΓòÉ 9.12. Certain Changes We Don't Want to Make ΓòÉΓòÉΓòÉ
This section lists changes that people frequently request, but which we do not
make because we think GNU CC is better without them.
Checking the number and type of arguments to a function which has an
old-fashioned definition and no prototype.
Such a feature would work only occasionally---only for calls that appear
in the same file as the called function, following the definition. The
only way to check all calls reliably is to add a prototype for the
function. But adding a prototype eliminates the motivation for this
feature. So the feature is not worthwhile.
Warning about using an expression whose type is signed as a shift count.
Shift count operands are probably signed more often than unsigned.
Warning about this would cause far more annoyance than good.
Warning about assigning a signed value to an unsigned variable.
Such assignments must be very common; warning about them would cause more
annoyance than good.
Warning about unreachable code.
It's very common to have unreachable code in machine-generated programs.
For example, this happens normally in some files of GNU C itself.
Warning when a non-void function value is ignored.
Coming as I do from a Lisp background, I balk at the idea that there is
something dangerous about discarding a value. There are functions that
return values which some callers may find useful; it makes no sense to
clutter the program with a cast to void whenever the value isn't useful.
Assuming (for optimization) that the address of an external symbol is
never zero.
This assumption is false on certain systems when '#pragma weak' is used.
Making '-fshort-enums' the default.
This would cause storage layout to be incompatible with most other C
compilers. And it doesn't seem very important, given that you can get
the same result in other ways. The case where it matters most is when
the enumeration-valued object is inside a structure, and in that case you
can specify a field width explicitly.
Making bitfields unsigned by default on particular machines where ``the
ABI standard'' says to do so.
The ANSI C standard leaves it up to the implementation whether a bitfield
declared plain int is signed or not. This in effect creates two
alternative dialects of C.
The GNU C compiler supports both dialects; you can specify the signed
dialect with '-fsigned-bitfields' and the unsigned dialect with
'-funsigned-bitfields'. However, this leaves open the question of which
dialect to use by default.
Currently, the preferred dialect makes plain bitfields signed, because
this is simplest. Since int is the same as signed int in every other
context, it is cleanest for them to be the same in bitfields as well.
Some computer manufacturers have published Application Binary Interface
standards which specify that plain bitfields should be unsigned. It is a
mistake, however, to say anything about this issue in an ABI. This is
because the handling of plain bitfields distinguishes two dialects of C.
Both dialects are meaningful on every type of machine. Whether a
particular object file was compiled using signed bitfields or unsigned is
of no concern to other object files, even if they access the same
bitfields in the same data structures.
A given program is written in one or the other of these two dialects. The
program stands a chance to work on most any machine if it is compiled
with the proper dialect. It is unlikely to work at all if compiled with
the wrong dialect.
Many users appreciate the GNU C compiler because it provides an
environment that is uniform across machines. These users would be
inconvenienced if the compiler treated plain bitfields differently on
certain machines.
Occasionally users write programs intended only for a particular machine
type. On these occasions, the users would benefit if the GNU C compiler
were to support by default the same dialect as the other compilers on
that machine. But such applications are rare. And users writing a
program to run on more than one type of machine cannot possibly benefit
from this kind of compatibility.
This is why GNU CC does and will treat plain bitfields in the same
fashion on all types of machines (by default).
There are some arguments for making bitfields unsigned by default on all
machines. If, for example, this becomes a universal de facto standard,
it would make sense for GNU CC to go along with it. This is something to
be considered in the future.
(Of course, users strongly concerned about portability should indicate
explicitly in each bitfield whether it is signed or not. In this way,
they write programs which have the same meaning in both C dialects.)
Undefining __STDC__ when '-ansi' is not used.
Currently, GNU CC defines __STDC__ as long as you don't use
'-traditional'. This provides good results in practice.
Programmers normally use conditionals on __STDC__ to ask whether it is
safe to use certain features of ANSI C, such as function prototypes or
ANSI token concatenation. Since plain 'gcc' supports all the features of
ANSI C, the correct answer to these questions is ``yes''.
Some users try to use __STDC__ to check for the availability of certain
library facilities. This is actually incorrect usage in an ANSI C
program, because the ANSI C standard says that a conforming freestanding
implementation should define __STDC__ even though it does not have the
library facilities. 'gcc -ansi -pedantic' is a conforming freestanding
implementation, and it is therefore required to define __STDC__, even
though it does not come with an ANSI C library.
Sometimes people say that defining __STDC__ in a compiler that does not
completely conform to the ANSI C standard somehow violates the standard.
This is illogical. The standard is a standard for compilers that claim
to support ANSI C, such as 'gcc -ansi'---not for other compilers such as
plain 'gcc'. Whatever the ANSI C standard says is relevant to the design
of plain 'gcc' without '-ansi' only for pragmatic reasons, not as a
requirement.
GNU CC normally defines __STDC__ to be 1, and in addition defines
__STRICT_ANSI__ if you specify the '-ansi' option. On some hosts, system
include files use a different convention, where __STDC__ is normally 0,
but is 1 if the user specifies strict conformance to the C Standard. GNU
CC follows the host convention when processing system include files, but
when processing user files it follows the usual GNU C convention.
Undefining __STDC__ in C++.
Programs written to compile with C++-to-C translators get the value of
__STDC__ that goes with the C compiler that is subsequently used. These
programs must test __STDC__ to determine what kind of C preprocessor that
compiler uses: whether they should concatenate tokens in the ANSI C
fashion or in the traditional fashion.
These programs work properly with GNU C++ if __STDC__ is defined. They
would not work otherwise.
In addition, many header files are written to provide prototypes in ANSI
C but not in traditional C. Many of these header files can work without
change in C++ provided __STDC__ is defined. If __STDC__ is not defined,
they will all fail, and will all need to be changed to test explicitly
for C++ as well.
Deleting ``empty'' loops.
GNU CC does not delete ``empty'' loops because the most likely reason you
would put one in a program is to have a delay. Deleting them will not
make real programs run any faster, so it would be pointless.
It would be different if optimization of a nonempty loop could produce an
empty one. But this generally can't happen.
Making side effects happen in the same order as in some other compiler.
It is never safe to depend on the order of evaluation of side effects.
For example, a function call like this may very well behave differently
from one compiler to another:
void func (int, int);
int i = 2;
func (i++, i++);
There is no guarantee (in either the C or the C++ standard language
definitions) that the increments will be evaluated in any particular order.
Either increment might happen first. func might get the arguments '2, 3', or
it might get '3, 2', or even '2, 2'.
Not allowing structures with volatile fields in registers.
Strictly speaking, there is no prohibition in the ANSI C standard against
allowing structures with volatile fields in registers, but it does not
seem to make any sense and is probably not what you wanted to do. So the
compiler will give an error message in this case.
ΓòÉΓòÉΓòÉ 9.13. Warning Messages and Error Messages ΓòÉΓòÉΓòÉ
The GNU compiler can produce two kinds of diagnostics: errors and warnings.
Each kind has a different purpose:
Errors report problems that make it impossible to compile your program.
GNU CC reports errors with the source file name and line number where the
problem is apparent.
Warnings report other unusual conditions in your code that may indicate a
problem, although compilation can (and does) proceed. Warning messages
also report the source file name and line number, but include the text
'warning:' to distinguish them from error messages.
Warnings may indicate danger points where you should check to make sure that
your program really does what you intend; or the use of obsolete features; or
the use of nonstandard features of GNU C or C++. Many warnings are issued
only if you ask for them, with one of the '-W' options (for instance, '-Wall'
requests a variety of useful warnings).
GNU CC always tries to compile your program if possible; it never gratuitously
rejects a program whose meaning is clear merely because (for instance) it
fails to conform to a standard. In some cases, however, the C and C++
standards specify that certain extensions are forbidden, and a diagnostic must
be issued by a conforming compiler. The '-pedantic' option tells GNU CC to
issue warnings in such cases; '-pedantic-errors' says to make them errors
instead. This does not mean that all non-ANSI constructs get warnings or
errors.
See Options to Request or Suppress Warnings, for more detail on these and
related command-line options.
ΓòÉΓòÉΓòÉ 10. Reporting Bugs ΓòÉΓòÉΓòÉ
Your bug reports play an essential role in making GNU CC reliable.
When you encounter a problem, the first thing to do is to see if it is already
known. See Trouble. If it isn't known, then you should report the problem.
Reporting a bug may help you by bringing a solution to your problem, or it may
not. (If it does not, look in the service directory; see Service.) In any
case, the principal function of a bug report is to help the entire community
by making the next version of GNU CC work better. Bug reports are your
contribution to the maintenance of GNU CC.
Since the maintainers are very overloaded, we cannot respond to every bug
report. However, if the bug has not been fixed, we are likely to send you a
patch and ask you to tell us whether it works.
In order for a bug report to serve its purpose, you must include the
information that makes for fixing the bug.
Bug Criteria Have you really found a bug?
Bug Lists Where to send your bug report.
Bug Reporting How to report a bug effectively.
Sending Patches How to send a patch for GNU CC.
Trouble Known problems.
Service Where to ask for help.
ΓòÉΓòÉΓòÉ 10.1. Have You Found a Bug? ΓòÉΓòÉΓòÉ
If you are not sure whether you have found a bug, here are some guidelines:
If the compiler gets a fatal signal, for any input whatever, that is a
compiler bug. Reliable compilers never crash.
