#!/usr/bin/perl
$| = 1;
sleep 10;
print "x\n";
while (1) { sleep 10; }
[download]

#!/usr/bin/perl

open my $fh, "./child.pl |";
close $fh;
[download]

Here's the relevant piece of the real code:

  ...
  my $active = 1;
  my $timeout = 20;
  my $an;

  open my $fh, "tcpdump -nn -l -e dst 224.111.111.112 and not igmp |";
  $read_set->add($fh);

  while ($active) {
    my ($rh_set) = IO::Select->select($read_set, undef, undef, 1);

    if (defined $rh_set) {
      if ($_ = <$fh>) {
        # do some parsing on the line
        $an = $1;
        last;
      } else {
        # tcpdump died. real code will sleep 2 seconds
        # and then close and reopen $fh here
      }
    }
    if (--$timeout == 0) { $active=0; }
  }
  close $fh;
[download]

What I want is to either capture the first packet that passes the tcpdump filter ($an set to the source IP), or if no packet comes in before the timeout is over, leave the loop with $an still undefined.

Currently I'm working around this problem by explicitely starting tcpdump in a fork()ed child and killing it after the timeout. This works but I have to use a temp file or a pipe to store the tcpdump output, and it's a lot more code than something like the above.

I'm open to whatever solutions the wise monks can come up with, even if it means going about it a whole other way.