How do I find what version of libc my application links to?

Question

When building an application using g++, I am not explicitly passing the libc library as a library to link to in the same way you would for other libraries (like passing -lpthread for example). I know that libc has the so name libc.so.6 but I am aware that this is not actually a library but something like a pointer to another version of libc (such as libc-2.15.so). My question is if I had multiple versions of libc on my computer, how can I tell which one actually gets linked to through the libc.so.6.

Answer 1

ldd should be the tool of your choice. That gives you the shared library actually linked.

confus@confusion:~/misc/test$ ldd -r -v testendian
    linux-vdso.so.1 =>  (0x00007fffbcfff000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f1a5a4c5000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f1a5a8a5000)

    Version information:
    ./testendian:
        libc.so.6 (GLIBC_2.2.5) => /lib/x86_64-linux-gnu/libc.so.6
    /lib/x86_64-linux-gnu/libc.so.6:
        ld-linux-x86-64.so.2 (GLIBC_2.3) => /lib64/ld-linux-x86-64.so.2
        ld-linux-x86-64.so.2 (GLIBC_PRIVATE) => /lib64/ld-linux-x86-64.so.2

In case of the libc you can simply run the .so file and will be told the library version.

confus@confusion:~/misc/test$ /lib/x86_64-linux-gnu/libc.so.6 
GNU C Library (Ubuntu EGLIBC 2.15-0ubuntu10) stable release version 2.15, by Roland McGrath et al.