".c" files are just plain text, they are source files like one you would write yourself. If you do a search for '.c' files in your compiler folder there is a good chance it will turn up lots of files you can read, often with comments in them.
Library files are an exception because they are normally pre-compiled object code and therefore not human readable. You should have documentation supplied with the compiler that tells you what parameters the library functions use and what values they return. As a final resort, if you produce a listing file when you compile a program, it will normally show the functions broken down to assembler level but often with comments still embedded in them.
Brian.