Catching SIGCHLD

When a child process stops or terminates, SIGCHLD is sent to the parent process. The default response to the signal is to ignore it. The signal can be caught and the exit status from the child process can be obtained by immediately calling wait(2) and wait3(3C). This allows zombie process entries to be removed as quickly as possible. Example 4-2 demonstrates installing a handler that catches SIGCHLD.

Example 4-2 Catching SIGCHLD

#include <stdio.h>
#include <signal.h>
#include <sys/wait.h>
#include <sys/resource.h>

void proc_exit()
{
		int wstat;
		union wait wstat;
		pid_t	pid;

		while (TRUE) {
			pid = wait3 (&wstat, WNOHANG, (struct rusage *)NULL );
			if (pid == 0)
				return;
			else if (pid == -1)
				return;
			else
				printf ("Return code: %d\n", wstat.w_retcode);
		}
}
main ()
{
		signal (SIGCHLD, proc_exit);
		switch (fork()) {
			case -1:
				perror ("main: fork");
				exit (0);
			case 0:
				printf ("I'm alive (temporarily)\n");
				exit (rand());
			default:
				pause();
		}
}

SIGCHLD catchers are usually set up as part of process initialization. They must be set before a child process is forked. A typical SIGCHLD handler retrieves the child process's exit status.