What is the difference between .a and .so file?

Question

As far as I understand they are libraries, but what is the difference between the two?

Answer 1

A .a file is a static library, while a .so file is a shared object (dynamic) library similar to a DLL on Windows. There's some detailed information about the differences between the two on this page.

Answer 2

As a follow on, a .a file is an "ar" archive. Not unlike a tar archive, it stores .o or object files, allowing them to be pulled out of the archive, and linked into a program, among other things. You could use ar to store other files if you wanted.

You can get a listing of the members of an ar file with the -t parameter, for instance:

ar -t /usr/lib/libc.a

A .so file is a "shared object" file, and has a lot more information available to the linker so that members can be linked in to a loading program as rapidly as possible.

For instance, try:

objdump -T /lib/libc-2.11.1.so

(or whatever version of libc.so you have in your /lib directory.) Note that a .so file could also just contain a linker script directing it to find the file elsewhere, or use something else.

Interestingly, a .so file can also be a full fledged program. For instance, trying running /lib/libc.so.6.