I have a forked ping program (mostly lifted from www.stonehenge.com, thanks merlyn). Anyway, I have modified his code to use Net::Ping and log the results to a file.

The problem I am having is this, I need the part that loads the array @hosts to only be executed in the parent not the child, but the fork comes below in the block that depends on @hosts being populated. The problem is that the loading of @hosts is being done in every child. Not only is this not efficient, but I am getting multiple reports of bad ip's written to the log file.

How can I, prior to having $pid set in the fork, make the block that loads @hosts execute only in the parent?

#!/usr/bin/perl -w

use strict;
use Net::Ping;


my $source_file = (shift || "/tmp/ping_list.txt");
my $good_out = "/tmp/good_ips.txt";
my $bad_out  = "/tmp/bad_ips.txt";
my %pid_to_host;
my %host_result;
my @hosts;

$|++;

open(GOOD,">$good_out")
    or die "Cannot open output file $good_out: $!\n";
open(BAD,">$bad_out")
    or die "Cannot open output file $bad_out: $!\n";
open(IPS,"$source_file") 
    or die "Cannot open source file $source_file: $!\n";

for (<IPS>)
{
    chomp;
    s/  *//;
    s/\cM//;
    if ($_ =~ m/^\d+\.\d+\.\d+\.\d+$/) 
    {
      push(@hosts,$_);
    }
    else 
    {
      print  BAD "$_ : NOT A VALID IP ADDRESS\n";
    }
}


for (@hosts)
{
      wait_for_a_kid() if keys %pid_to_host > 50;
      if ( my $pid = fork ) {
        # parent 
        $pid_to_host{$pid} = $_;
      } else { 
        # child
        exit !ping_a_host($_);
      }
}

1 while wait_for_a_kid();
for (sort keys %host_result)
{
      print GOOD "$_\n" if ($host_result{$_});
      print BAD "$_\n" if (!$host_result{$_});
}


sub ping_a_host
{
      my $host = shift;
      my $p    = Net::Ping->new('tcp',5);
      $p->ping($host) ? 1 : 0;
}

sub wait_for_a_kid
{
      my $pid = wait;
      return 0 if $pid < 0;
      my $host = delete $pid_to_host{$pid}
        or warn("Why did I see $pid ($?)\n"), next;
      $host_result{$host} = $? ? 0 : 1;
      1;
}
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