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Processes terminate in one of two ways:
On receiving a signal, a process looks for a signal-handling function. Failure to find a signal-handling function forces the process to call exit, and therefore to terminate. The functions _exit, exit and abort terminate a process with the same effects except that abort makes available to wait or waitpid the status of a process terminated by the signal SIGABRT (see exit(S) and abort(S)).
As a process terminates, it can set an eight-bit exit status code available to its parent. Usually, this code indicates success (zero) or failure (non-zero), but it can be used in any manner the user wishes. If a signal terminated the process, the system first tries to dump an image of core, then modifies the exit code to indicate which signal terminated the process and whether core was dumped. This is provided that the signal is one that produces a core dump (see signal(M)). Next, all signals are set to be ignored, and resources owned by the process are released, including open files and the working directory. The terminating process is now a ``zombie'' process, with only its process-table entry remaining; and that is unavailable for use until the process has finally terminated. Next, the process-table is searched for any child or zombie processes belonging to the terminating process. Those children are then adopted by init by changing their parent process ID to 1). This is necessary since there must be a parent to record the death of the child. The last actions of exit are to record the accounting information and exit code for the terminated process in the zombie process-table entry and to send the parent the death-of-child signal, SIGCHLD (see ``Signals, job control and pipes'').
If the parent wants to wait until a child terminates before continuing execution, the parent can call wait, which causes the parent to sleep until a child zombie is found (meaning the child terminated). When the child terminates, the death-of-child signal is sent to the parent although the parent ignores this signal. (Ignore is the default disposition. Applications that fork children and need to know the return status should set this signal to other than ignore.) The search for child zombies continues until the terminated child is found; at which time, the child's exit status and accounting information is reported to the parent (remember the call to exit in the child put this information in the child's process-table entry) and the zombie process-table entry is freed. Now the parent can wake up and continue executing.