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From: ahb@ccice5.UUCP (Al Brumm)
Newsgroups: net.unix-wizards,net.unix
Subject: Re: UNIX question
Message-ID: <974@ccice5.UUCP>
Date: Wed, 11-Dec-85 18:49:00 EST
Article-I.D.: ccice5.974
Posted: Wed Dec 11 18:49:00 1985
Date-Received: Fri, 13-Dec-85 20:34:30 EST
References: <156@uw-june>
Reply-To: ahb@ccice5.UUCP (Al Brumm)
Organization: CCI, Central Engineering, Rochester, NY
Lines: 49
Keywords: zombies
Xref: watmath net.unix-wizards:16083 net.unix:6618

In article <156@uw-june> pjs@uw-june (Philip J. Schneider) writes:
>My question: Is there any way to kill off these zombies so I can get
>	     more processes ?  Or, failing that, is there any other
>	     way to do what I want ?

A clean way to handle this problem on Sys3 was to use the following
system call in the parent process:
	signal(SIGCLD, SIG_IGN);

Then when a child process exited, a zombie would not be created.
Note that this would not allow you to examine the child's exit
status.  However, you could examine the exit status by doing the 
following:
	int
	sigcld()
	{
		int pid, status;
		pid = wait(&status);
		.
		. (do stuff)
		.
	}
	main()
	{
		int	(*sigcld)();

		signal(SIGCLD, sigcld);
	}

The example immediately above is also possible in 4.2BSD,
only SIGCLD is called SIGCHLD.

Then again, there is always the double fork() trick which goes
something like this:
	if (fork()) {			/* parent */
		wait((int *)0);		/* no zombies please */
	}
	else {
		if (fork()) {		/* child */
			exit(0);	/* satisfy parent's wait */
		}
		else {			/* grandchild */
			do_stuff();		/* since my parent exit'ed */
			.			/* I am inherited by init */
			.
			.
		}
	}
The above trick is used quite heavily by the UNET servers.