If the compiler produces invalid assembly code, for any input whatever
(except an asm statement), that is a compiler bug, unless the compiler
reports errors (not just warnings) which would ordinarily prevent the
assembler from being run.
If the compiler produces valid assembly code that does not correctly
execute the input source code, that is a compiler bug.
However, you must double-check to make sure, because you may have run
into an incompatibility between GNU C and traditional C (see
Incompatibilities). These incompatibilities might be considered bugs,
but they are inescapable consequences of valuable features.
Or you may have a program whose behavior is undefined, which happened by
chance to give the desired results with another C or C++ compiler.
For example, in many nonoptimizing compilers, you can write 'x;' at the
end of a function instead of 'return x;', with the same results. But the
value of the function is undefined if return is omitted; it is not a bug
when GNU CC produces different results.
Problems often result from expressions with two increment operators, as
in f (*p++, *p++). Your previous compiler might have interpreted that
expression the way you intended; GNU CC might interpret it another way.
Neither compiler is wrong. The bug is in your code.
After you have localized the error to a single source line, it should be
easy to check for these things. If your program is correct and well
defined, you have found a compiler bug.
If the compiler produces an error message for valid input, that is a
compiler bug.
If the compiler does not produce an error message for invalid input, that
is a compiler bug. However, you should note that your idea of ``invalid
input'' might be my idea of ``an extension'' or ``support for traditional
practice''.
If you are an experienced user of C or C++ compilers, your suggestions
for improvement of GNU CC or GNU C++ are welcome in any case.
ΓòÉΓòÉΓòÉ 10.2. Where to Report Bugs ΓòÉΓòÉΓòÉ
Send bug reports for GNU C to 'bug-gcc@prep.ai.mit.edu'.
Send bug reports for GNU C++ to 'bug-g++@prep.ai.mit.edu'. If your bug
involves the C++ class library libg++, send mail instead to the address
'bug-lib-g++@prep.ai.mit.edu'. If you're not sure, you can send the bug
report to both lists.
Do not send bug reports to 'help-gcc@prep.ai.mit.edu' or to the newsgroup
'gnu.gcc.help'. Most users of GNU CC do not want to receive bug reports.
Those that do, have asked to be on 'bug-gcc' and/or 'bug-g++'.
The mailing lists 'bug-gcc' and 'bug-g++' both have newsgroups which serve as
repeaters: 'gnu.gcc.bug' and 'gnu.g++.bug'. Each mailing list and its
newsgroup carry exactly the same messages.
Often people think of posting bug reports to the newsgroup instead of mailing
them. This appears to work, but it has one problem which can be crucial: a
newsgroup posting does not contain a mail path back to the sender. Thus, if
maintainers need more information, they may be unable to reach you. For this
reason, you should always send bug reports by mail to the proper mailing list.
As a last resort, send bug reports on paper to:
GNU Compiler Bugs
Free Software Foundation
59 Temple Place - Suite 330
Boston, MA 02111-1307, USA
ΓòÉΓòÉΓòÉ 10.3. How to Report Bugs ΓòÉΓòÉΓòÉ
The fundamental principle of reporting bugs usefully is this: report all the
facts. If you are not sure whether to state a fact or leave it out, state it!
Often people omit facts because they think they know what causes the problem
and they conclude that some details don't matter. Thus, you might assume that
the name of the variable you use in an example does not matter. Well, probably
it doesn't, but one cannot be sure. Perhaps the bug is a stray memory
reference which happens to fetch from the location where that name is stored
in memory; perhaps, if the name were different, the contents of that location
would fool the compiler into doing the right thing despite the bug. Play it
safe and give a specific, complete example. That is the easiest thing for you
to do, and the most helpful.
Keep in mind that the purpose of a bug report is to enable someone to fix the
bug if it is not known. It isn't very important what happens if the bug is
already known. Therefore, always write your bug reports on the assumption
that the bug is not known.
Sometimes people give a few sketchy facts and ask, ``Does this ring a bell?''
This cannot help us fix a bug, so it is basically useless. We respond by
asking for enough details to enable us to investigate. You might as well
expedite matters by sending them to begin with.
Try to make your bug report self-contained. If we have to ask you for more
information, it is best if you include all the previous information in your
response, as well as the information that was missing.
Please report each bug in a separate message. This makes it easier for us to
track which bugs have been fixed and to forward your bugs reports to the
appropriate maintainer.
Do not compress and encode any part of your bug report using programs such as
'uuencode'. If you do so it will slow down the processing of your bug. If
you must submit multiple large files, use 'shar', which allows us to read your
message without having to run any decompression programs.
To enable someone to investigate the bug, you should include all these things:
The version of GNU CC. You can get this by running it with the '-v'
option.
Without this, we won't know whether there is any point in looking for the
bug in the current version of GNU CC.
A complete input file that will reproduce the bug. If the bug is in the
C preprocessor, send a source file and any header files that it requires.
If the bug is in the compiler proper ('cc1'), run your source file
through the C preprocessor by doing 'gcc -E sourcefile > outfile', then
include the contents of outfile in the bug report. (When you do this,
use the same '-I', '-D' or '-U' options that you used in actual
compilation.)
A single statement is not enough of an example. In order to compile it,
it must be embedded in a complete file of compiler input; and the bug
might depend on the details of how this is done.
Without a real example one can compile, all anyone can do about your bug
report is wish you luck. It would be futile to try to guess how to
provoke the bug. For example, bugs in register allocation and reloading
frequently depend on every little detail of the function they happen in.
Even if the input file that fails comes from a GNU program, you should
still send the complete test case. Don't ask the GNU CC maintainers to
do the extra work of obtaining the program in question---they are all
overworked as it is. Also, the problem may depend on what is in the
header files on your system; it is unreliable for the GNU CC maintainers
to try the problem with the header files available to them. By sending
CPP output, you can eliminate this source of uncertainty and save us a
certain percentage of wild goose chases.
The command arguments you gave GNU CC or GNU C++ to compile that example
and observe the bug. For example, did you use '-O'? To guarantee you
won't omit something important, list all the options.
If we were to try to guess the arguments, we would probably guess wrong
and then we would not encounter the bug.
The type of machine you are using, and the operating system name and
version number.
The operands you gave to the configure command when you installed the
compiler.
A complete list of any modifications you have made to the compiler
source. (We don't promise to investigate the bug unless it happens in an
unmodified compiler. But if you've made modifications and don't tell us,
then you are sending us on a wild goose chase.)
Be precise about these changes. A description in English is not
enough---send a context diff for them.
Adding files of your own (such as a machine description for a machine we
don't support) is a modification of the compiler source.
Details of any other deviations from the standard procedure for
installing GNU CC.
A description of what behavior you observe that you believe is incorrect.
For example, ``The compiler gets a fatal signal,'' or, ``The assembler
instruction at line 208 in the output is incorrect.''
Of course, if the bug is that the compiler gets a fatal signal, then one
can't miss it. But if the bug is incorrect output, the maintainer might
not notice unless it is glaringly wrong. None of us has time to study
all the assembler code from a 50-line C program just on the chance that
one instruction might be wrong. We need you to do this part!
Even if the problem you experience is a fatal signal, you should still
say so explicitly. Suppose something strange is going on, such as, your
copy of the compiler is out of synch, or you have encountered a bug in
the C library on your system. (This has happened!) Your copy might
crash and the copy here would not. If you said to expect a crash, then
when the compiler here fails to crash, we would know that the bug was not
happening. If you don't say to expect a crash, then we would not know
whether the bug was happening. We would not be able to draw any
conclusion from our observations.
If the problem is a diagnostic when compiling GNU CC with some other
compiler, say whether it is a warning or an error.
Often the observed symptom is incorrect output when your program is run.
Sad to say, this is not enough information unless the program is short
and simple. None of us has time to study a large program to figure out
how it would work if compiled correctly, much less which line of it was
compiled wrong. So you will have to do that. Tell us which source line
it is, and what incorrect result happens when that line is executed. A
person who understands the program can find this as easily as finding a
bug in the program itself.
If you send examples of assembler code output from GNU CC or GNU C++,
please use '-g' when you make them. The debugging information includes
source line numbers which are essential for correlating the output with
the input.
If you wish to mention something in the GNU CC source, refer to it by
context, not by line number.
The line numbers in the development sources don't match those in your
sources. Your line numbers would convey no useful information to the
maintainers.
Additional information from a debugger might enable someone to find a
problem on a machine which he does not have available. However, you need
to think when you collect this information if you want it to have any
chance of being useful.
For example, many people send just a backtrace, but that is never useful
by itself. A simple backtrace with arguments conveys little about GNU CC
because the compiler is largely data-driven; the same functions are
called over and over for different RTL insns, doing different things
depending on the details of the insn.
Most of the arguments listed in the backtrace are useless because they
are pointers to RTL list structure. The numeric values of the pointers,
which the debugger prints in the backtrace, have no significance
whatever; all that matters is the contents of the objects they point to
(and most of the contents are other such pointers).
In addition, most compiler passes consist of one or more loops that scan
the RTL insn sequence. The most vital piece of information about such a
loop---which insn it has reached---is usually in a local variable, not in
an argument.
