When I run apt-get with an option that installs packages, such as apt-get install firefox or apt-get dist-upgrade, the last line of output that I see is usually ldconfig deferred processing now taking place. What is going on here? What is ldconfig, and what does it do?
ldconfig creates the necessary links and cache to the most recent shared libraries found in the directories specified on the command line, in the file /etc/ld.so.conf, and in the trusted directories (/lib and /usr/lib). The cache is used by the run-time linker, ld.so or ld-linux.so. ldconfig checks the header and filenames of the libraries it encounters when determining which versions should have their links updated.
ldconfig will attempt to deduce the type of ELF libs (i.e., libc5 or libc6/glibc) based on what C libs, if any, the library was linked against.
http://linux.die.net/man/8/ldconfig