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This is Info file gcc.info, produced by Makeinfo-1.54 from the input
file gcc.texi.
This file documents the use and the internals of the GNU compiler.
Published by the Free Software Foundation 675 Massachusetts Avenue
Cambridge, MA 02139 USA
Copyright (C) 1988, 1989, 1992, 1993 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" 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
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" 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.
File: gcc.info, Node: Overall Options, Next: Invoking G++, Prev: Option Summary, Up: Invoking GCC
Options Controlling the Kind of Output
======================================
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.
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.
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.
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.
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.
File: gcc.info, Node: Invoking G++, Next: C Dialect Options, Prev: Overall Options, Up: Invoking GCC
Compiling C++ Programs
======================
C++ source files conventionally use one of the suffixes `.C', `.cc',
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 GNU class library
libg++. (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. *Note
Options Controlling C Dialect: C Dialect Options, for explanations of
options for languages related to C. *Note Options Controlling C++
Dialect: C++ Dialect Options, for explanations of options that are
meaningful only for C++ programs.
---------- Footnotes ----------
(1) 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.
File: gcc.info, Node: C Dialect Options, Next: C++ Dialect Options, Prev: Invoking G++, Up: Invoking GCC
Options Controlling C Dialect
=============================
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 disallows `$' as part of
identifiers.
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'. *Note 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. These
words may then be used as identifiers. You can use the keywords
`__asm__', `__inline__' and `__typeof__' instead. `-ansi' implies
`-fno-asm'.
`-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.
`-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:
* All `extern' declarations take effect globally even if they
are written inside of a function definition. This includes
implicit declarations of functions.
* 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.)
* Comparisons between pointers and integers are always allowed.
* Integer types `unsigned short' and `unsigned char' promote to
`unsigned int'.
* Out-of-range floating point literals are not an error.
* Certain constructs which ANSI regards as a single invalid
preprocessing number, such as `0xe-0xd', are treated as
expressions instead.
* 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'.)
* All automatic variables not declared `register' are preserved
by `longjmp'. Ordinarily, GNU C follows ANSI C: automatic
variables not declared `volatile' may be clobbered.
* 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.
* 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.
In the preprocessor, comments convert to nothing at all, rather
than to a space. This allows traditional token concatenation.
In preprocessor directive, the `#' symbol must appear as the first
character of a line.
In the preprocessor, 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.
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. *Note Standard Predefined Macros: (cpp.info)Standard
Predefined, for more discussion of these and other predefined
macros.
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.)
`-traditional-cpp'
Attempt to support some aspects of traditional C preprocessors.
This includes the last five items in the table immediately above,
but none of the other effects of `-traditional'.
`-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'.
`-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).
File: gcc.info, Node: C++ Dialect Options, Next: Warning Options, Prev: C Dialect Options, Up: Invoking GCC
Options Controlling C++ Dialect
===============================
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:
`-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.
`-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 some target systems but not others.)
Traditional C allowed the character `$' to form part of
identifiers. However, ANSI C and C++ forbid `$' in identifiers.
`-felide-constructors'
Elide constructors when this seems plausible. With this option,
GNU C++ initializes `y' directly from the call to `foo' without
going through a temporary in the following code:
A foo ();
A y = foo ();
Without this option, GNU C++ (1) initializes `y' by calling the
appropriate constructor for type `A'; (2) assigns the result of
`foo' to a temporary; and, finally, (3) replaces the initial value
of `y' with the temporary.
The default behavior (`-fno-elide-constructors') is specified by
the draft ANSI C++ standard. If your program's constructors have
side effects, `-felide-constructors' can change your program's
behavior, since some constructor calls may be omitted.
`-fenum-int-equiv'
Permit implicit conversion of `int' to enumeration types. Normally
GNU C++ allows conversion of `enum' to `int', but not the other
way around.
`-fexternal-templates'
Produce smaller code for template declarations, by generating only
a single copy of each template function where it is defined. To
use this option successfully, you must also mark all files that
use templates with either `#pragma implementation' (the
definition) or `#pragma interface' (declarations). *Note
Declarations and Definitions in One Header: C++ Interface, for more
discussion of these pragmas.
When your code is compiled with `-fexternal-templates', all
template instantiations are external. You must arrange for all
necessary instantiations to appear in the implementation file; you
can do this with a `typedef' that references each instantiation
needed. Conversely, when you compile using the default option
`-fno-external-templates', all template instantiations are
explicitly internal.
