GNU Info
(gcc-300.info)Interoperation
Interoperation
==============
This section lists various difficulties encountered in using GCC
together with other compilers or with the assemblers, linkers,
libraries and debuggers on certain systems.
* G++ 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 GCC. 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 (Note: Installation).
* 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 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 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 ISO C.
* The `libstdc++.a' library in GCC relies on the SVR4 dynamic linker
semantics which merges global symbols between libraries and
applications, especially necessary for C++ streams functionality.
This is not the default behavior of AIX shared libraries and
dynamic linking. `libstdc++.a' is built on AIX with
"runtime-linking" enabled so that symbol merging can occur. To
utilize this feature, the application linked with `libstdc++.a'
must include the `-Wl,-brtl' flag on the link line. G++ cannot
impose this because this option may interfere with the semantics
of the user program and users may not always use `g++' to link his
or her application. Applications are not required to use the
`-Wl,-brtl' flag on the link line--the rest of the `libstdc++.a'
library which is not dependent on the symbol merging semantics
will continue to function correctly.
* An application can interpose its own definition of functions for
functions invoked by `libstdc++.a' with "runtime-linking" enabled
on AIX. To accomplish this the application must be linked with
"runtime-linking" option and the functions explicitly must be
exported by the application (`-Wl,-brtl,-bE:exportfile').
* 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.
* 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. 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 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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