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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.

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1 Answer 1

up vote 8 down vote accepted

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:
        libc.so.6 (GLIBC_2.2.5) => /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.
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I did know about ldd, didn't know you could just run a library though - thanks. Unfortunately my output from running the library doesn't show the version.... –  mathematician1975 Jul 13 '12 at 13:18
Disregard that I was being an idiot - thanks for the answer! –  mathematician1975 Jul 13 '12 at 13:19

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