#!/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 running\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";