exit(0)

You may not have known it, but every program that runs on Unix (and many other modern operating systems) has an exit status, telling whether it was successful or not. Programs that run other programs (like the make utility program) look at that exit status to see that everything happened correctly. The exit status is just a single byte, so it can't say much; traditionally, it is 0 for success and a nonzero value for failure. Perhaps 1 means a syntax error in the command arguments, while 2 means that something went wrong during processing, and 3 means the configuration file couldn't be found; the details differ from one command to the next. But 0 always means that everything worked. When the exit status shows failure, a program like make knows not to go on to the next step.