What is a .so.2 file?

I compiled Intel TBB from source using GCC. It generates a libtbb.so and littbb.so.2. Looks like the .so.2 file is the real shared library, libtbb.so just contains a line of text INPUT (libtbb.so.2)

What is the purpose to generate these two files instead of one? For the INPUT (libtbb.so.2), what is the syntax? I want to know more about it.

Answer

Usually when you build shared objects (.so) then you also take care of versions by adding suffixes such as mylib.so.2.3.1. To make sure your programs can load this lib or other later versions you create links with names

mylib.so -> mylib.so.2.3.1
mylib.so.2 -> mylib.so.2.3.1
mylib.so.2.3 -> mylib.so.2.3.1

So, everything after .so represents version.sub-version.build (or similar) Also, it is possible for more than one version of the same lib to coexist with this scheme, and all that is necessary to switch programs to using a particular version is to have the appropriate links in place.