What you need to provide in addition to a backtrace are the values of the
local variables for several stack frames up. When a local variable or an
argument is an RTX, first print its value and then use the GDB command pr
to print the RTL expression that it points to. (If GDB doesn't run on
your machine, use your debugger to call the function debug_rtx with the
RTX as an argument.) In general, whenever a variable is a pointer, its
value is no use without the data it points to.
Here are some things that are not necessary:
A description of the envelope of the bug.
Often people who encounter a bug spend a lot of time investigating which
changes to the input file will make the bug go away and which changes
will not affect it.
This is often time consuming and not very useful, because the way we will
find the bug is by running a single example under the debugger with
breakpoints, not by pure deduction from a series of examples. You might
as well save your time for something else.
Of course, if you can find a simpler example to report instead of the
original one, that is a convenience. Errors in the output will be easier
to spot, running under the debugger will take less time, etc. Most GNU CC
bugs involve just one function, so the most straightforward way to
simplify an example is to delete all the function definitions except the
one where the bug occurs. Those earlier in the file may be replaced by
external declarations if the crucial function depends on them.
(Exception: inline functions may affect compilation of functions defined
later in the file.)
However, simplification is not vital; if you don't want to do this,
report the bug anyway and send the entire test case you used.
In particular, some people insert conditionals '#ifdef BUG' around a
statement which, if removed, makes the bug not happen. These are just
clutter; we won't pay any attention to them anyway. Besides, you should
send us cpp output, and that can't have conditionals.
A patch for the bug.
A patch for the bug is useful if it is a good one. But don't omit the
necessary information, such as the test case, on the assumption that a
patch is all we need. We might see problems with your patch and decide
to fix the problem another way, or we might not understand it at all.
Sometimes with a program as complicated as GNU CC it is very hard to
construct an example that will make the program follow a certain path
through the code. If you don't send the example, we won't be able to
construct one, so we won't be able to verify that the bug is fixed.
And if we can't understand what bug you are trying to fix, or why your
patch should be an improvement, we won't install it. A test case will
help us to understand.
See Sending Patches, for guidelines on how to make it easy for us to
understand and install your patches.
A guess about what the bug is or what it depends on.
Such guesses are usually wrong. Even I can't guess right about such
things without first using the debugger to find the facts.
A core dump file.
We have no way of examining a core dump for your type of machine unless
we have an identical system---and if we do have one, we should be able to
reproduce the crash ourselves.
ΓòÉΓòÉΓòÉ 10.4. Sending Patches for GNU CC ΓòÉΓòÉΓòÉ
If you would like to write bug fixes or improvements for the GNU C compiler,
that is very helpful. Send suggested fixes to the bug report mailing list,
bug-gcc@prep.ai.mit.edu.
Please follow these guidelines so we can study your patches efficiently. If
you don't follow these guidelines, your information might still be useful, but
using it will take extra work. Maintaining GNU C is a lot of work in the best
of circumstances, and we can't keep up unless you do your best to help.
Send an explanation with your changes of what problem they fix or what
improvement they bring about. For a bug fix, just include a copy of the
bug report, and explain why the change fixes the bug.
(Referring to a bug report is not as good as including it, because then
we will have to look it up, and we have probably already deleted it if
we've already fixed the bug.)
Always include a proper bug report for the problem you think you have
fixed. We need to convince ourselves that the change is right before
installing it. Even if it is right, we might have trouble judging it if
we don't have a way to reproduce the problem.
Include all the comments that are appropriate to help people reading the
source in the future understand why this change was needed.
Don't mix together changes made for different reasons. Send them
individually.
If you make two changes for separate reasons, then we might not want to
install them both. We might want to install just one. If you send them
all jumbled together in a single set of diffs, we have to do extra work
to disentangle them---to figure out which parts of the change serve which
purpose. If we don't have time for this, we might have to ignore your
changes entirely.
If you send each change as soon as you have written it, with its own
explanation, then the two changes never get tangled up, and we can
consider each one properly without any extra work to disentangle them.
Ideally, each change you send should be impossible to subdivide into
parts that we might want to consider separately, because each of its
parts gets its motivation from the other parts.
Send each change as soon as that change is finished. Sometimes people
think they are helping us by accumulating many changes to send them all
together. As explained above, this is absolutely the worst thing you
could do.
Since you should send each change separately, you might as well send it
right away. That gives us the option of installing it immediately if it
is important.
Use 'diff -c' to make your diffs. Diffs without context are hard for us
to install reliably. More than that, they make it hard for us to study
the diffs to decide whether we want to install them. Unidiff format is
better than contextless diffs, but not as easy to read as '-c' format.
If you have GNU diff, use 'diff -cp', which shows the name of the
function that each change occurs in.
Write the change log entries for your changes. We get lots of changes,
and we don't have time to do all the change log writing ourselves.
Read the 'ChangeLog' file to see what sorts of information to put in, and
to learn the style that we use. The purpose of the change log is to show
people where to find what was changed. So you need to be specific about
what functions you changed; in large functions, it's often helpful to
indicate where within the function the change was.
On the other hand, once you have shown people where to find the change,
you need not explain its purpose. Thus, if you add a new function, all
you need to say about it is that it is new. If you feel that the purpose
needs explaining, it probably does---but the explanation will be much
more useful if you put it in comments in the code.
If you would like your name to appear in the header line for who made the
change, send us the header line.
When you write the fix, keep in mind that we can't install a change that
would break other systems.
People often suggest fixing a problem by changing machine-independent
files such as 'toplev.c' to do something special that a particular system
needs. Sometimes it is totally obvious that such changes would break GNU
CC for almost all users. We can't possibly make a change like that. At
best it might tell us how to write another patch that would solve the
problem acceptably.
Sometimes people send fixes that might be an improvement in general---but
it is hard to be sure of this. It's hard to install such changes because
we have to study them very carefully. Of course, a good explanation of
the reasoning by which you concluded the change was correct can help
convince us.
The safest changes are changes to the configuration files for a
particular machine. These are safe because they can't create new bugs on
other machines.
Please help us keep up with the workload by designing the patch in a form
that is good to install.
ΓòÉΓòÉΓòÉ 11. How To Get Help with GNU CC ΓòÉΓòÉΓòÉ
If you need help installing, using or changing GNU CC, there are two ways to
find it:
Send a message to a suitable network mailing list. First try
bug-gcc@prep.ai.mit.edu, and if that brings no response, try
help-gcc@prep.ai.mit.edu.
Look in the service directory for someone who might help you for a fee.
The service directory is found in the file named 'SERVICE' in the GNU CC
distribution.
ΓòÉΓòÉΓòÉ 12. Contributing to GNU CC Development ΓòÉΓòÉΓòÉ
If you would like to help pretest GNU CC releases to assure they work well, or
if you would like to work on improving GNU CC, please contact the maintainers
at bug-gcc@gnu.ai.mit.edu. A pretester should be willing to try to
investigate bugs as well as report them.
If you'd like to work on improvements, please ask for suggested projects or
suggest your own ideas. If you have already written an improvement, please
tell us about it. If you have not yet started work, it is useful to contact
bug-gcc@prep.ai.mit.edu before you start; the maintainers may be able to
suggest ways to make your extension fit in better with the rest of GNU CC and
with other development plans.
ΓòÉΓòÉΓòÉ 13. Using GNU CC on VMS ΓòÉΓòÉΓòÉ
Here is how to use GNU CC on VMS.
Include Files and VMS Where the preprocessor looks for the include
files.
Global Declarations How to do globaldef, globalref and globalvalue
with GNU CC.
VMS Misc Misc information.
ΓòÉΓòÉΓòÉ 13.1. Include Files and VMS ΓòÉΓòÉΓòÉ
Due to the differences between the filesystems of Unix and VMS, GNU CC
attempts to translate file names in '#include' into names that VMS will
understand. The basic strategy is to prepend a prefix to the specification of
the include file, convert the whole filename to a VMS filename, and then try
to open the file. GNU CC tries various prefixes one by one until one of them
succeeds:
1. The first prefix is the 'GNU_CC_INCLUDE:' logical name: this is where GNU
C header files are traditionally stored. If you wish to store header
files in non-standard locations, then you can assign the logical
'GNU_CC_INCLUDE' to be a search list, where each element of the list is
suitable for use with a rooted logical.
2. The next prefix tried is 'SYS$SYSROOT:[SYSLIB.]'. This is where VAX-C
header files are traditionally stored.
3. If the include file specification by itself is a valid VMS filename, the
preprocessor then uses this name with no prefix in an attempt to open the
include file.
4. If the file specification is not a valid VMS filename (i.e. does not
contain a device or a directory specifier, and contains a '/' character),
the preprocessor tries to convert it from Unix syntax to VMS syntax.
Conversion works like this: the first directory name becomes a device,
and the rest of the directories are converted into VMS-format directory
names. For example, the name 'X11/foobar.h' is translated to
'X11:[000000]foobar.h' or 'X11:foobar.h', whichever one can be opened.
This strategy allows you to assign a logical name to point to the actual
location of the header files.
5. If none of these strategies succeeds, the '#include' fails.
Include directives of the form:
#include foobar
are a common source of incompatibility between VAX-C and GNU CC. VAX-C treats
this much like a standard #include <foobar.h> directive. That is incompatible
with the ANSI C behavior implemented by GNU CC: to expand the name foobar as a
macro. Macro expansion should eventually yield one of the two standard
formats for #include:
#include "file"
#include <file>
If you have this problem, the best solution is to modify the source to convert
the #include directives to one of the two standard forms. That will work with
either compiler. If you want a quick and dirty fix, define the file names as
macros with the proper expansion, like this:
#define stdio <stdio.h>
This will work, as long as the name doesn't conflict with anything else in the
program.
