Detect library features at runtime in C

Question

I am trying to build a program in C which has a lot of optional features that depend on various shared libraries.

In our heterogeneous computing cluster not all of those libraries are available (or up to date) on all systems.

Examples are symbols from newer glibc (sched_getcpu@@GLIBC_2.6, __sched_cpucount@@GLIBC_2.6) or whole shared libraries which may or may not be available (libnuma, libR, libpbs).

I know that I can use libdl to load the symbols with dlopen and dlsym, but doing this for an ever growing number of symbols (around 30 at the moment) is tedious at best.

As far as I understand shared libraries in Linux are lazy-loaded by default, so a symbol should not be needed until it is actually used.

But if I try to check for that in advance then it fails at execution start:

#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
#include <dlfcn.h>
#include <sched.h>

int main() {

    void *lib_handle;
    int (*fn)(void);
    int x;
    char *error;

    lib_handle = dlopen("libc.so.6", RTLD_LAZY);
    if (!lib_handle) 
    {
       fprintf(stderr, "%s\n", dlerror());
       exit(1);
    }

    fn = dlsym(lib_handle, "sched_getcpu");
    if ((error = dlerror()) != NULL)  
    {
       fprintf(stderr, "%s\n", error);
       exit(1);
    }

    printf("%d\n", sched_getcpu());

    return 0;
}

On compile system which has all libraries:

$ icc test.c
$ ./a.out
10

On another system which has a less recent version of GLIBC:

$ ./a.out 
./a.out: /lib64/libc.so.6: version `GLIBC_2.6' not found (required by ./a.out)

If I comment out the line that actually calls sched_getcpu then I get instead on the lesser system:

$ ./a.out 
/lib64/libc.so.6: undefined symbol: sched_getcpu

So, is there a way to force libraries only to be loaded on use and have checks like these before blocks that use them?

Answer 1

Not with glibc. This is a fail-safe and it's in place so that you won't shoot yourself in the foot. If the GLIBC_2.6 symbol wasn't defined and looked up, even if there were no other missing symbols, you could get garbage results from glibc (data corruption and crashes,) since it's not forwards compatible.

If you need compatibility on the glibc level, you need to build against the lowest common version.