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Interoperation

The following documentation 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 compiler.
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 compilersbut 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 we 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 wont 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 previous options:

    -I/usr/local/lib/g++-include

- On some SGI systems, when you use -lgl_s as an option, it gets translated magically to -lgl_s -lX11_s -lc_s. Naturally, this does not happen when you use GNU CC. You must specify all three options explicitly.
- On a SPARC, GNU CC aligns all values of type double on an 8-byte boundary, and it expects every double to be so aligned. The Sun compiler usually gives double values 8-byte alignment, with one exception: function arguments of type double may not be aligned.
As a result, if a function compiled with Sun CC takes the address of an argument of type double and passes this pointer of type double* to a function compiled with GNU CC, de-referencing 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 de-reference it using 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. Instead, use malloc and related functions from libc.a; they do not have this problem.
- Sun forgot to include a static version of libdl.a with some versions of SunOS (mainly 4.1). This results in undefined symbols when linking static binaries (that is, if you use -static). If you see undefined symbols _dlclose, _dlsym or _dlopen when linking, compile and link against the file, mit/util/misc/dlsym.c, from the MIT version of X windows.
- The 128-bit long double format that the Sparc port supports currently works by using the architecturally defined quad-word floating point instructions. Since there is no hardware that supports these instructions they must be emulated by the operating system.
Long doubles do not work in Sun OS versions 4.0.3 and earlier, because the kernel emulator uses an obsolete and incompatible format. Long doubles do not work in Sun OS version 4.1.1 due to a problem in a Sun library. Long doubles do work on Sun OS versions 4.1.2 and higher, but GNU CC does not enable them by default. Long doubles appear to work in Sun OS 5.x (Solaris 2.x).
- On HPUX version 9.01 on the HPPA, the HP compiler, cc, does not compile GNU CC correctly. We do not yet know why. However, GNU CC compiled on earlier HPUX versions works properly on HPUX 9.01 and can compile itself properly on 9.01.
- On the HPPA 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 doesnt generate HPUX 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 Installing GNU CC).
- Taking the address of a label may generate errors from the HPUX PA assembler. GAS for the PA does not have this problem.
- Using floating point parameters for indirect calls to static functions will not work when using the HP assembler. There simply is no way for GCC to specify what registers hold arguments for static functions when using the HP assembler. GAS for the PA does not have this problem.
- In extremely rare cases involving some very large functions you may receive errors from the HP linker complaining about an out of bounds unconditional branch offset. This used to occur more often in previous versions of GNU CC, but is now exceptionally rare. If you should run into it, you can work around by making your function smaller.
- GNU CC compiled code sometimes emits warnings from the HPUX assembler of the following 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 presence 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 following form will cause the linker to report an undefined symbol, foo.

    extern int foo;

    ... foo ...

    static int foo;

Although this behavior differs from most other systems, it is not a bug because redefining an extern variable as static is undefined in ANSI C.
- AIX on the RS/6000 provides support (NLS) for environments outside of the United States. Compilers and assemblers use NLS to support locale-specific representations of various objects including floating-point numbers (. vs , for separating decimal fractions). There have been problems reported where the library linked with GCC does not produce the same floating-point formats that the assembler accepts. If you have this problem, set the LANG environment variable to C or En US.

- Even if you specify -fdollars-in-identifiers, you cannot successfully use $ in identifiers on the RS/6000 due to a restriction in the IBM assembler. GAS supports these identifiers.
- On the RS/6000, XLC version 1.3.0.0 will miscompile jump.c. XLC version 1.3.0.1 or later fixes this problem. You can obtain XLC-1.3.0.2 by requesting PTF 421749 from IBM.
- There is an assembler bug in versions of DG/UX prior to 5.4.2.01 that occurs when the fldcr instruction is used. GNU CC uses fldcr on the 88100 to serialize volatile memory references. Use the option, -mno-serialize-volatile, if your version of the assembler has this bug.
- On VMS, GAS versions 1.38.1 and earlier may cause spurious warning messages from the linker. These warning messages complain of mismatched psect attributes. You can ignore them. See Installing GNU CC on VMS on page 154.
- 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 the following lines around the definition of size_t.

    #ifndef _SIZE_T
    #define _SIZE_T
    actual typedef here
    #endif

- On the Alliant, the systems own convention for returning structures and unions is unusual, and is not compatible with GNU CC no matter what options are used.
- On the IBM RT PC, the MetaWare HighC compiler (hc) uses a different convention for structure and union returning.
Use the option, -mhc-struct-return, to tell GNU CC to use a convention compatible with it.
- On Ultrix, the Fortran compiler expects registers 2 through 5 to be saved by function calls. However, the C compiler uses conventions compatible with BSD Unix: registers 2 through 5 may be clobbered by function calls. GNU CC uses the same convention as the Ultrix C compiler. Use the following options to produce code compatible with the Fortran compiler.

    -fcall-saved-r2      -fcall-saved-r3
    -fcall-saved-r4      -fcall-saved-r5

- On the WE32k, you may find that programs compiled with GNU CC do not work with the standard shared C library. You may need to link with the ordinary C compiler. If you do so, you must specify the following options:

    -L/usr/local/lib/gcc-lib/we32k-att-sysv/2.7.1 -lgcc -lc_s

The first specifies where to find the library, libgcc.a, specified with the -lgcc option.
GNU CC does linking by invoking ld, just as cc does, and there is no reason why it should matter which compilation program you use to invoke ld. If someone tracks this problem down, it can probably be fixed easily.
- On the Alpha, you may get assembler errors about invalid syntax as a result of floating point constants. This is due to a bug in the C library functions ecvt, fcvt and gcvt. Given valid floating point numbers, they sometimes print NaN.
- On Irix 4.0.5F (and perhaps in some other versions), an assembler bug sometimes reorders instructions incorrectly when optimization is turned on. If you think this may be happening to you, try using the GNU assembler; GAS version 2.1 supports ECOFF on Irix.
Or use the -noasmopt option when you compile GNU CC with itself, and then again when you compile your program. (This is a temporary kludge to turn off assembler optimization on Irix.) If this proves to be what you need, edit the assembler spec in the file, specs so that it unconditionally passes -O0 to the assembler, and never passes -O2 or -O3.