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From: richl@penguin.USS.TEK.COM (Rick Lindsley)
Newsgroups: comp.unix.questions
Subject: Re: SIGSTOP - what is it?
Message-ID: <47@puffin.USS.TEK.COM>
Date: Fri, 3-Jul-87 12:05:38 EDT
Article-I.D.: puffin.47
Posted: Fri Jul  3 12:05:38 1987
Date-Received: Sat, 4-Jul-87 15:31:38 EDT
References: <3786@spool.WISC.EDU>
Sender: nobody@puffin.USS.TEK.COM
Reply-To: richl@penguin.USS.TEK.COM (Rick Lindsley)
Organization: Tektronix, Inc., Beaverton, OR.
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In article <3786@spool.WISC.EDU> lm@cottage.WISC.EDU (Larry McVoy) writes:
> Does anyone know what the SIGSTOP signal is - it's not the keyboard (^Z) stop
> signal.  So what's it there for?

It's analogous to SIGKILL in that it can't be caught or ignored. That makes it
handy when you may not want to kill a process, just stop it. One use of it:
we had a user once who wanted to practice playing with fork(). So he wrote
a simple C program ... basically "for (i=0;i<20;i++) while (fork()<0);" He
thought this would create 20 processes. Of course it doesn't; it creates
a LOT more. He quickly reached his process limit, thankfully, but killing
one of these didn't do any good -- as soon as there was room, one of the forks
would succeed and we'd be off to the races. There was one very easy solution,
though -- SIGSTOP them all, THEN SIGKILL them all. (A second solution would be
nice them all to 20 before killing them, though that's not as certain.)

Another use for it would be stopping processes for backups on a "live"
filesystem. We used to do that on one or two busy machines. The processes
cannot ignore the signal, so you can essentially grind things to a halt without
destroying context.

Rick