a.c:
#include <stdio.h>
#include <dlfcn.h>
#include <assert.h>

int external()
{
        return 5;
}

int main()
{
        int (*ptr)();
        void *handle = dlopen("./libb.so", RTLD_LAZY|RTLD_GLOBAL);
        if (!handle)
        {
                fprintf(stderr, "%s\n", dlerror());
                return 1;
        }
#ifdef __FreeBSD__
        ptr = (int (*)())dlfunc(handle, "fun");
#else
        ptr = (int (*)())dlsym(handle, "fun");
#endif
        printf("%d\n", (*ptr)());
        return 0;
}

b.c:
extern int external();

int fun()
{
        return external();
}

FreeBSD:
[sysnet80:/tmp] steve$ uname -s -r
FreeBSD 9.1-PRERELEASE
[sysnet80:/tmp] steve$ clang -fPIC a.c
[sysnet80:/tmp] steve$ clang -fPIC -c b.c
[sysnet80:/tmp] steve$ clang -shared -o libb.so b.o
[sysnet80:/tmp] steve$ ./a.out
./libb.so: Undefined symbol "external"

Linux:
[hilbert:/tmp] steve$ uname -s -r
Linux 2.6.35-28-generic
[hilbert:/tmp] steve$ clang -fPIC a.c -ldl
[hilbert:/tmp] steve$ clang -fPIC -c b.c
[hilbert:/tmp] steve$ clang -shared -o libb.so b.o
[hilbert:/tmp] steve$ ./a.out
./a.out: symbol lookup error: ./libb.so: undefined symbol: external