## System Libraries

 1. We will first look at some of the system libraries we have
    mentioned in the lecture. Let's look at ‹/usr/lib› first:
    
        $ ls /usr/lib/*.a
        $ ls /usr/lib/*.so

 2. I want to show you an interesting new command. It is called
    ‹file› and its job is to identify the type of a file based on
    the content (not the file name). Let's try:
    
        $ file /usr/lib/libc_nonshared.a
        /usr/lib/libc_nonshared.a: current ar archive
        $ file /usr/lib/libc-2.17.so
        /usr/lib/libc-2.17.so: ELF 32-bit LSB shared object, [...]

 3. Using ‹file›, we learned a few things. Even before that,
    however, we notice that the file names are all wrong.
    Unfortunately, there is no ‹libc.a› on ‹aisa›, because many
    people consider static libraries obsolete. The
    ‹libc_nonshared.a› is a helper library that only contains a
    small fraction of ‹libc›.
 4. The shared version of the library has a version number in the
    file name.  This is unusual, but ‹libc› on this computer is
    somewhat special. If you are interested, you can peek at the
    file named ‹libc.so›, which is not an actual shared library, as
    confirmed by ‹file›:
    
        $ file /usr/lib/libc.so
        $ ls -l /usr/lib/libc.so.6

 5. The text file ‹libc.so› instructs the linker to look for
    ‹libc.so.6›, which is a symbolic link (we don't know those yet,
    but think of it as an alias for a different file). It points to
    our ‹libc-2.17.so›.
 6. The other kind of versioning -- the 6 in ‹libc.so.6› is common
    and this is how most shared libraries are. It means that this is
    the 6th major revision of ‹libc›.
 7. The second thing we learned is that the shared version of ‹libc›
    is 32-bit.  But surely, ‹aisa› is a 64b machine? Yes, and for
    historical reasons, the actual system libraries on ‹aisa› are
    located in ‹/usr/lib64›. I am sorry, life is complicated.

This was a little disappointing. Let's move on to making our own
(static) libraries instead:

    $ cat archives.txt
