Using and Porting the GNU Compiler Collection (GCC): Trouble

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# 7. Known Causes of Trouble with GCC

This section describes known problems that affect users of GCC. Most of these are not GCC 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.

> 7.1 Actual Bugs We Haven't Fixed Yet |  | Bugs we will fix later.
> 7.2 Installation Problems |  | Problems that manifest when you install GCC.
> 7.3 Cross-Compiler Problems |  | Common problems of cross compiling with GCC.
> 7.4 Interoperation |  | Problems using GCC with other compilers, and with certain linkers, assemblers and debuggers.
> 7.5 Problems Compiling Certain Programs |  | Problems compiling certain programs.
> 7.6 Incompatibilities of GCC |  | GCC is incompatible with traditional C.
> 7.7 Fixed Header Files |  | GNU C uses corrected versions of system header files. This is necessary, but doesn't always work smoothly.
> 7.8 Standard Libraries |  | GNU C uses the system C library, which might not be compliant with the ISO/ANSI C standard.
> 7.9 Disappointments and Misunderstandings |  | Regrettable things we can't change, but not quite bugs.
> 7.10 Common Misunderstandings with GNU C++ |  | Common misunderstandings with GNU C++.
> 7.11 Caveats of using `protoize` |  | Things to watch out for when using `protoize`.
> 7.12 Certain Changes We Don't Want to Make |  | Things we think are right, but some others disagree.
> 7.13 Warning Messages and Error Messages |  | Which problems in your code get warnings, and which get errors.

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## 7.1 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.

- 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, GCC 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.

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## 7.2 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. See section 3.3 Compilation in a Separate Directory.

- If you build GCC 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 GCC.

- In previous versions of GCC, the `gcc` driver program looked for `as` and `ld` in various places; for example, in files beginning with ``/usr/local/lib/gcc-'`. GCC 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 GCC. The fix is to get rid of the file `real-ld` which purify installs--so that GCC 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, GCC 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 GCC 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, GCC 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 GCC with the system's compiler, do not use ``-O'`. Some versions of the system's compiler miscompile GCC with ``-O'`.

- Sometimes on a Sun 4 you may observe a crash in the program `genflags` or `genoutput` while building GCC. 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 GCC version 2.0.2 are commonly available, but they have a bug that shows up when compiling current versions of GCC: undefined symbol errors occur during assembly if you use ``-g'`.

  The solution is to compile the current version of GCC 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 GCC fully. If you did not install all optional packages when installing Solaris, you will need to verify that the packages that GCC 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, GCC needs six packages: ``SUNWarc'`, ``SUNWbtool'`, ``SUNWesu'`, ``SUNWhea'`, ``SUNWlibm'`, and ``SUNWtoo'`.

  For Solaris 2.2, GCC needs an additional seventh package: ``SUNWsprot'`.

- On Solaris 2, trying to use the linker and other tools in ``/usr/ucb'` to install GCC 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 GCC. Once GCC 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, 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 GCC 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. GCC 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 GCC 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 GCC. You must use an older version of GCC for bootstrapping. One indication of this problem is if you get a crash when GCC compiles the function `muldi3` in file ``libgcc2.c'`.

  You may be able to succeed by getting GCC version 1, installing it, and using it to compile GCC version 2. The bug in the Pyramid C compiler does not seem to affect GCC 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 GCC.

  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 GCC 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 section 3.2 Configurations Supported by GNU CC.

- 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

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## 7.3 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. See section 17.18 Cross Compilation and Floating Point.

- At present, the program ``mips-tfile'` which adds debug support to object files on MIPS systems does not work in a cross compile environment.

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## 7.4 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.

- 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 GCC version 2. If you have trouble, get GDB version 4.4 or later.

- DBX rejects some files produced by GCC, 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 GCC unless corrected by `fixincludes`. The corrected header files go in a new directory; GCC searches this directory before ``/usr/include'`. If you use ``-I/usr/include'`, this tells GCC 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, GCC 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 GCC. You must specify all three options explicitly.

- On a Sparc, GCC 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 GCC, 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 GCC 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 GCC 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 GCC correctly. We do not yet know why. However, GCC 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 GCC. Specifically, it fails to work on functions that use `alloca` or variable-size arrays. This is because GCC 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 section 3. Installing GNU CC).

- 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 GCC, but is now exceptionally rare. If you should run into it, you can work around by making your function smaller.

- GCC 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. GCC 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 section 3.6 Installing GNU CC on VMS.

- 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 GCC 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 GCC 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.

  GCC 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 GCC 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.

  GCC 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 GCC 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'`.

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## 7.5 Problems Compiling Certain Programs

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 GCC 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 GCC:

      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 GCC:

      MALLOC=gmalloc.o

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## 7.6 Incompatibilities of GCC

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.

- GCC normally makes string constants read-only. If several identical-looking string constants are used, GCC 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 GCC 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.