Another source of incompatibility is that VAX-C assumes that:
#include "foobar"
is actually asking for the file 'foobar.h'. GNU CC does not make this
assumption, and instead takes what you ask for literally; it tries to read the
file 'foobar'. The best way to avoid this problem is to always specify the
desired file extension in your include directives.
GNU CC for VMS is distributed with a set of include files that is sufficient
to compile most general purpose programs. Even though the GNU CC distribution
does not contain header files to define constants and structures for some VMS
system-specific functions, there is no reason why you cannot use GNU CC with
any of these functions. You first may have to generate or create header
files, either by using the public domain utility UNSDL (which can be found on
a DECUS tape), or by extracting the relevant modules from one of the system
macro libraries, and using an editor to construct a C header file.
A #include file name cannot contain a DECNET node name. The preprocessor
reports an I/O error if you attempt to use a node name, whether explicitly, or
implicitly via a logical name.
ΓòÉΓòÉΓòÉ 13.2. Global Declarations and VMS ΓòÉΓòÉΓòÉ
GNU CC does not provide the globalref, globaldef and globalvalue keywords of
VAX-C. You can get the same effect with an obscure feature of GAS, the GNU
assembler. (This requires GAS version 1.39 or later.) The following macros
allow you to use this feature in a fairly natural way:
#ifdef __GNUC__
#define GLOBALREF(TYPE,NAME) \
TYPE NAME \
asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME)
#define GLOBALDEF(TYPE,NAME,VALUE) \
TYPE NAME \
asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME) \
= VALUE
#define GLOBALVALUEREF(TYPE,NAME) \
const TYPE NAME[1] \
asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME)
#define GLOBALVALUEDEF(TYPE,NAME,VALUE) \
const TYPE NAME[1] \
asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME) \
= {VALUE}
#else
#define GLOBALREF(TYPE,NAME) \
globalref TYPE NAME
#define GLOBALDEF(TYPE,NAME,VALUE) \
globaldef TYPE NAME = VALUE
#define GLOBALVALUEDEF(TYPE,NAME,VALUE) \
globalvalue TYPE NAME = VALUE
#define GLOBALVALUEREF(TYPE,NAME) \
globalvalue TYPE NAME
#endif
(The _$$PsectAttributes_GLOBALSYMBOL prefix at the start of the name is
removed by the assembler, after it has modified the attributes of the symbol).
These macros are provided in the VMS binaries distribution in a header file
'GNU_HACKS.H'. An example of the usage is:
GLOBALREF (int, ijk);
GLOBALDEF (int, jkl, 0);
The macros GLOBALREF and GLOBALDEF cannot be used straightforwardly for
arrays, since there is no way to insert the array dimension into the
declaration at the right place. However, you can declare an array with these
macros if you first define a typedef for the array type, like this:
typedef int intvector[10];
GLOBALREF (intvector, foo);
Array and structure initializers will also break the macros; you can define
the initializer to be a macro of its own, or you can expand the GLOBALDEF
macro by hand. You may find a case where you wish to use the GLOBALDEF macro
with a large array, but you are not interested in explicitly initializing each
element of the array. In such cases you can use an initializer like: {0,},
which will initialize the entire array to 0.
A shortcoming of this implementation is that a variable declared with
GLOBALVALUEREF or GLOBALVALUEDEF is always an array. For example, the
declaration:
GLOBALVALUEREF(int, ijk);
declares the variable ijk as an array of type int [1]. This is done because a
globalvalue is actually a constant; its ``value'' is what the linker would
normally consider an address. That is not how an integer value works in C,
but it is how an array works. So treating the symbol as an array name gives
consistent results---with the exception that the value seems to have the wrong
type. Don't try to access an element of the array. It doesn't have any
elements. The array ``address'' may not be the address of actual storage.
The fact that the symbol is an array may lead to warnings where the variable
is used. Insert type casts to avoid the warnings. Here is an example; it
takes advantage of the ANSI C feature allowing macros that expand to use the
same name as the macro itself.
GLOBALVALUEREF (int, ss$_normal);
GLOBALVALUEDEF (int, xyzzy,123);
#ifdef __GNUC__
#define ss$_normal ((int) ss$_normal)
#define xyzzy ((int) xyzzy)
#endif
Don't use globaldef or globalref with a variable whose type is an enumeration
type; this is not implemented. Instead, make the variable an integer, and use
a globalvaluedef for each of the enumeration values. An example of this would
be:
#ifdef __GNUC__
GLOBALDEF (int, color, 0);
GLOBALVALUEDEF (int, RED, 0);
GLOBALVALUEDEF (int, BLUE, 1);
GLOBALVALUEDEF (int, GREEN, 3);
#else
enum globaldef color {RED, BLUE, GREEN = 3};
#endif
ΓòÉΓòÉΓòÉ 13.3. Other VMS Issues ΓòÉΓòÉΓòÉ
GNU CC automatically arranges for main to return 1 by default if you fail to
specify an explicit return value. This will be interpreted by VMS as a status
code indicating a normal successful completion. Version 1 of GNU CC did not
provide this default.
GNU CC on VMS works only with the GNU assembler, GAS. You need version 1.37
or later of GAS in order to produce value debugging information for the VMS
debugger. Use the ordinary VMS linker with the object files produced by GAS.
Under previous versions of GNU CC, the generated code would occasionally give
strange results when linked to the sharable 'VAXCRTL' library. Now this should
work.
A caveat for use of const global variables: the const modifier must be
specified in every external declaration of the variable in all of the source
files that use that variable. Otherwise the linker will issue warnings about
conflicting attributes for the variable. Your program will still work despite
the warnings, but the variable will be placed in writable storage.
Although the VMS linker does distinguish between upper and lower case letters
in global symbols, most VMS compilers convert all such symbols into upper case
and most run-time library routines also have upper case names. To be able to
reliably call such routines, GNU CC (by means of the assembler GAS) converts
global symbols into upper case like other VMS compilers. However, since the
usual practice in C is to distinguish case, GNU CC (via GAS) tries to preserve
usual C behavior by augmenting each name that is not all lower case. This
means truncating the name to at most 23 characters and then adding more
characters at the end which encode the case pattern of those 23. Names which
contain at least one dollar sign are an exception; they are converted directly
into upper case without augmentation.
Name augmentation yields bad results for programs that use precompiled
libraries (such as Xlib) which were generated by another compiler. You can
use the compiler option '/NOCASE_HACK' to inhibit augmentation; it makes
external C functions and variables case-independent as is usual on VMS.
Alternatively, you could write all references to the functions and variables
in such libraries using lower case; this will work on VMS, but is not portable
to other systems. The compiler option '/NAMES' also provides control over
global name handling.
Function and variable names are handled somewhat differently with GNU C++.
The GNU C++ compiler performs name mangling on function names, which means
that it adds information to the function name to describe the data types of
the arguments that the function takes. One result of this is that the name of
a function can become very long. Since the VMS linker only recognizes the
first 31 characters in a name, special action is taken to ensure that each
function and variable has a unique name that can be represented in 31
characters.
If the name (plus a name augmentation, if required) is less than 32 characters
in length, then no special action is performed. If the name is longer than 31
characters, the assembler (GAS) will generate a hash string based upon the
function name, truncate the function name to 23 characters, and append the
hash string to the truncated name. If the '/VERBOSE' compiler option is used,
the assembler will print both the full and truncated names of each symbol that
is truncated.
The '/NOCASE_HACK' compiler option should not be used when you are compiling
programs that use libg++. libg++ has several instances of objects (i.e.
Filebuf and filebuf) which become indistinguishable in a case-insensitive
environment. This leads to cases where you need to inhibit augmentation
selectively (if you were using libg++ and Xlib in the same program, for
example). There is no special feature for doing this, but you can get the
result by defining a macro for each mixed case symbol for which you wish to
inhibit augmentation. The macro should expand into the lower case equivalent
of itself. For example:
#define StuDlyCapS studlycaps
These macro definitions can be placed in a header file to minimize the number
of changes to your source code.
Here is a list of all the passes of the compiler and their source files. Also
included is a description of where debugging dumps can be requested with '-d'
options.
Parsing. This pass reads the entire text of a function definition,
constructing partial syntax trees. This and RTL generation are no longer
truly separate passes (formerly they were), but it is easier to think of
them as separate.
The tree representation does not entirely follow C syntax, because it is
intended to support other languages as well.
Language-specific data type analysis is also done in this pass, and every
tree node that represents an expression has a data type attached.
Variables are represented as declaration nodes.
Constant folding and some arithmetic simplifications are also done during
this pass.
The language-independent source files for parsing are 'stor-layout.c',
'fold-const.c', and 'tree.c'. There are also header files 'tree.h' and
'tree.def' which define the format of the tree representation.
The source files to parse C are 'c-parse.in', 'c-decl.c', 'c-typeck.c',
'c-aux-info.c', 'c-convert.c', and 'c-lang.c' along with header files
'c-lex.h', and 'c-tree.h'.
