Problem with children (out of conrol forking vs. zombies)

belg4mit has asked for the wisdom of the Perl Monks concerning the following question:

Okay so the following code is supposed to:


loop infinitely
 have a nested loop that iterates n times (in this case 10) 
  within the nested loop there is a conditional fork (fork on 6th loop)
  sleep after completing the nested loop

Instead it proceeds as expected initially but after a few (2) iterations there are mutliple children which execute simultaneously... The problem goes away if I remove the CHLD handler, but then I have zombie children... If I set CHLD to IGNORE the problem goes away (though I always have *one* zombie)

It seems to be related to the fact that it is somehow running through the nested loop multiple times?

Looping 6 (28680) ...
I am child 28682 [0]
Looping 6 (28680) ...

Any ideas?

sub REAPER {
    $waitedpid = wait;
    print "GOT PID:   $waitedpid\n";
    $SIG{CHLD} = \&REAPER;
}
$SIG{CHLD} = \&REAPER;

while(1){
    my $i=0;
    print scalar localtime(time()), "\n";
    for(;$i<10;$i++){
    print "Looping $i ($$) ...\n";
    if( $i == 5 ){
        if( ! defined($pid = fork) ){
        print "Danger danger Will Robinson!\n";
        }
        unless( $pid ){
        print "I am child $$ [$pid]\n";
        exit 0;
        }
    }
    }
    sleep 5;
}
[download]

Comment on Problem with children (out of conrol forking vs. zombies) Download Code

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Re: Problem with children (out of conrol forking vs. zombies) by merlyn (Sage) on Oct 12, 2001 at 01:53 UTC
You will get one SIGCHLD for any appearance of one or more kids. You have to reap kids until there are no kids to reap. Something like this: `use POSIX qw(WNOHANG); while ((my $kid = waitpid(-1, WNOHANG)) > 0) { warn "$kid reaped\n" if $trace; delete $kids{$kid}; }` [download] Ripped off from my column about stuff like that. The best way is to simply do this polling at some opportune time in your main loop, since having it in a signal handler will break eventually. -- Randal L. Schwartz, Perl hacker	[reply] [d/l]

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Re: Problem with children (out of conrol forking vs. zombies)
by merlyn (Sage) on Oct 12, 2001 at 01:53 UTC

or more

      use POSIX qw(WNOHANG);

      while ((my $kid = waitpid(-1, WNOHANG)) > 0) {
        warn "$kid reaped\n" if $trace;
        delete $kids{$kid};
      }
[download]

my column about stuff like that

The best way is to simply do this polling at some opportune time in your main loop, since having it in a signal handler will break eventually.

-- Randal L. Schwartz, Perl hacker

[reply]
[d/l]