The actual code that implements the functions in stadnard libary are compiler-dependent -- the standards do not dictate how the functions are implemented, only what the functions should do. So one compiler might implement the functions differently then another compiler. You can get the source code for all compiler functions from GNU, which is open-source and free. But be prepared for some really nasty ( ) reading!

Some of the functions, like strlen() are pretty simple. Others, like printf() family of functions, get pretty complicated and difficult for newbes to follow. So I'm not sure how useful the source code would be to you at your level of understanding. Many, if not most, of the functions in the C standard library were written over 20 years ago and tested in thousands, if not millions, of programs. So you can be well assured that they work correctly as designed and documented. This doesn't mean they are good functions to use, just that they work ok.