The source files for parsing C++ are 'cp-parse.y', 'cp-class.c',
'cp-cvt.c', 'cp-decl.c', 'cp-decl2.c', 'cp-dem.c', 'cp-except.c',
'cp-expr.c', 'cp-init.c', 'cp-lex.c', 'cp-method.c', 'cp-ptree.c',
'cp-search.c', 'cp-tree.c', 'cp-type2.c', and 'cp-typeck.c', along with
header files 'cp-tree.def', 'cp-tree.h', and 'cp-decl.h'.
The special source files for parsing Objective C are 'objc-parse.y',
'objc-actions.c', 'objc-tree.def', and 'objc-actions.h'. Certain
C-specific files are used for this as well.
The file 'c-common.c' is also used for all of the above languages.
RTL generation. This is the conversion of syntax tree into RTL code. It
is actually done statement-by-statement during parsing, but for most
purposes it can be thought of as a separate pass.
This is where the bulk of target-parameter-dependent code is found, since
often it is necessary for strategies to apply only when certain standard
kinds of instructions are available. The purpose of named instruction
patterns is to provide this information to the RTL generation pass.
Optimization is done in this pass for if-conditions that are comparisons,
boolean operations or conditional expressions. Tail recursion is
detected at this time also. Decisions are made about how best to arrange
loops and how to output switch statements.
The source files for RTL generation include 'stmt.c', 'calls.c',
'expr.c', 'explow.c', 'expmed.c', 'function.c', 'optabs.c' and
'emit-rtl.c'. Also, the file 'insn-emit.c', generated from the machine
description by the program genemit, is used in this pass. The header
file 'expr.h' is used for communication within this pass.
The header files 'insn-flags.h' and 'insn-codes.h', generated from the
machine description by the programs genflags and gencodes, tell this pass
which standard names are available for use and which patterns correspond
to them.
Aside from debugging information output, none of the following passes
refers to the tree structure representation of the function (only part of
which is saved).
The decision of whether the function can and should be expanded inline in
its subsequent callers is made at the end of rtl generation. The
function must meet certain criteria, currently related to the size of the
function and the types and number of parameters it has. Note that this
function may contain loops, recursive calls to itself (tail-recursive
functions can be inlined!), gotos, in short, all constructs supported by
GNU CC. The file 'integrate.c' contains the code to save a function's
rtl for later inlining and to inline that rtl when the function is
called. The header file 'integrate.h' is also used for this purpose.
The option '-dr' causes a debugging dump of the RTL code after this pass.
This dump file's name is made by appending '.rtl' to the input file name.
Jump optimization. This pass simplifies jumps to the following
instruction, jumps across jumps, and jumps to jumps. It deletes
unreferenced labels and unreachable code, except that unreachable code
that contains a loop is not recognized as unreachable in this pass. (Such
loops are deleted later in the basic block analysis.) It also converts
some code originally written with jumps into sequences of instructions
that directly set values from the results of comparisons, if the machine
has such instructions.
Jump optimization is performed two or three times. The first time is
immediately following RTL generation. The second time is after CSE, but
only if CSE says repeated jump optimization is needed. The last time is
right before the final pass. That time, cross-jumping and deletion of
no-op move instructions are done together with the optimizations
described above.
The source file of this pass is 'jump.c'.
The option '-dj' causes a debugging dump of the RTL code after this pass
is run for the first time. This dump file's name is made by appending
'.jump' to the input file name.
Register scan. This pass finds the first and last use of each register,
as a guide for common subexpression elimination. Its source is in
'regclass.c'.
Jump threading. This pass detects a condition jump that branches to an
identical or inverse test. Such jumps can be 'threaded' through the
second conditional test. The source code for this pass is in 'jump.c'.
This optimization is only performed if '-fthread-jumps' is enabled.
Common subexpression elimination. This pass also does constant
propagation. Its source file is 'cse.c'. If constant propagation causes
conditional jumps to become unconditional or to become no-ops, jump
optimization is run again when CSE is finished.
The option '-ds' causes a debugging dump of the RTL code after this pass.
This dump file's name is made by appending '.cse' to the input file name.
Loop optimization. This pass moves constant expressions out of loops,
and optionally does strength-reduction and loop unrolling as well. Its
source files are 'loop.c' and 'unroll.c', plus the header 'loop.h' used
for communication between them. Loop unrolling uses some functions in
'integrate.c' and the header 'integrate.h'.
The option '-dL' causes a debugging dump of the RTL code after this pass.
This dump file's name is made by appending '.loop' to the input file
name.
If '-frerun-cse-after-loop' was enabled, a second common subexpression
elimination pass is performed after the loop optimization pass. Jump
threading is also done again at this time if it was specified.
The option '-dt' causes a debugging dump of the RTL code after this pass.
This dump file's name is made by appending '.cse2' to the input file
name.
Stupid register allocation is performed at this point in a nonoptimizing
compilation. It does a little data flow analysis as well. When stupid
register allocation is in use, the next pass executed is the reloading
pass; the others in between are skipped. The source file is 'stupid.c'.
Data flow analysis ('flow.c'). This pass divides the program into basic
blocks (and in the process deletes unreachable loops); then it computes
which pseudo-registers are live at each point in the program, and makes
the first instruction that uses a value point at the instruction that
computed the value.
This pass also deletes computations whose results are never used, and
combines memory references with add or subtract instructions to make
autoincrement or autodecrement addressing.
The option '-df' causes a debugging dump of the RTL code after this pass.
This dump file's name is made by appending '.flow' to the input file
name. If stupid register allocation is in use, this dump file reflects
the full results of such allocation.
Instruction combination ('combine.c'). This pass attempts to combine
groups of two or three instructions that are related by data flow into
single instructions. It combines the RTL expressions for the
instructions by substitution, simplifies the result using algebra, and
then attempts to match the result against the machine description.
The option '-dc' causes a debugging dump of the RTL code after this pass.
This dump file's name is made by appending '.combine' to the input file
name.
Instruction scheduling ('sched.c'). This pass looks for instructions
whose output will not be available by the time that it is used in
subsequent instructions. (Memory loads and floating point instructions
often have this behavior on RISC machines). It re-orders instructions
within a basic block to try to separate the definition and use of items
that otherwise would cause pipeline stalls.
Instruction scheduling is performed twice. The first time is immediately
after instruction combination and the second is immediately after reload.
The option '-dS' causes a debugging dump of the RTL code after this pass
is run for the first time. The dump file's name is made by appending
'.sched' to the input file name.
Register class preferencing. The RTL code is scanned to find out which
register class is best for each pseudo register. The source file is
'regclass.c'.
Local register allocation ('local-alloc.c'). This pass allocates hard
registers to pseudo registers that are used only within one basic block.
Because the basic block is linear, it can use fast and powerful
techniques to do a very good job.
The option '-dl' causes a debugging dump of the RTL code after this pass.
This dump file's name is made by appending '.lreg' to the input file
name.
Global register allocation ('global.c'). This pass allocates hard
registers for the remaining pseudo registers (those whose life spans are
not contained in one basic block).
Reloading. This pass renumbers pseudo registers with the hardware
registers numbers they were allocated. Pseudo registers that did not get
hard registers are replaced with stack slots. Then it finds instructions
that are invalid because a value has failed to end up in a register, or
has ended up in a register of the wrong kind. It fixes up these
instructions by reloading the problematical values temporarily into
registers. Additional instructions are generated to do the copying.
The reload pass also optionally eliminates the frame pointer and inserts
instructions to save and restore call-clobbered registers around calls.
Source files are 'reload.c' and 'reload1.c', plus the header 'reload.h'
used for communication between them.
The option '-dg' causes a debugging dump of the RTL code after this pass.
This dump file's name is made by appending '.greg' to the input file
name.
Instruction scheduling is repeated here to try to avoid pipeline stalls
due to memory loads generated for spilled pseudo registers.
The option '-dR' causes a debugging dump of the RTL code after this pass.
This dump file's name is made by appending '.sched2' to the input file
name.
Jump optimization is repeated, this time including cross-jumping and
deletion of no-op move instructions.
The option '-dJ' causes a debugging dump of the RTL code after this pass.
This dump file's name is made by appending '.jump2' to the input file
name.
Delayed branch scheduling. This optional pass attempts to find
instructions that can go into the delay slots of other instructions,
usually jumps and calls. The source file name is 'reorg.c'.
The option '-dd' causes a debugging dump of the RTL code after this pass.
This dump file's name is made by appending '.dbr' to the input file name.
Conversion from usage of some hard registers to usage of a register stack
may be done at this point. Currently, this is supported only for the
floating-point registers of the Intel 80387 coprocessor. The source file
name is 'reg-stack.c'.
The options '-dk' causes a debugging dump of the RTL code after this
pass. This dump file's name is made by appending '.stack' to the input
file name.
Final. This pass outputs the assembler code for the function. It is
also responsible for identifying spurious test and compare instructions.
Machine-specific peephole optimizations are performed at the same time.
The function entry and exit sequences are generated directly as assembler
code in this pass; they never exist as RTL.
The source files are 'final.c' plus 'insn-output.c'; the latter is
generated automatically from the machine description by the tool
'genoutput'. The header file 'conditions.h' is used for communication
between these files.
Debugging information output. This is run after final because it must
output the stack slot offsets for pseudo registers that did not get hard
registers. Source files are 'dbxout.c' for DBX symbol table format, '
sdbout.c' for SDB symbol table format, and 'dwarfout.c' for DWARF symbol
table format.
