System V and SIGCLD

[Doug Gwyn]

In article <709 at cheviot.uucp> lindsay at 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 */
>	}
>
>	main()
>	{
>		signal(SIGCLD, trap);
>		switch ( fork() )
>		{
>		case 0 : /* child */
>			sleep(5);
>			exit(0);
>		case -1 :
>			printf("error\n");
>			exit(1);
>		default :
>			pause();
>		}
>		exit(0);
>	}
>
>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...... 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. Anyone have any thoughts, information etc. on
>this problem??

The reason SIGCLD keeps recurring is that you continue to have an
unwaited-for terminated child process.  A wait() must be done to
lay the zombie to rest.

As to the window of vulnerability:  Yes, all generally-available
UNIXes except 4.2BSD have this problem.  AT&T has said that they
plan to change to Berkeley-like "reliable signals" in some future
release of UNIX System V.