Depends on the convention you follow. If you just want to know if the function executed without errors or not, you could return any value you like. General convention in most libraries I have come across is that functions return 0 on success and -1 if there was an error.
if you call a function, that say does a quick calculation, but doesnt pass anything back to the function taht called it, should it return 1 or 0?
C is not obsolete. But, it is not the language of choice for commercial product development either. Your choice of language depends strongly on the kind of product you are planning to develop. C is a good way to start off with learning how to program. I am pretty sure your university will have courses in C++ in higher semesters.
Is C language obselete? University is teaching us C, but why not C++? Its difficult in C to do things like basic graphics, etc, would you ever nowadays write a commercial program in C??