Some additional files are used by all or many passes:
Every pass uses 'machmode.def' and 'machmode.h' which define the machine
modes.
Several passes use 'real.h', which defines the default representation of
floating point constants and how to operate on them.
All the passes that work with RTL use the header files 'rtl.h' and
'rtl.def', and subroutines in file 'rtl.c'. The tools gen* also use
these files to read and work with the machine description RTL.
Several passes refer to the header file 'insn-config.h' which contains a
few parameters (C macro definitions) generated automatically from the
machine description RTL by the tool genconfig.
Several passes use the instruction recognizer, which consists of
'recog.c' and 'recog.h', plus the files 'insn-recog.c' and
'insn-extract.c' that are generated automatically from the machine
description by the tools 'genrecog' and 'genextract'.
Several passes use the header files 'regs.h' which defines the
information recorded about pseudo register usage, and 'basic-block.h'
which defines the information recorded about basic blocks.
'hard-reg-set.h' defines the type HARD_REG_SET, a bit-vector with a bit
for each hard register, and some macros to manipulate it. This type is
just int if the machine has few enough hard registers; otherwise it is an
array of int and some of the macros expand into loops.
Several passes use instruction attributes. A definition of the
attributes defined for a particular machine is in file 'insn-attr.h',
which is generated from the machine description by the program 'genattr'.
The file 'insn-attrtab.c' contains subroutines to obtain the attribute
values for insns. It is generated from the machine description by the
program 'genattrtab'.
ΓòÉΓòÉΓòÉ 14. Index ΓòÉΓòÉΓòÉ
#pragma implementation, implied Declarations and Definitions
in One Header
#pragma, reason for not using Declaring Attributes of
Functions
$ Dollar Signs in Identifier
Names
'!' in constraint Multiple Alternative
Constraints
'#' in constraint Constraint Modifier
Characters
'%' in constraint Constraint Modifier
Characters
'&' in constraint Constraint Modifier
Characters
'+' in constraint Constraint Modifier
Characters
'0' in constraint Simple Constraints
'<' in constraint Simple Constraints
'=' in constraint Constraint Modifier
Characters
'>' in constraint Simple Constraints
'?' in constraint Multiple Alternative
Constraints
'd' in constraint Simple Constraints
'E' in constraint Simple Constraints
'F' in constraint Simple Constraints
'G' in constraint Simple Constraints
Simple Constraints
'H' in constraint Simple Constraints
'i' in constraint Simple Constraints
Simple Constraints
'm' in constraint Simple Constraints
'n' in constraint Simple Constraints
'o' in constraint Simple Constraints
'p' in constraint Simple Constraints
'Q', in constraint Simple Constraints
'r' in constraint Simple Constraints
's' in constraint Simple Constraints
'stdarg.h' and RT PC IBM RT Options
'V' in constraint Simple Constraints
'varargs.h' and RT PC IBM RT Options
'VAXCRTL' Other VMS Issues
'X' in constraint Simple Constraints
'_' in variables in macros Naming an Expression's Type
┬╖sdata/.sdata2 references (PowerPC) IBM RS/6000 and PowerPC
Options
' Incompatibilities of GNU CC
-lgcc, use with -nodefaultlibs Options for Linking
-lgcc, use with -nostdlib Options for Linking
-nodefaultlibs and unresolved references Options for Linking
-nostdlib and unresolved references Options for Linking
// C++ Style Comments
?: extensions Generalized Lvalues
Conditionals with Omitted
Operands
?: side effect Conditionals with Omitted
Operands
address constraints Simple Constraints
address of a label Labels as Values
alias attribute Declaring Attributes of
Functions
aligned attribute Specifying Attributes of
Variables
Specifying Attributes of
Types
alignment Inquiring on Alignment of
Types or Variables
Alliant Interoperation
alloca and SunOS Installing GNU CC
alloca vs variable-length arrays Arrays of Variable Length
alloca, for SunOS Installing GNU CC on the Sun
alloca, for Unos Configurations Supported by
GNU CC
alternate keywords Alternate Keywords
AMD29K options AMD29K Options
analysis, data flow Other VMS Issues
ANSI support Options Controlling C
Dialect
apostrophes Incompatibilities of GNU CC
arguments in frame (88k) M88K Options
arithmetic simplifications Other VMS Issues
ARM options ARM Options
arrays of length zero Arrays of Length Zero
arrays of variable length Arrays of Variable Length
arrays, non-lvalue Non-Lvalue Arrays May Have
Subscripts
asm constraints Constraints for asm Operands
asm expressions Assembler Instructions with
C Expression Operands
assembler instructions Assembler Instructions with
C Expression Operands
assembler names for identifiers Controlling Names Used in
Assembler Code
assembler syntax, 88k M88K Options
assembly code, invalid Have You Found a Bug?
attribute of types Specifying Attributes of
Types
attribute of variables Specifying Attributes of
Variables
autoincrement/decrement addressing Simple Constraints
autoincrement/decrement analysis Other VMS Issues
automatic inline for C++ member fns An Inline Function is As
Fast As a Macro
backtrace for bug reports How to Report Bugs
basic blocks Other VMS Issues
Bison parser generator Installing GNU CC
bit shift overflow (88k) M88K Options
bug criteria Have You Found a Bug?
bug report mailing lists Where to Report Bugs
bugs Reporting Bugs
bugs, known Known Causes of Trouble with
GNU CC
builtin functions Options Controlling C
Dialect
byte writes (29k) AMD29K Options
C compilation options GNU CC Command Options
C intermediate output, nonexistent Compile C, C++, or Objective
C
C language extensions Extensions to the C Language
Family
C language, traditional Options Controlling C
Dialect
C++ Compile C, C++, or Objective
C
C++ comments C++ Style Comments
C++ compilation options GNU CC Command Options
C++ interface and implementation headers Declarations and Definitions
in One Header
C++ language extensions Extensions to the C++
Language
C++ member fns, automatically inline An Inline Function is As
Fast As a Macro
C++ misunderstandings Common Misunderstandings
with GNU C++
C++ named return value Named Return Values in C++
C++ options, command line Options Controlling C++
Dialect
C++ pragmas, effect on inlining Declarations and Definitions
in One Header
C++ runtime library Installing GNU CC
C++ signatures Type Abstraction using
Signatures
C++ source file suffixes Compiling C++ Programs
C++ static data, declaring and defining Declare and Define Static
Members
C++ subtype polymorphism Type Abstraction using
Signatures
C++ type abstraction Type Abstraction using
Signatures
calling functions through the function vector on the H8/300 processors
Declaring Attributes of
Functions
case labels in initializers Labeled Elements in
Initializers
case ranges Case Ranges
case sensitivity and VMS Other VMS Issues
cast to a union Cast to a Union Type
casts as lvalues Generalized Lvalues
code generation conventions Options for Code Generation
Conventions
code motion Other VMS Issues
command options GNU CC Command Options
comments, C++ style C++ Style Comments
common subexpression elimination Other VMS Issues
comparison of signed and unsigned values, warning Options to Request or
Suppress Warnings
compilation in a separate directory Compilation in a Separate
Directory
compiler bugs, reporting How to Report Bugs
compiler compared to C++ preprocessor Compile C, C++, or Objective
C
compiler options, C++ Options Controlling C++
Dialect
compiler version, specifying Specifying Target Machine
and Compiler Version
complex numbers Complex Numbers
compound expressions as lvalues Generalized Lvalues
computed gotos Labels as Values
conditional expressions as lvalues Generalized Lvalues
conditional expressions, extensions Conditionals with Omitted
Operands
configurations supported by GNU CC Configurations Supported by
GNU CC
conflicting types Disappointments and
Misunderstandings
const applied to function Declaring Attributes of
Functions
const function attribute Declaring Attributes of
Functions
constant folding Other VMS Issues
constant propagation Other VMS Issues
constants in constraints Simple Constraints
constraint modifier characters Constraint Modifier
Characters
constraint, matching Simple Constraints
constraints, asm Constraints for asm Operands
constraints, machine specific Constraints for Particular
Machines
constructing calls Constructing Function Calls
constructor expressions Constructor Expressions
constructor function attribute Declaring Attributes of
Functions
constructors vs goto goto and Destructors in GNU
C++
constructors, automatic calls collect2
Convex options Convex Options
core dump Have You Found a Bug?
