Deamon: Need Child to exit if parent dies

aennen has asked for the wisdom of the Perl Monks concerning the following question:

Perl 5.8 Redhat Linux I need a forked function that more or less is an infinite heartbeat to unconditionaly terminate if parent dies. If parent is hard killed (-9) child does not terminate, Code example below.

#!/usr/bin/perl

#heart beat funtion, infinite

sub beat() {
    print "IM THE CHILD\n";
    while(1) {
        print "Beat $i\n";
        sleep 2;
    }
    exit(0);
}


#put heartbeat in background so whatever app is doing doesnt effect th
+is.

my $cpid = fork();

if ($cpid == 0) {
    &beat();
}


## parent stuff here ..... 

print "IM THE PARENT\n";

sleep 10;

print "Bye\n";

#I can kill the child directly but if parent is killed child still run
+s. Need child to exit when parent exits.

kill TERM, $cpid;

exit(0);
[download]

Comment on Deamon: Need Child to exit if parent dies Download Code

Replies are listed 'Best First'.
Re: Deamon: Need Child to exit if parent dies by JavaFan (Canon) on Jan 04, 2010 at 22:42 UTC
Swap the role of the parent and child (that is, let the parent do the heartbeating). Assuming you're using Unix (perhaps other OSses do the same, but I don't know them well enough), if a child process terminates, the parent receives a SIGCHLD. Then add to the parent: `$SIG{CHLD} = sub {exit};` [download] Otherwise, you may be able to use a pipe between the parent and the child. The child should receive a SIGPIPE if it tries to read from the pipe after the parent died. But I haven't tried that in code.	[reply] [d/l]
Re: Deamon: Need Child to exit if parent dies by MidLifeXis (Monsignor) on Jan 04, 2010 at 22:38 UTC
SIGKILL (9) cannot be trapped by a process - the OS just stops scheduling the process. You will need to have the child monitor the parent process to see if it still exists (perhaps with `kill 0, ...` (see kill), and take action based on that. See also documentation for $PID, specifically the information about `getppid`. --MidLifeXis	[reply] [d/l] [select]
Re^2: Deamon: Need Child to exit if parent dies by bot403 (Beadle) on Jan 05, 2010 at 20:33 UTC
Agreed. I would check to see if my parent ID changed to init (1). Then the child knows it became an orphan.	[reply]
Re: Daemon: Need Child to exit if parent dies by shmem (Chancellor) on Jan 04, 2010 at 23:00 UTC
See kill and getppid. Here's a recipe: `#!/usr/bin/perl print "process $$ started\n"; if ((my $pid = fork) == 0) { my $ppid = getppid; while (1) { sleep 1; unless (kill 0, $ppid) { print "child: parent($ppid) gone, exiting.\n"; exit; } else { print "child: parent($ppid) still alive...\n"; } } } else { my $secs = int rand 20; print "parent: sleeping $secs seconds\n"; sleep $secs; } print "parent: exiting after 1 second\n"; sleep 1;` [download]	[reply] [d/l]
Re^2: Daemon: Need Child to exit if parent dies by JavaFan (Canon) on Jan 05, 2010 at 11:43 UTC
That may not work correctly. The parent may exit, and its PID reused before the child gets a change to check, or either process may have changed UID (which is not uncommon for daemons) - the `kill 0` will then fail, but all `kill 0` tells you is whether the current process can send a signal to the other one - not existence is only one reason why a signal cannot be send.	[reply] [d/l] [select]