I got the alarm to work. I did encounter one problem: after the pcap_breakloop() executed, the pcap_loop() return code would indicate that no packets were captured, even if there were some. I may have an error, or it may be an older pcap library.

Versions I used:

• os: Ubuntu 8.04 LTS
• perl: v5.8.8 built for i486-linux-gnu-thread-multi
• Net-Pcap: 0.16 (latest & greatest)
• pcap: 0.9.8 (aka libpcap0.8 under Ubuntu 8.04)

Test code:

#!/usr/bin/perl

use strict;
use warnings;
use diagnostics;
use Net::Pcap;

$| = 1;     # autoflush stdout

my $dev = "eth1";
my $err = '';

my $pcap = Net::Pcap::open_live($dev, 1500, 1, 0, \$err);
die("open_live error upon open of $dev: $err") if !defined $pcap;

my $packet_counter = 0;
my $maxcap = 10;

# First try: just capture a few packets without any timeout:
loopy($maxcap, \&capture, \$packet_counter, "1");

# Second try: capture a few packets with timeout inside an eval
eval
{
    local $SIG{'ALRM'} = \&AlarmHandler;
    alarm 10;
    loopy($maxcap, \&capture, \$packet_counter, "2");
    alarm 0;
};

sub loopy
{
    my ($cnt, $callback, $udata, $id) = @_;
    print tim()."loop $id: listening...\n";
    my $looprc = Net::Pcap::loop($pcap, $cnt, $callback, $udata);
    if    ($looprc ==  0) { print tim()."loop $id: $cnt packets captur
+ed\n"; }
    elsif ($looprc == -1) { print tim()."loop $id: error\n"; }
    elsif ($looprc == -2) { print tim()."loop $id: breakloop (nothing 
+captured)\n"; }
    elsif ($looprc >   0) { print tim()."loop $id: breakloop ($looprc 
+captured)\n"; }
    else                  { print tim()."loop $id: unknown code $loopr
+c\n"; }
}

sub capture
{
    my ($user_data_ref, $header, $packet) = @_;
    $$user_data_ref++;
    print tim()."capture: got packet $$user_data_ref\n";
}

sub tim { localtime().": "; }

sub AlarmHandler
{
    my $signal = shift;
    print STDERR tim()."AlarmHandler: $signal signal received\n";
    Net::Pcap::breakloop($pcap) if defined($pcap);
}
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Sample output (case one - nothing captured with timeout):

Mon Nov  9 11:09:16 2009: loop 1: listening...
Mon Nov  9 11:10:00 2009: capture: got packet 1
Mon Nov  9 11:10:00 2009: capture: got packet 2
Mon Nov  9 11:10:05 2009: capture: got packet 3
Mon Nov  9 11:10:05 2009: capture: got packet 4
Mon Nov  9 11:10:19 2009: capture: got packet 5
Mon Nov  9 11:10:19 2009: capture: got packet 6
Mon Nov  9 11:10:22 2009: capture: got packet 7
Mon Nov  9 11:10:22 2009: capture: got packet 8
Mon Nov  9 11:10:24 2009: capture: got packet 9
Mon Nov  9 11:10:24 2009: capture: got packet 10
Mon Nov  9 11:10:24 2009: loop 1: 10 packets captured
Mon Nov  9 11:10:24 2009: loop 2: listening...
Mon Nov  9 11:10:34 2009: AlarmHandler: ALRM signal received
Mon Nov  9 11:10:34 2009: loop 2: breakloop (nothing captured)
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Sample output (case two - something captured, then timeout):

Mon Nov  9 11:12:49 2009: loop 1: listening...
Mon Nov  9 11:13:12 2009: capture: got packet 1
Mon Nov  9 11:13:12 2009: capture: got packet 2
Mon Nov  9 11:13:17 2009: capture: got packet 3
Mon Nov  9 11:13:17 2009: capture: got packet 4
Mon Nov  9 11:14:15 2009: capture: got packet 5
Mon Nov  9 11:14:15 2009: capture: got packet 6
Mon Nov  9 11:14:20 2009: capture: got packet 7
Mon Nov  9 11:14:20 2009: capture: got packet 8
Mon Nov  9 11:15:20 2009: capture: got packet 9
Mon Nov  9 11:15:20 2009: capture: got packet 10
Mon Nov  9 11:15:20 2009: loop 1: 10 packets captured
Mon Nov  9 11:15:20 2009: loop 2: listening...
Mon Nov  9 11:15:25 2009: capture: got packet 11
Mon Nov  9 11:15:25 2009: capture: got packet 12
Mon Nov  9 11:15:30 2009: AlarmHandler: ALRM signal received
Mon Nov  9 11:15:30 2009: loop 2: breakloop (nothing captured)    <== 
+problem: should say 2 captured
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