fork and zombies

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

Hi, Monks!

I need your help again. My question is why i have zombies process when i run my program?

Zombies looks like so:

 3614 pts/1    D      0:00 /usr/bin/perl /root/USS/uss.pl
 3616 pts/1    D      0:00 /usr/bin/perl /root/USS/uss.pl
 3617 pts/1    Z      0:00 uss.pl <defunct>
 3618 pts/1    Z      0:00 uss.pl <defunct>
 3619 pts/1    Z      0:00 uss.pl <defunct>
 3620 pts/1    Z      0:00 uss.pl <defunct>
 3621 pts/1    D      0:00 /usr/bin/perl /root/USS/uss.pl

Here is part of program:

foreach my $row (@$sections) {
    
    if (my $pid = fork) {
    push @pids, $pid;
    }
    else {
    open BIDS, ">>", $CONFIG{BIDS_FOLDER} . "{currentsite}_" . $row->[
+0] . ".svd" or die "Could't open bids file!";
    flock(BIDS, 2);
    
    print "$site_id, $row->[0], $row->[1]\n";
    $found += search_bids( $site_id, $row->[0], $row->[1] );

    flock(BIDS, 8);
    close BIDS;
    exit;
    }
}
for (my $i = 0; $i <= $#pids; $i++) {
    waitpid($pids[$i], 0);
}
[download]

In search_bids sub i'm printing into BIDS and doing some LWP::UserAgent (getting pages and parse it) stuff.

Comment on fork and zombies Download Code

Replies are listed 'Best First'.
Re: fork and zombies by ikegami (Patriarch) on Oct 24, 2007 at 06:49 UTC
It means the process has ended, but it hasn't been reaped (using `waitpid`) by its parent yet. The following code should reap the children as they end, instead of in the order they were created: `foreach my $row (@$sections) { if (my $pid = fork) { $pids{$pid}++; } else { ... } } while (%pids) { my $pid = waitpid(-1, 0); delete $pids{$pid}; }` [download] By the way, have you considered Parallel::ForkManager? It seems ideal in this situation. Update: Added code.	[reply] [d/l] [select]
Re^2: fork and zombies by Gangabass (Vicar) on Oct 24, 2007 at 07:52 UTC
Thanks for Parallel::ForkManager. I'm mandatory look at it.	[reply]
Re: fork and zombies by Zaxo (Archbishop) on Oct 24, 2007 at 07:11 UTC
waitpid looks for a particular child, and other signals may be lost. I like to keep the pid's as keys in a hash and delete them as they die: `delete $pid{+wait} while %pid;` [download] Trying to do that in a for loop assumes that child processes will halt in a particular order. After Compline, Zaxo	[reply] [d/l]
Re: fork and zombies by Somni (Friar) on Oct 24, 2007 at 06:51 UTC
There's only one reason you ever get zombies: you were supposed to wait on a child process after it died, and you didn't. Given you're calling waitpid it means you either haven't gotten to that part of your code, or it's sitting there waiting on one pid that isn't ready. If you truly don't care what your child processes return, just set `$SIG{CHLD} = "IGNORE"`. If you do care, then you should setup an actual signal handler, `$SIG{CHLD} = sub { my $pid = wait; $exits{$pid} = $?; }`, or something similar. Waiting on a specific pid is usually only done when you know that specific process has died. Also, `foreach my $pid (@pids) { ... }`, you don't need a C-style for loop to iterate over an array.	[reply] [d/l] [select]
Re^2: fork and zombies by wolfger (Deacon) on Oct 24, 2007 at 16:52 UTC
I had the same problem once before and inserting `$SIG{CHLD} = 'IGNORE';` worked for me. -- Nothing to see here. Move along.	[reply] [d/l]


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