In fact, under normal termination, exit() may be called either directly (as
shown above) or indirectly (by a return statement in main()).
A parent process may wait for the termination of a child process by using
the wait() system call. The wait() system call is passed a parameter that
allows the parent to obtain the exit status of the child. This system call also
returns the process identifier of the terminated child so that the parent can tell
which of its children has terminated: