If you have ever done a long listing of /usr/lib on a Linux system, you probably choked and asked yourself what the f$%^ is this mess? After reading through Peter Seebach’s article Dissecting Shared Libraries, things don’t seem so bad, and the large number of files actually starts to make sense. Step one in sorting out the library madness requires making sense of the digits (the major and minor revision numbers) that appear in the shared library file name. Peter’s article clarifies this with the following description:
“One of the potential advantages of dynamic linking, however, is in fixing bugs. It’d be nice if you could fix a bug in the library and not have to recompile a thousand programs to take advantage of that fix. So sometimes, you want to link to a newer version. Unfortunately, that creates some cases where you want to link to the newer version and some cases where you’d rather stick with an older version. There is a solution, though — two kinds of version numbers:
- A major number indicates a potential incompatibility between library versions.
- A minor number indicates only bug fixes.
So under most circumstances, it is safe to load a library with the same major number and a higher minor number; consider it an unsafe practice to load a library with a higher major number.”
This makes sense, and seems like a good thing for systems that don’t want to break applications. Now what about all those links in /usr/lib? Peter provides the following description:
To prevent users (and programmers) from needing to track library numbers and updates, the system comes with a large number of symbolic links. In general, the pattern is that
libexample.so
will be a link to
libexample.so.N
in which N is the highest major version number found on the system. For every major version number supported,
libexample.so.N
will be a link in turn to
libexample.so.N.M
in which M is the largest minor version number.
If you haven’t checked out IBM’s redbook or Linux developerworks sites, I highly recommend doing so. There are numerous awesome pieces of documentation spanning numerous technologies.
