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.
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Objective C does not work on the RS/6000.
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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.
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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.
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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.
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The GNU assembler (GAS) does not support PIC. To generate PIC code, you
must use some other assembler, such as `/bin/as'.
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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.
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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
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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.
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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.
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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.
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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.
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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 eumulator 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).
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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.
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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.
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Debugging (`-g') is not supported on the HP PA machine, unless you use
the preliminary GNU tools (see section Installing GNU CC).
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Taking the address of a label may generate errors from the HP-UX
PA assembler. GAS for the PA does not have this problem.
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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.
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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.
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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.
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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.
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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".
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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.
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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.
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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.
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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 Installing GNU CC on VMS.
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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
s@end smallexample
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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.
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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.
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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
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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/2.7.0 -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.
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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'.
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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'.