You do not need to specify `-fexternal-templates' when compiling a
file that does not define and instantiate templates used in other
files, even if your file *uses* templates defined in other files
that are compiled with `-fexternal-templates'. The only side
effect is an increase in object size for each file that you
compile without `-fexternal-templates'.
`-fhandle-signatures'
Recognize the `signature' and `sigof' keywords for specifying
abstract types. The default (`-fno-handle-signatures') is not to
recognize them. *Note Type Abstraction using Signatures: C++
Signatures.
`-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.
`-fno-strict-prototype'
Treat a function declaration with no arguments, such as `int foo
();', as C would treat it--as saying nothing about the number of
arguments or their types. Normally, such a declaration in C++
means that the function `foo' takes no arguments.
`-fnonnull-objects'
Assume that objects reached through references are not null.
Normally, GNU C++ makes conservative assumptions about objects
reached through references. For example, the compiler must check
that `a' is not null in code like the following:
obj &a = g ();
a.f (2);
Checking that references of this sort have non-null values requires
extra code, however, and it is unnecessary for many programs. You
can use `-fnonnull-objects' to omit the checks for null, if your
program doesn't require checking.
`-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 the
type of `this' in a member function of class `X' to be `X *const'.
However, for backwards compatibility, you can make it valid with
`-fthis-is-variable'.
`-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 libg++.)
`-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'. *Note
Options Controlling C Dialect: C Dialect Options.
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.
*Note Options That Control Optimization: Optimize Options.
`-Wenum-clash'
`-Woverloaded-virtual'
`-Wtemplate-debugging'
Warnings that apply only to C++ programs. *Note Options to
Request or Suppress Warnings: Warning Options.
`+eN'
Control how virtual function definitions are used, in a fashion
compatible with `cfront' 1.x. *Note Options for Code Generation
Conventions: Code Gen Options.
File: gcc.info, Node: Warning Options, Next: Debugging Options, Prev: C++ Dialect Options, Up: Invoking GCC
Options to Request or Suppress Warnings
=======================================
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.
Inhibit all warning messages.
`-Wno-import'
Inhibit warning messages about the use of `#import'.
`-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.
*Note 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.
Print extra warning messages for these events:
* 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.
* 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;
}
* An expression-statement contains no side effects.
* An unsigned value is compared against zero with `>' or `<='.
* 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.
* Storage-class specifiers like `static' are not the first
things in a declaration. According to the C Standard, this
usage is obsolescent.
* 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 };
`-Wimplicit'
Warn whenever a function or parameter is implicitly declared.
`-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'.
`-Wunused'
Warn whenever a local 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.
`-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.
`-Wcomment'
Warn whenever a comment-start sequence `/*' appears in a comment.
`-Wtrigraphs'
Warn if any trigraphs are encountered (assuming they are enabled).
`-Wformat'
Check calls to `printf' and `scanf', etc., to make sure that the
arguments supplied have types appropriate to the format string
specified.
`-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.
`-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 `volatile'. *Note Function
Attributes::.
`-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.
`-Wenum-clash'
Warn about conversion between different enumeration types. (C++
only).
`-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. These are all the options
which pertain to usage that we recommend avoiding and that we
believe is easy to avoid, even in conjunction with macros.
The remaining `-W...' options are not implied by `-Wall' because
they warn about constructions that we consider reasonable to use, on
occasion, in clean programs.
`-Wtraditional'
Warn about certain constructs that behave differently in
traditional and ANSI C.
* 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.
* A function declared external in one block and then used after
the end of the block.
* A `switch' statement has an operand of type `long'.
`-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'.
`-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.
`-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.
`-Werror'
Make all warnings into errors.
File: gcc.info, Node: Debugging Options, Next: Optimize Options, Prev: Warning Options, Up: Invoking GCC
Options for Debugging Your Program or GNU CC
============================================
GNU CC has various special options that are used for debugging
either your program or GCC:
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+', or
`-gdwarf' (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 in the native format (if that is
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 and Alpha systems this option
produces embedded stabs debugging output which is not understood
by DBX.
`-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.
`-gdwarf'
Produce debugging information in DWARF 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 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.
`-gLEVEL'
`-ggdbLEVEL'
`-gstabsLEVEL'
`-gcoffLEVEL'
`-gxcoffLEVEL'
`-gdwarfLEVEL'
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'.
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.
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.
`-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'.
`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'.
`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.
`-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' but you do want to link
with `libgcc.a'. You can do
gcc -nostdlib FILES... `gcc -print-libgcc-file-name`
(.txt)
(.txt)