Re: Child process inter communication

[tybalt89]

Here's my guess.

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1192662

use strict;
use warnings;
use IO::Select;

my $childcount = 3;
my $hasterminal = 1;
my %who;
my %pipes;

for my $from (1 .. $childcount)
  {
  for my $to (1 .. $from - 1, $from + 1 .. $childcount)
    {
    pipe( my $readhandle, my $writehandle) or die "$! on pipe";
    $writehandle->autoflush;
    $pipes{$from}{$to}{rh} = $readhandle;
    $pipes{$from}{$to}{wh} = $writehandle;
    }
  }

for my $me (1 .. $childcount)
  {
  if( my $pid = fork ) # parent
    {
    $who{$pid} = $me;
    }
  elsif( defined $pid ) # child
    {
    my $sel = IO::Select->new;
    $me == $hasterminal and $sel->add(*STDIN);
    for my $from (1 .. $me - 1, $me + 1 .. $childcount)
      {
      $sel->add($pipes{$from}{$me}{rh});
      close $pipes{$from}{$me}{wh};
      }
    while(1)
      {
      for my $handle ($sel->can_read)
        {
        defined( my $command = <$handle> ) or exit;
        print "$me got $command";
        $command =~ /^(\d+)\s+(.*\n)/ and $1 != $me and
          print { $pipes{$me}{$1}{wh} } $2;
        }
      }
    }
  else
    {
    die "fork failed with $!";
    }
  }

use Data::Dump 'pp'; pp \%who;
my $pid = wait; # on first exit, kill rest
print "$who{$pid} exited\n";
kill 15, keys %who;
print "$who{$pid} exited\n" while ($pid = wait) > 0;

$hasterminal determines which child listens to the terminal for commands.
Commands with a leading number are forwarded to that child.
If you type in

2 3 foobar

you get back

1 got 2 3 foobar
2 got 3 foobar
3 got foobar

showing that child 1 got the message, then forwarded it to child 2, who then forwarded it to child 3.

Any child can send directly to any other child using the pipes set up in the hash %pipes before the children are forked.

Note that this is just a prototype and should not be considered production code (there's a bit of cheating going on in it :)