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From: richard@aiai.ed.ac.uk (Richard Tobin)
Newsgroups: comp.unix.questions
Subject: Re: How can a child learn of its parents termination?
Message-ID: <4428@skye.ed.ac.uk>
Date: 4 Apr 91 13:14:43 GMT
Article-I.D.: skye.4428
References: <94559@lll-winken.LLNL.GOV>
Reply-To: richard@aiai.UUCP (Richard Tobin)
Organization: AIAI, University of Edinburgh, Scotland
Lines: 64

In article <94559@lll-winken.LLNL.GOV> booloo@lll-crg.llnl.gov (Mark Boolootian) writes:
>The subject pretty much says it all:  Is it possible for a child to receive
>notification of its parent's termination?  One possibility would be to check
>the parent process id from time to time but this won't help me.  I have a
>child sleeping on a semaphore, normally awakened by the parent.  If the parent
>terminates, the child is going to sleep forever.  I need some way of waking
>the child up upon parent's death so that the child may exit.

Why does your parent terminate without clearing the semaphore?

One way for a process to detect another process dying is to have a
pipe between them.  The one that wants to be informed closes the write
side of the pipe, and does a read() (or select()) from the pipe.  When
the other process dies, the read() will return.  If necessary, it can
then send a signal to a third process that was too busy to do a read()
itself:

#include <stdio.h>
#include <signal.h>

main()
{
    int pid, p[2];

    pipe(p);

    if(fork() == 0)
    {
	close(p[1]);
	pid = fork();
	if(pid == 0)
	    work();
	else
	{
	    char buf[1];

	    read(p[0], buf, 1);
	    kill(pid, SIGUSR1);
	    exit(0);
	}
    }

    sleep(5);
    exit(0);    
}

void parent_died()
{
    printf("parent died\n");
    exit(0);
}

work()
{
    signal(SIGUSR1, parent_died);
    while(1)
	;    
}

-- Richard
-- 
Richard Tobin,                       JANET: R.Tobin@uk.ac.ed             
AI Applications Institute,           ARPA:  R.Tobin%uk.ac.ed@nsfnet-relay.ac.uk
Edinburgh University.                UUCP:  ...!ukc!ed.ac.uk!R.Tobin