When dynamically linking, where will it search for the library, and how to change this?

Question

When I'm dynamically linking on a Unix/Linux based system, where will it try to find the library files when I execute the program, and how do I change this? I am using the GNU/G++ compiler.

Answer 1

The system the program runs on defines where to search for the dynamic libraries. The program itself just names the libraries (without path).

On Linux systems there is a system-wide default where to search and you can override this by setting the environment variable LD_LIBRARY_PATH to a colon-separated list of paths to the libraries:

$ LD_LIBRARY_PATH=/path/to/my/libs/:/another/path/to/libs/of/mine/
$ export LD_LIBRARY_PATH
$ ./path/to/my/executable

or shorter:

$ LD_LIBRARY_PATH=/path/to/my/libs/:/another/path/to/libs/of/mine/ ./path/to/my/executable

The system default is configured using ldconfig and typically based on the files found at /etc/ld.so.conf (pointing to /etc/ld.so.conf.d) and /etc/ld.so.cache.

Which dynamic libraries a given executable is going to need can be examined using ldd:

$ ldd /bin/ls
    linux-vdso.so.1 =>  (0x00007fffc83ff000)
    libselinux.so.1 => /lib/x86_64-linux-gnu/libselinux.so.1 (0x00007fbe1306b000)
    librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007fbe12e63000)
    libacl.so.1 => /lib/x86_64-linux-gnu/libacl.so.1 (0x00007fbe12c5a000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fbe1289b000)
    libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fbe12697000)
    /lib64/ld-linux-x86-64.so.2 (0x00007fbe132ae000)
    libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fbe12479000)
    libattr.so.1 => /lib/x86_64-linux-gnu/libattr.so.1 (0x00007fbe12274000)

You can also use the output of strace when starting the executable to see where the system looks for libraries:

$ strace ls 2>&1 | grep '^open.*\.so'
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libselinux.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/librt.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libacl.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libpthread.so.0", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libattr.so.1", O_RDONLY|O_CLOEXEC) = 3