Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!allegra!mit-eddie!think!harvard!caip!nike!styx!lll-crg!gymble!umcp-cs!seismo!mcvax!ukc!stc!inset!dave
From: dave@inset.UUCP (Dave Lukes)
Newsgroups: net.unix-wizards
Subject: Re: System V and SIGCLD
Message-ID: <979@inset.UUCP>
Date: Fri, 9-May-86 11:33:20 EDT
Article-I.D.: inset.979
Posted: Fri May  9 11:33:20 1986
Date-Received: Tue, 13-May-86 01:09:09 EDT
References: <709@cheviot.uucp>
Reply-To: dave@inset.UUCP (Dave Lukes)
Organization: The Instruction Set Ltd., London, UK.
Lines: 42

In article <709@cheviot.uucp> lindsay@cheviot.newcastle.ac.uk (Lindsay F. Marshall) writes:
>The following code goes into an infinite loop on System V :-
>
>	trap(sig)
>	int sig;
>	{
>		printf("trapped SIGCLD\n");
>		signal(SIGCLD, trap);	/* reset handler */
>	}
>
>	...
>
>The problem is that resetting the SIGCLD trap inside the handler causes the
>signal to be raised again and the handler to be re-entered......

Yes, this is because you still have an unwait()ed for child!!
What you have to do is wait() for the child in the SIGCLD handler,
THEN reset the handler: this works fine.

>This is not documented in the manual page and seems to me to be a bug as if you
>do not reset the handler the system seems to set it to SIG_DFL, meaning that
>you will loose any SIGCLD signals between the handler's exit and your getting
>a chance to call signal again.

WRONG!!! (``It's not a bug: it's a feature'')
If you catch SIGCLD you will get sent SIGCLD whenever you have ANY
zombie children around (whether newly zombified or not):
the same thing happens when you re-catch it.
Yes, the manual is wrong (as well as totally unclear): it should say that
any pending SIGCLD signals are queued until you call signal(SIGCLD, ...) again
				      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
it should also remind you that you still MUST call wait() to dispose of the
children.

Still, in defence of SIGCLD:
it IS safe (you NEVER lose any children), AND usable (if you know how!).

Hope this helps.
-- 
		Dave Lukes. (...!inset!dave)

``Fox hunting: the unspeakable chasing the inedible'' -- Oscar Wilde