access a POSIX function using dlopen

Question

POSIX 2008 introduces several file system functions, which rely on directory descriptor when determining a path to the file (I'm speaking about -at functions, such as openat, renameat, symlinkat, etc.). I doubt if all POSIX platforms support it (well, at least the most recent versions seem to support) and I'm looking for a way to determine if platform supports such functions. Of course one may use autoconf and friends for compile-time determination, but I'm looking for a possibility to find out whether implementation supports -at functions dynamically.

The first that comes to my mind is a dlopen()/dlsym()/dlclose() combo; at least I've successfully loaded the necessary symbols from /usr/libc.so.6 shared library. However, libc may be (or is?) named differently on various platforms. Is there a list of standard locations to find libc? At least on Linux /lib/libc.so appears to be not a symbolic link to shared library, but a ld script. May be there exist some other way to examine during runtime if a POSIX function is supported? Thanks in advance!

Answer 1

#define _GNU_SOURCE 1
#include <dlfcn.h>
#include <stdio.h>

int main ()
{
    void * funcaddr = dlsym(RTLD_DEFAULT, "symlinkat");
    /* -----------------------^ magic! */
    printf ("funcaddr = %p\n", funcaddr);
}

Output:

funcaddr = 0x7fb62e44c2c0

Magic explanation: your program is already linked with libc, no need to load it again.

Note, this is actually GNU libc feature, as hinted by _GNU_SOURCE. POSIX reserves RTLD_DEFAULT "for future use", and then proceeds to define it exactly like GNU libc does. So strictly speaking it is not guaranteed to work on all POSIX systems.