Weak symbol in an executable, overridable by shared libraries at runtime

Weak symbol in an executable, overridable by shared libraries at runtime

[Lorenzo Pistone]

Hello,
Is there a way to define a weak symbol in an executable (not a .so!), 
that can be overridden by the runtime linking process by a symbol found 
in another library?
The aim is to provide a fallback implementation, within my executable, 
for systems where a specific function in a standard library cannot be 
found. More exactly, if I compile my program with -ffast-math, some 
calls to __pow_finite are generated: on recent systems this function is 
in libm.so, but on older systems this function simply does not exist. 
The idea is then to have a weak symbol "__pow_infinite" in my 
executable, whose implementation just calls the normal "pow": this way, 
on a recent system, the weak symbol is overridden and the libm.so 
version is used, whereas on an old system there's no other symbol than 
the weak one, that would be then picked up.

Cheers.

Re: Weak symbol in an executable, overridable by shared libraries at runtime

[Alan Modra]

On Sat, Nov 02, 2013 at 10:44:32PM +0100, Lorenzo Pistone wrote:
> Is there a way to define a weak symbol in an executable (not a
> .so!), that can be overridden by the runtime linking process by a
> symbol found in another library?

No, not without making changes to linker operation.  Make your
executable a shared library, then have a "stub" executable that just
consists of startup files, like so.  Also note the tweak needed to
ld.so symbol resolution via LD_DYNAMIC_WEAK, and you may need to use
-Wl,-no-as-needed for some of the linking stages.

$ cat > weakexe.c <<EOF
#include <stdio.h>

int foo (void) __attribute__ ((weak));

int
foo (void)
{
  return 1;
}

int
main (void)
{
  printf ("foo: %d\n", foo ());
  return 0;
}
EOF
$ cat > weakso.c <<EOF
int
foo (void)
{
  return 2;
}
EOF
$ gcc -O2 -fpic -c weakexe.c 
$ gcc -O2 -fpic -c weakso.c 
$ gcc -shared -o weak.so weakso.o
$ gcc -shared -o weakmain.so weakexe.o weak.so
$ gcc -o weak weakmain.so weak.so
$ LD_LIBRARY_PATH=. ./weak
foo: 1
$ LD_LIBRARY_PATH=. LD_DYNAMIC_WEAK=1 ./weak
foo: 2


-- 
Alan Modra
Australia Development Lab, IBM

Re: Weak symbol in an executable, overridable by shared libraries at runtime

[H.J. Lu]

On Sat, Nov 2, 2013 at 2:44 PM, Lorenzo Pistone <blaffablaffa@gmail.com> wrote:
> Hello,
> Is there a way to define a weak symbol in an executable (not a .so!), that
> can be overridden by the runtime linking process by a symbol found in
> another library?
> The aim is to provide a fallback implementation, within my executable, for
> systems where a specific function in a standard library cannot be found.
> More exactly, if I compile my program with -ffast-math, some calls to
> __pow_finite are generated: on recent systems this function is in libm.so,
> but on older systems this function simply does not exist. The idea is then
> to have a weak symbol "__pow_infinite" in my executable, whose
> implementation just calls the normal "pow": this way, on a recent system,
> the weak symbol is overridden and the libm.so version is used, whereas on an
> old system there's no other symbol than the weak one, that would be then
> picked up.
>
> Cheers.

The secondary symbol is what you are looking for. But it isn't supported
everywhere.

-- 
H.J.