#!/usr/bin/perl

# forktest.pl
#
# I needed a script that launched three instancies of the nmap
# network scanner to scan all our network in parallel. Each time
# a subnet is scanned and nmap finishes, another instance gets
# started to scan another subnet. This way I always have three
# parallel processes that do the job.
#
# To learn how to do it I created this small script that could
# be a starting point for accomplishing similar tasks.

use strict ;
use warnings ;

my %childpid ;
my $maxchild = 3 ;

my $subnet   = 1 ;

do {
  while (keys(%childpid) < $maxchild and $subnet <= 254) {
    my $pid = fork ;

    # The following instruction comes from node 237098
    die "Cannot fork: $!" unless defined $pid ;

    if ($pid) {
      # Parent process
      print "Child $pid started to scan subnet $subnet\n" ;
      $childpid{$pid} = $subnet++ ;
    } else {
      # Child process
      sleep rand 5 ;
      exit ;
    }
  }

  my $dead = wait ;

  # This die added after merlyn's suggestion
  # If you want to check a waitpid solution, see the code
  # posted by merlyn and zentara
  die "Something weird happened while wait!" if $dead == -1 ;

  print "Subnet $childpid{$dead} scan completed by child $dead\n" ;
  delete $childpid{$dead} ;
} while (keys(%childpid) > 0) ;