What happens if main does not return 0?

If main returns a non-zero value you will get a Run Time Error. However, with a C99 compiler this should not happen
as long as you (a) declare main to return int, and (b) do not explicitly return a non-zero value. If you declare main to return, e.g., float or void you will get undefined behavior, likely to result in a Run Time error. It never hurts to have an explicit return 0 at the end of main, though.