Thank you for your reply xpi0t0s.

That you have explained I am aware of but what I want to know is lest say I write a code that loads a video file. It is in wrapped in some format and compressed with some codec, for arguments sake mpeg4 for the video and mpeg4-L3 for audio. and in order to decode this I am using the library libffmpeg.so. Now, that library is already compiled and in binary. How do find out what functions are contained in that library? How do I use the functions contained in that library and how do I link to it?

Again, thanks in advance.