Someone sent in this question by email: The main() function may return (to the operating system) by calling the exit() function, simply calling return, or 'dropping off' the bottom curly bracket. In the last 2 cases, no call to exit() is required at all. We an assume that the anticipated exit status of a process is success - which we can indicate with exit(EXIT_SUCCESS) or even return EXIT_SUCCESS. A program that can never fail, presumably a very simple one or one that doesn't check enough things, will not even have a call to exit(EXIT_FAILURE). And there's a program, named false, that only ever exits with failure, never with success. So there's no general rule, nor requirement, about how any calls to exit() a program must have.