cross compiling Specifying Target Machine
and Compiler Version
cross-compiler, installation Building and Installing a
Cross-Compiler
cross-jumping Other VMS Issues
data flow analysis Other VMS Issues
DBX Interoperation
dead code Other VMS Issues
deallocating variable length arrays Arrays of Variable Length
debugging information generation Other VMS Issues
debugging information options Options for Debugging Your
Program or GNU CC
debugging, 88k OCS M88K Options
declaration scope Incompatibilities of GNU CC
declarations inside expressions Statements and Declarations
in Expressions
declaring attributes of functions Declaring Attributes of
Functions
declaring static data in C++ Declare and Define Static
Members
default implementation, signature member function Type Abstraction using
Signatures
defining static data in C++ Declare and Define Static
Members
delayed branch scheduling Other VMS Issues
dependencies for make as output Environment Variables
Affecting GNU CC
dependencies, make Options Controlling the
Preprocessor
destructor function attribute Declaring Attributes of
Functions
destructors vs goto goto and Destructors in GNU
C++
detecting '-traditional' Options Controlling C
Dialect
dialect options Options Controlling C
Dialect
digits in constraint Simple Constraints
directory options Options for Directory Search
divide instruction, 88k M88K Options
dollar signs in identifier names Dollar Signs in Identifier
Names
double-word arithmetic Double-Word Integers
downward funargs Nested Functions
DW bit (29k) AMD29K Options
earlyclobber operand Constraint Modifier
Characters
eight bit data on the H8/300 and H8/300H Declaring Attributes of
Functions
environment variables Environment Variables
Affecting GNU CC
error messages Warning Messages and Error
Messages
escape sequences, traditional Options Controlling C
Dialect
exclamation point Multiple Alternative
Constraints
exit status and VMS Other VMS Issues
explicit register variables Variables in Specified
Registers
expressions containing statements Statements and Declarations
in Expressions
expressions, compound, as lvalues Generalized Lvalues
expressions, conditional, as lvalues Generalized Lvalues
expressions, constructor Constructor Expressions
extended asm Assembler Instructions with
C Expression Operands
extensible constraints Simple Constraints
extensions, ?: Generalized Lvalues
Conditionals with Omitted
Operands
extensions, C language Extensions to the C Language
Family
extensions, C++ language Extensions to the C++
Language
external declaration scope Incompatibilities of GNU CC
fatal signal Have You Found a Bug?
file name suffix Options Controlling the Kind
of Output
file names Options for Linking
final pass Other VMS Issues
float as function value type Incompatibilities of GNU CC
floating point precision Options That Control
Optimization
Disappointments and
Misunderstandings
format function attribute Declaring Attributes of
Functions
format_arg function attribute Declaring Attributes of
Functions
forwarding calls Constructing Function Calls
fscanf, and constant strings Incompatibilities of GNU CC
function addressability on the M32R/D Declaring Attributes of
Functions
function attributes Declaring Attributes of
Functions
function pointers, arithmetic Arithmetic on void- and
Function-Pointers
function prototype declarations Prototypes and Old-Style
Function Definitions
function, size of pointer to Arithmetic on void- and
Function-Pointers
functions called via pointer on the RS/6000 and PowerPC M32R/D Options
Declaring Attributes of
Functions
Declaring Attributes of
Functions
functions in arbitrary sections Declaring Attributes of
Functions
functions that are passed arguments in registers on the 386 Declaring
Attributes of Functions
Declaring Attributes of
Functions
functions that do not pop the argument stack on the 386 Declaring Attributes
of Functions
functions that do pop the argument stack on the 386 Declaring Attributes of
Functions
functions that have no side effects Declaring Attributes of
Functions
functions that never return Declaring Attributes of
Functions
functions that pop the argument stack on the 386 Declaring Attributes of
Functions
Declaring Attributes of
Functions
functions which are exported from a dll on PowerPC Windows NT Declaring
Attributes of Functions
functions which are imported from a dll on PowerPC Windows NT Declaring
Attributes of Functions
functions which specify exception handling on PowerPC Windows NT Declaring
Attributes of Functions
functions with printf or scanf style arguments Declaring Attributes of
Functions
G++ Compile C, C++, or Objective
C
g++ 1.xx Compiling C++ Programs
g++ older version Compiling C++ Programs
g++, separate compiler Compiling C++ Programs
GCC Compile C, C++, or Objective
C
generalized lvalues Generalized Lvalues
genflags, crash on Sun 4 Installation Problems
global offset table Options for Code Generation
Conventions
global register after longjmp Defining Global Register
Variables
global register allocation Other VMS Issues
global register variables Defining Global Register
Variables
GNU CC command options GNU CC Command Options
goto in C++ goto and Destructors in GNU
C++
goto with computed label Labels as Values
gp-relative references (MIPS) MIPS Options
gprof Options for Debugging Your
Program or GNU CC
grouping options GNU CC Command Options
hardware models and configurations, specifying Hardware Models and
Configurations
header files and VMS Include Files and VMS
hosted environment Options Controlling C
Dialect
Options Controlling C
Dialect
HPPA Options HPPA Options
i386 Options Intel 386 Options
IBM RS/6000 and PowerPC Options IBM RS/6000 and PowerPC
Options
IBM RT options IBM RT Options
IBM RT PC Interoperation
identifier names, dollar signs in Dollar Signs in Identifier
Names
identifiers, names in assembler code Controlling Names Used in
Assembler Code
identifying source, compiler (88k) M88K Options
implicit argument: return value Named Return Values in C++
implied #pragma implementation Declarations and Definitions
in One Header
include files and VMS Include Files and VMS
incompatibilities of GNU CC Incompatibilities of GNU CC
increment operators Have You Found a Bug?
initializations in expressions Constructor Expressions
initializers with labeled elements Labeled Elements in
Initializers
initializers, non-constant Non-Constant Initializers
inline automatic for C++ member fns An Inline Function is As
Fast As a Macro
inline functions An Inline Function is As
Fast As a Macro
inline functions, omission of An Inline Function is As
Fast As a Macro
inline, automatic Other VMS Issues
inlining and C++ pragmas Declarations and Definitions
in One Header
installation trouble Known Causes of Trouble with
GNU CC
installing GNU CC Installing GNU CC
installing GNU CC on the Sun Installing GNU CC on the Sun
installing GNU CC on VMS Installing GNU CC on VMS
instruction combination Other VMS Issues
instruction recognizer Other VMS Issues
instruction scheduling Other VMS Issues
Other VMS Issues
integrating function code An Inline Function is As
Fast As a Macro
Intel 386 Options Intel 386 Options
interface and implementation headers, C++ Declarations and Definitions
in One Header
intermediate C version, nonexistent Compile C, C++, or Objective
C
interrupt handler functions on the H8/300 processors Declaring Attributes of
Functions
interrupt handlers on the M32R/D Declaring Attributes of
Functions
introduction Introduction
invalid assembly code Have You Found a Bug?
invalid input Have You Found a Bug?
invoking g++ Compiling C++ Programs
jump optimization Other VMS Issues
jump threading Other VMS Issues
kernel and user registers (29k) AMD29K Options
keywords, alternate Alternate Keywords
known causes of trouble Known Causes of Trouble with
GNU CC
labeled elements in initializers Labeled Elements in
Initializers
labels as values Labels as Values
language dialect options Options Controlling C
Dialect
large bit shifts (88k) M88K Options
length-zero arrays Arrays of Length Zero
Libraries Options for Linking
libstdc++ Installing GNU CC
link options Options for Linking
load address instruction Simple Constraints
local labels Locally Declared Labels
local register allocation Other VMS Issues
local variables in macros Naming an Expression's Type
local variables, specifying registers Specifying Registers for
Local Variables
long long data types Double-Word Integers
longjmp and automatic variables Options Controlling C
Dialect
longjmp incompatibilities Incompatibilities of GNU CC
longjmp warnings Options to Request or
Suppress Warnings
loop optimization Other VMS Issues
lvalues, generalized Generalized Lvalues
M32R/D options M32R/D Options
M680x0 options M680x0 Options
M88k options M88K Options
machine dependent options Hardware Models and
Configurations
machine specific constraints Constraints for Particular
Machines
macro with variable arguments Macros with Variable Numbers
of Arguments
macros containing asm Assembler Instructions with
C Expression Operands
macros, inline alternative An Inline Function is As
Fast As a Macro
macros, local labels Locally Declared Labels
macros, local variables in Naming an Expression's Type
macros, statements in expressions Statements and Declarations
in Expressions
macros, types of arguments Referring to a Type with
typeof
main and the exit status Other VMS Issues
make Options Controlling the
Preprocessor
matching constraint Simple Constraints
maximum operator Minimum and Maximum
Operators in C++
member fns, automatically inline An Inline Function is As
Fast As a Macro
memory model (29k) AMD29K Options
memory references in constraints Simple Constraints
messages, warning Options to Request or
Suppress Warnings
messages, warning and error Warning Messages and Error
Messages
middle-operands, omitted Conditionals with Omitted
Operands
minimum operator Minimum and Maximum
Operators in C++
MIPS options MIPS Options
misunderstandings in C++ Common Misunderstandings
with GNU C++
mktemp, and constant strings Incompatibilities of GNU CC
MN10300 options MN10300 Options
mode attribute Specifying Attributes of
Variables
modifiers in constraints Constraint Modifier
Characters
multiple alternative constraints Multiple Alternative
Constraints
multiprecision arithmetic Double-Word Integers
name augmentation Other VMS Issues
named return value in C++ Named Return Values in C++
names used in assembler code Controlling Names Used in
Assembler Code
naming convention, implementation headers Declarations and Definitions
in One Header
naming types Naming an Expression's Type
nested functions Nested Functions
newline vs string constants Options Controlling C
Dialect
no-op move instructions Other VMS Issues
nocommon attribute Specifying Attributes of
Variables
