library path when dynamically loaded?

The dynamic linker actually searches several places to find each dynamic library. These include (from man ld.so):

• Paths given by the environment variable LD_LIBRARY_PATH
• Paths baked into the binary load the library under the DT_RUNPATH entry
• The cache file /etc/ld.so.cache
• /lib and /usr/lib

If you want to get the path for a specific shared library, I would recommend the dladdr function. From the man page:

The function dladdr() takes a function pointer and tries to resolve
name and file where it is located. Information is stored in the
Dl_info structure:

typedef struct {
    const char *dli_fname;  /* Pathname of shared object that
                               contains address */
    void       *dli_fbase;  /* Address at which shared object
                               is loaded */
    const char *dli_sname;  /* Name of nearest symbol with address
                               lower than addr */
    void       *dli_saddr;  /* Exact address of symbol named
                               in dli_sname */
} Dl_info;

If no symbol matching addr could be found, then dli_sname and
dli_saddr are set to NULL.

dladdr() returns 0 on error, and non-zero on success.

So you just give it a function pointer, and it will give you the name of the file which supplies it and a bunch of other information. So for instance, you could have a constructor in a library call this on itself to find out the full path of the library:

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

__attribute__((constructor))
void on_load(void) {
    Dl_info dl_info;
    dladdr((void *)on_load, &dl_info);
    fprintf(stderr, "module %s loaded\n", dl_info.dli_fname);
}

This function also works on OS X with the same semantics.