This perl script tests the use of fork(). I inadvertantly discovered that adding the problem code causes oddities with process id's, wait's, and such. When the problem code is uncommented, the child count goes to -1 instead of 0. I would greatly appreciate an explanation of why get_pidof() screws up wait().

Thank you. 
#!/usr/bin/perl -w
use strict;

use Unix::PID;

## problem code:
my $pid = Unix::PID->new();
#my @pids = $pid->get_pidof('fork.pl');
#die "one copy of me is already running\n" if scalar(@pids) > 0;
## :problem code

print "<parent>: my process id: $$\n";

my $childcnt = 0;

foreach my $process (@ARGV) {
    my $pid = fork;
    if (defined $pid )     {
        if ($pid ) {    # parent process
            if (++$childcnt >= 2) {    # hit child limit
                print "<parent>: must wait to start more\n";
                my $completed_pid = wait;
                $childcnt--;
                print "<parent>: $completed_pid is done; $childcnt run
+ning\n";
            } else {    # more children ok
                print "<parent>: $childcnt running; can start more\n";
            }
        } else {    # child process
            print "<$process>: my id: $$\n";
            sleep int(rand(10));
            print "<$process>:   end: $$\n";
            exit 0;
        }
    } else {                    # fork failed
        warn "<parent>: fork failed $!\n";
        last;
    }
}

print "<parent>: waiting for $childcnt to complete\n";
until ((my $completed_pid = wait) == -1) {
        $childcnt--;
        print "<parent>: $completed_pid is done; $childcnt running\n";
};
print "<parent>: complete; $childcnt process(es) left\n";

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In reply to get_pidof() messes with wait()? by rhumbliner

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