- GCC does not substitute macro arguments when they appear inside of string constants. For example, the following macro in GCC

      #define foo(a) "a"

  will produce output `"a"` regardless of what the argument a is.

  The ``-traditional'` option directs GCC 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 GCC 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 GCC. 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 ``+='`. GCC, following the ANSI standard, does not allow this. The difficulty described immediately above applies here too.

- GCC 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, GCC 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. GCC 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, GCC output code normally uses a method different from that used on most versions of Unix. As a result, code compiled with GCC cannot call a structure-returning function compiled with PCC, and vice versa.

  The method used by GCC 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 GCC 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. GCC 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. GCC 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 GCC 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.

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## 7.7 Fixed Header Files

GCC 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 GCC unless they are changed. Some have bugs, some are incompatible with ANSI C, and some depend on special features of other compilers.

Installing GCC 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 GCC are not automatically updated. The easiest way to update them is to reinstall GCC. (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, GCC 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 GCC's fault, but it does mean that there's nothing for us to do about them.

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## 7.8 Standard Libraries

GCC 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 GCC 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.

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## 7.9 Disappointments and Misunderstandings

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 GCC 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 GCC 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 GCC 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.

- GCC 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 GCC 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 GCC 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 section 2.8 Options That Control Optimization).

- 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.

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## 7.10 Common Misunderstandings with GNU C++

C++ is a complex language and an evolving one, and its standard definition (the ISO C++ standard) was only recently completed. 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.

> 7.10.1 Declare and Define Static Members |  | Static member declarations are not definitions
> 7.10.2 Temporaries May Vanish Before You Expect |  | Temporaries may vanish before you expect
> 7.10.3 Implicit Copy-Assignment for Virtual Bases |  | Copy Assignment operators copy virtual bases twice

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### 7.10.1 Declare and Define Static Members

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.

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### 7.10.2 Temporaries May Vanish Before You Expect

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 string classes, especially ones that define a conversion function to type `char *` or `const char *` -- which is one reason why the standard `string` class requires you to call the `c_str` member function. 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 *);

    void
    f ()
    {
      const char *p = strfunc().c_str();
      ...
      charfunc (p);
      ...
      charfunc (p);
    }

In this situation, it may seem reasonable to save a pointer to the C string returned by the `c_str` member function and use that rather than call `c_str` repeatedly. However, the temporary string created by the call to `strfunc` is destroyed after `p` is initialized, at which point `p` is left pointing to freed memory.

Code like this may run successfully under some other compilers, particularly obsolete cfront-based compilers that delete temporaries along with normal local variables. However, the GNU C++ behavior is standard-conforming, so if your program depends on late destruction of temporaries it is not portable.

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.c_str ());

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### 7.10.3 Implicit Copy-Assignment for Virtual Bases

When a base class is virtual, only one subobject of the base class belongs to each full object. Also, the constructors and destructors are invoked only once, and called from the most-derived class. However, such objects behave unspecified when being assigned. For example:

    struct Base{
      char *name;
      Base(char *n) : name(strdup(n)){}
      Base& operator= (const Base& other){
       free (name);
       name = strdup (other.name);
      }
    };

    struct A:virtual Base{
      int val;
      A():Base("A"){}
    };

    struct B:virtual Base{
      int bval;
      B():Base("B"){}
    };

    struct Derived:public A, public B{
      Derived():Base("Derived"){}
    };

    void func(Derived &d1, Derived &d2)
    {
      d1 = d2;
    }

The C++ standard specifies that ``Base::Base'` is only called once when constructing or copy-constructing a Derived object. It is unspecified whether ``Base::operator='` is called more than once when the implicit copy-assignment for Derived objects is invoked (as it is inside ``func'` in the example).

g++ implements the "intuitive" algorithm for copy-assignment: assign all direct bases, then assign all members. In that algorithm, the virtual base subobject can be encountered many times. In the example, copying proceeds in the following order: ``val'`, ``name'` (via `strdup`), ``bval'`, and ``name'` again.

If application code relies on copy-assignment, a user-defined copy-assignment operator removes any uncertainties. With such an operator, the application can define whether and how the virtual base subobject is assigned.

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## 7.11 Caveats of using `protoize`

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.)

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## 7.12 Certain Changes We Don't Want to Make

This section lists changes that people frequently request, but which we do not make because we think GCC 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 GCC 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 GCC 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, GCC 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.

  GCC 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. GCC 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.

  Historically, GCC has not deleted "empty" loops under the assumption that the most likely reason you would put one in a program is to have a delay, so deleting them will not make real programs run any faster.

  However, the rationale here is that optimization of a nonempty loop cannot produce an empty one, which holds for C but is not always the case for C++.

  Moreover, with ``-funroll-loops'` small "empty" loops are already removed, so the current behavior is both sub-optimal and inconsistent and will change in the future.

- 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.

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## 7.13 Warning Messages and Error Messages

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. GCC 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).

GCC 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 GCC 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 section Options to Request or Suppress Warnings, for more detail on these and related command-line options.

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