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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. 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 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" 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: Function Names, Prev: Incomplete Enums, Up: C Extensions Function Names as Strings ========================= 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) File: gcc.info, Node: C++ Extensions, Next: Trouble, Prev: C Extensions, Up: Top Extensions to the C++ Language ****************************** 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++ (*note Standard Predefined Macros: (cpp.info)Standard Predefined.). * Menu: * 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. File: gcc.info, Node: Naming Results, Next: Min and Max, Up: C++ Extensions Named Return Values in C++ ========================== 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 as 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. File: gcc.info, Node: Min and Max, Next: Destructors and Goto, Prev: Naming Results, Up: C++ Extensions Minimum and Maximum Operators in C++ ==================================== 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 (*note Naming an Expression's Type: Naming Types.). However, writing `MIN' and `MAX' as macros also forces you to use function-call notation 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. File: gcc.info, Node: Destructors and Goto, Next: C++ Interface, Prev: Min and Max, Up: C++ Extensions `goto' and Destructors in GNU C++ ================================= 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. (In ANSI C++, `goto' is restricted to targets within the current block.) The compiler still forbids using `goto' to *enter* a scope that requires constructors. File: gcc.info, Node: C++ Interface, Prev: Destructors and Goto, Up: C++ Extensions Declarations and Definitions in One Header ========================================== 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' 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. `#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. `#pragma implementation' is *implied* whenever the basename(1) of your source file matches the basename of a header file it includes. There is no way to turn this off (other than using a different name for one of the two files). In the same vein, if you use `#pragma implementation' with no argument, it applies to an include file with the same basename as your source file. For example, in `allclass.cc', `#pragma implementation' by itself is equivalent to `#pragma implementation "allclass.h"'; but even if you do not say `#pragma implementation' at all, `allclass.h' is treated as an implementation file whenever you include it from `allclass.cc'. 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). ---------- Footnotes ---------- (1) A file's "basename" is the name stripped of all leading path information and of trailing suffixes, such as `.h' or `.C' or `.cc'. File: gcc.info, Node: Trouble, Next: Bugs, Prev: C++ Extensions, Up: Top Known Causes of Trouble with GNU CC *********************************** 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. * Menu: * 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. * 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. File: gcc.info, Node: Actual Bugs, Next: Installation Problems, Up: Trouble Actual Bugs We Haven't Fixed Yet ================================ * 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. * Loop unrolling doesn't work properly for certain C++ programs. This is because of difficulty in updating the debugging information within the loop being unrolled. We plan to revamp the representation of debugging information so that this will work properly, but we have not done this in version 2.4 because we don't want to delay it any further. File: gcc.info, Node: Installation Problems, Next: Cross-Compiler Problems, Prev: Actual Bugs, Up: Trouble Installation Problems ===================== 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. *Note Other Dir::. * In previous versions of GNU CC, the `gcc' driver program looked for `as' and `ld' in various places such as 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. * On 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. * 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. * 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. Six 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 these six packages are installed. The six packages that GNU CC needs are: `SUNWarc', `SUNWbtool', `SUNWesu', `SUNWhea', `SUNWlibm', and `SUNWtoo'. 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. * 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. * 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. * 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, GCC 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 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_0800. * To build GCC for HP PA model 1.1 machines running HP-UX versions earlier than 8.07, you have to configure for HP PA model 1.0. This is because a bug in the PA configuration that probably will be fixed in the next release of the compiler. * 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. * Another assembler problem on the HP PA results in an error message like this while compiling part of `libgcc2.a': as: /usr/tmp/cca08196.s @line#30 [err#1060] Argument 1 or 3 in FARG upper - lookahead = RTNVAL=GR This happens because HP changed the assembler syntax after system release 8.02. GNU CC assumes the newer syntax; if your assembler wants the older syntax, comment out this line in the file `pa1-hpux.h': #define HP_FP_ARG_DESCRIPTOR_REVERSED * 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 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. *Note WE32K Install::. File: gcc.info, Node: Cross-Compiler Problems, Next: Interoperation, Prev: Installation Problems, Up: Trouble Cross-Compiler Problems ======================= 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. *Note Cross-compilation::. * At present, the program `mips-tfile' which adds debug support to object files on MIPS systems does not work in a cross compile environment. File: gcc.info, Node: Interoperation, Next: External Bugs, Prev: Cross-Compiler Problems, Up: Trouble Interoperation ============== 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. * If you are using version 2.3 of libg++, you need to rebuild it with `make CC=gcc' to avoid mismatches in the definition of `size_t'. * 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 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. * On a Sun, linking using GNU CC fails to find a shared library and reports that the library doesn't exist at all. This happens if you are using the GNU linker, because it does only static linking and looks only for unshared libraries. If you have a shared library with no unshared counterpart, the GNU linker won't find anything. We hope to make a linker which supports Sun shared libraries, but please don't ask when it will be finished--we don't know. * 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. * 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 (*note Installation::.). * The HP-UX linker has a bug which can cause programs which make use of `const' variables to fail in unusual ways. If your program makes use of global `const' variables, we suggest you compile with the following additional options: -Dconst="" -D__const="" -D__const__="" -fwritable-strings This will force the `const' variables into the DATA subspace which will avoid the linker bug. Another option one might use to work around this problem is `-mkernel'. `-mkernel' changes how the address of variables is computed to a sequence less likely to tickle the HP-UX linker bug. We hope to work around this problem in a later version, if HP does not fix it. * Taking the address of a label may generate errors from the HP-UX PA assembler. GAS for the PA does not have this problem. * GNU CC produced code will not yet link against HP-UX 8.0 shared libraries. We expect to fix this problem in GNU CC 2.4. * 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". * 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 `-fno-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. *Note 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 yet another convention for structure and union returning. Use `-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 ilbrary. 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/2.4 -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'. File: gcc.info, Node: External Bugs, Next: Incompatibilities, Prev: Interoperation, Up: Trouble Problems Compiling Certain Programs =================================== * 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 File: gcc.info, Node: Incompatibilities, Next: Disappointments, Prev: External Bugs, Up: Trouble Incompatibilities of GNU CC =========================== 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 preprocessor 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 preprocessor 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'.