non-constant initializers Non-Constant Initializers
non-static inline function An Inline Function is As
Fast As a Macro
noreturn function attribute Declaring Attributes of
Functions
Objective C Compile C, C++, or Objective
C
Objective C threads Installing GNU CC
OCS (88k) M88K Options
offsettable address Simple Constraints
old-style function definitions Prototypes and Old-Style
Function Definitions
omitted middle-operands Conditionals with Omitted
Operands
open coding An Inline Function is As
Fast As a Macro
operand constraints, asm Constraints for asm Operands
optimize options Options That Control
Optimization
options to control warnings Options to Request or
Suppress Warnings
options, C++ Options Controlling C++
Dialect
options, code generation Options for Code Generation
Conventions
options, debugging Options for Debugging Your
Program or GNU CC
options, dialect Options Controlling C
Dialect
options, directory search Options for Directory Search
options, GNU CC command GNU CC Command Options
options, grouping GNU CC Command Options
options, linking Options for Linking
options, optimization Options That Control
Optimization
options, order GNU CC Command Options
options, preprocessor Options Controlling the
Preprocessor
order of evaluation, side effects Certain Changes We Don't
Want to Make
order of options GNU CC Command Options
other directory, compilation in Compilation in a Separate
Directory
output file option Options Controlling the Kind
of Output
overloaded virtual fn, warning Options to Request or
Suppress Warnings
packed attribute Specifying Attributes of
Variables
parameter forward declaration Arrays of Variable Length
parser generator, Bison Installing GNU CC
peephole optimization Other VMS Issues
PIC Options for Code Generation
Conventions
pointer arguments Declaring Attributes of
Functions
portions of temporary objects, pointers to Temporaries May Vanish
Before You Expect
pragma, reason for not using Declaring Attributes of
Functions
pragmas in C++, effect on inlining Declarations and Definitions
in One Header
pragmas, interface and implementation Declarations and Definitions
in One Header
preprocessing numbers Incompatibilities of GNU CC
preprocessing tokens Incompatibilities of GNU CC
preprocessor options Options Controlling the
Preprocessor
processor selection (29k) AMD29K Options
prof Options for Debugging Your
Program or GNU CC
promotion of formal parameters Prototypes and Old-Style
Function Definitions
push address instruction Simple Constraints
qsort, and global register variables Defining Global Register
Variables
question mark Multiple Alternative
Constraints
r0-relative references (88k) M88K Options
ranges in case statements Case Ranges
read-only strings Incompatibilities of GNU CC
register allocation Other VMS Issues
register allocation, stupid Other VMS Issues
register class preference pass Other VMS Issues
register positions in frame (88k) M88K Options
M88K Options
register use analysis Other VMS Issues
register variable after longjmp Defining Global Register
Variables
register-to-stack conversion Other VMS Issues
registers Assembler Instructions with
C Expression Operands
registers for local variables Specifying Registers for
Local Variables
registers in constraints Simple Constraints
registers, global allocation Variables in Specified
Registers
registers, global variables in Defining Global Register
Variables
reloading Other VMS Issues
reordering, warning Options to Request or
Suppress Warnings
reporting bugs Reporting Bugs
rest argument (in macro) Macros with Variable Numbers
of Arguments
return value of main Other VMS Issues
return value, named, in C++ Named Return Values in C++
return, in C++ function header Named Return Values in C++
RS/6000 and PowerPC Options IBM RS/6000 and PowerPC
Options
RT options IBM RT Options
RT PC Interoperation
RTL generation Other VMS Issues
run-time options Options for Code Generation
Conventions
scanf, and constant strings Incompatibilities of GNU CC
scheduling, delayed branch Other VMS Issues
scheduling, instruction Other VMS Issues
Other VMS Issues
scope of a variable length array Arrays of Variable Length
scope of declaration Disappointments and
Misunderstandings
scope of external declarations Incompatibilities of GNU CC
search path Options for Directory Search
second include path Options Controlling the
Preprocessor
section function attribute Declaring Attributes of
Functions
section variable attribute Specifying Attributes of
Variables
separate directory, compilation in Compilation in a Separate
Directory
sequential consistency on 88k M88K Options
setjmp incompatibilities Incompatibilities of GNU CC
shared strings Incompatibilities of GNU CC
shared VMS run time system Other VMS Issues
side effect in ?: Conditionals with Omitted
Operands
side effects, macro argument Statements and Declarations
in Expressions
side effects, order of evaluation Certain Changes We Don't
Want to Make
signature in C++, advantages Type Abstraction using
Signatures
signature member function default implementation Type Abstraction using
Signatures
signatures, C++ Type Abstraction using
Signatures
signed and unsigned values, comparison warning Options to Request or
Suppress Warnings
simple constraints Simple Constraints
simplifications, arithmetic Other VMS Issues
smaller data references M32R/D Options
smaller data references (88k) M88K Options
smaller data references (MIPS) MIPS Options
smaller data references (PowerPC) IBM RS/6000 and PowerPC
Options
SPARC options SPARC Options
specified registers Variables in Specified
Registers
specifying compiler version and target machine Specifying Target Machine
and Compiler Version
specifying hardware config Hardware Models and
Configurations
specifying machine version Specifying Target Machine
and Compiler Version
specifying registers for local variables Specifying Registers for
Local Variables
sscanf, and constant strings Incompatibilities of GNU CC
stack checks (29k) AMD29K Options
stage1 Installing GNU CC
start files Tools and Libraries for a
Cross-Compiler
statements inside expressions Statements and Declarations
in Expressions
static data in C++, declaring and defining Declare and Define Static
Members
storem bug (29k) AMD29K Options
strength-reduction Other VMS Issues
string constants Incompatibilities of GNU CC
string constants vs newline Options Controlling C
Dialect
structure passing (88k) M88K Options
structures Incompatibilities of GNU CC
structures, constructor expression Constructor Expressions
stupid register allocation Other VMS Issues
submodel options Hardware Models and
Configurations
subscripting Non-Lvalue Arrays May Have
Subscripts
subscripting and function values Non-Lvalue Arrays May Have
Subscripts
subtype polymorphism, C++ Type Abstraction using
Signatures
suffixes for C++ source Compiling C++ Programs
Sun installation Installing GNU CC on the Sun
suppressing warnings Options to Request or
Suppress Warnings
surprises in C++ Common Misunderstandings
with GNU C++
SVr4 M88K Options
syntax checking Options to Request or
Suppress Warnings
synthesized methods, warning Options to Request or
Suppress Warnings
tail recursion optimization Other VMS Issues
target machine, specifying Specifying Target Machine
and Compiler Version
target options Specifying Target Machine
and Compiler Version
target-parameter-dependent code Other VMS Issues
tcov Options for Debugging Your
Program or GNU CC
template debugging Options to Request or
Suppress Warnings
template instantiation Where's the Template?
temporaries, lifetime of Temporaries May Vanish
Before You Expect
threads, Objective C Installing GNU CC
thunks Nested Functions
tiny data section on the H8/300H Declaring Attributes of
Functions
traditional C language Options Controlling C
Dialect
type abstraction, C++ Type Abstraction using
Signatures
type alignment Inquiring on Alignment of
Types or Variables
type attributes Specifying Attributes of
Types
typedef names as function parameters Incompatibilities of GNU CC
Ultrix calling convention Interoperation
undefined behavior Have You Found a Bug?
undefined function value Have You Found a Bug?
underscores in variables in macros Naming an Expression's Type
underscores, avoiding (88k) M88K Options
union, casting to a Cast to a Union Type
unions Incompatibilities of GNU CC
unreachable code Other VMS Issues
unresolved references and -nodefaultlibs Options for Linking
unresolved references and -nostdlib Options for Linking
V850 Options V850 Options
value after longjmp Defining Global Register
Variables
variable addressability on the M32R/D Specifying Attributes of
Variables
variable alignment Inquiring on Alignment of
Types or Variables
variable attributes Specifying Attributes of
Variables
variable number of arguments Macros with Variable Numbers
of Arguments
variable-length array scope Arrays of Variable Length
variable-length arrays Arrays of Variable Length
variables in specified registers Variables in Specified
Registers
variables, local, in macros Naming an Expression's Type
Vax calling convention Interoperation
VAX options VAX Options
VMS and case sensitivity Other VMS Issues
VMS and include files Include Files and VMS
VMS installation Installing GNU CC on VMS
void pointers, arithmetic Arithmetic on void- and
Function-Pointers
void, size of pointer to Arithmetic on void- and
Function-Pointers
volatile applied to function Declaring Attributes of
Functions
warning for comparison of signed and unsigned values Options to Request or
Suppress Warnings
warning for overloaded virtual fn Options to Request or
Suppress Warnings
warning for reordering of member initializers Options to Request or
Suppress Warnings
warning for synthesized methods Options to Request or
Suppress Warnings
warning messages Options to Request or
Suppress Warnings
warnings vs errors Warning Messages and Error
Messages
weak attribute Declaring Attributes of
Functions
whitespace Incompatibilities of GNU CC
zero division on 88k M88K Options
zero-length arrays Arrays of Length Zero
ΓòÉΓòÉΓòÉ <hidden> ΓòÉΓòÉΓòÉ
Prior to release 2 of the compiler, there was a separate g++ compiler. That
version was based on GNU CC, but not integrated with it. Versions of g++ with
a '1.xx' version number---for example, g++ version 1.37 or 1.42---are much less
reliable than the versions integrated with GCC 2. Moreover, combining G++
'1.xx' with a version 2 GCC will simply not work.
ΓòÉΓòÉΓòÉ <hidden> ΓòÉΓòÉΓòÉ
The analogous feature in Fortran is called an assigned goto, but that name
seems inappropriate in C, where one can do more than simply store label
addresses in label variables.
ΓòÉΓòÉΓòÉ <hidden> ΓòÉΓòÉΓòÉ
A file's basename was the name stripped of all leading path information and of
trailing suffixes, such as '.h' or '.C' or