Figuring out location of a loaded shared library

Question

I'm maintaining a C-program, that uses Oracle client libraries. The libraries expect the ORACLE_HOME environment variable to be set, and fail to work without it -- in many cases.

In our situation, the value of the variable is always derived from the location of the shared libraries themselves anyway -- and I'd like to implement setting it from the C-code (using putenv(3)).

But, for that, I need to know, from where a particular shared library used by the program was loaded. So, if ldd, when applied to the executable file, outputs:

libc.so.7 => /lib/libc.so.7 (0x1021000)
libmeow.so.1 => /opt/meow/lib/libmeow.so.1

How can the running process itself find out, that libmeow.so, which it is using, is loaded from /opt/meow/lib?

The answer needn't be universally-portable -- something, that'd work for just Linux and BSD will suffice.

Answer 1

For Linux you can read and search through pseudo file /proc/<pid>/maps, where <pid> would be self from a program looking at itself. It has lines for each memory mapped item, which should include each shared library, eg

7f63c2c23000-7f63c2dd0000 r-xp 00000000 08:01 136321  /usr/lib64/libc-2.26.so

See man proc for details.

---

There is also a C api for a program to walk through the list of shared objects it is using. See man dl_iterate_phdr. It provides an example of its use. For your use case it is as simple as this:

#define _GNU_SOURCE
#include <link.h>
#include <stdlib.h>
#include <stdio.h>
static int callback(struct dl_phdr_info *info, size_t size, void *data){
    if(info->dlpi_name[0]!='\0')printf("%s\n", info->dlpi_name);
    return 0;
}
int main(int argc, char *argv[]) {
    dl_iterate_phdr(callback, NULL);
    exit(EXIT_SUCCESS);
}

Answer 2

If you want to get the libraries, linked in particular binary can be found with command ldd. For example:

$ldd /bin/cp
    linux-vdso.so.1 =>  (0x00007fffaf3ff000)
    libselinux.so.1 => /lib64/libselinux.so.1 (0x0000003a06a00000)
    librt.so.1 => /lib64/librt.so.1 (0x0000003a06200000)
    libacl.so.1 => /lib64/libacl.so.1 (0x0000003a13000000)
    libattr.so.1 => /lib64/libattr.so.1 (0x0000003a0ea00000)
    libc.so.6 => /lib64/libc.so.6 (0x0000003a05200000)
    libdl.so.2 => /lib64/libdl.so.2 (0x0000003a05a00000)
    /lib64/ld-linux-x86-64.so.2 (0x0000003a04a00000)
    libpthread.so.0 => /lib64/libpthread.so.0 (0x0000003a05600000)

From this place you can filter the Oracle libraries and set accordingly ORACLE_HOME variable. All this can be done in script which will run your program with already set variable.