Windows fork problem or my ignorance?

romandas has asked for the wisdom of the Perl Monks concerning the following question:

Or both? :)

I am trying to use fork() for the first time, via merlyn's column Doing many things, like pings. However, I am trying to do this on ActiveState Perl (5.8.8) on Windows XP, and am not sure if that is the cause of my problem.

Here is my code (well, merlyn's with OS-specific modifications):

#!/perl/bin/perl
use warnings;
use strict;

sub ping_a_host {
  my $host = shift;
  `ping -n 1 -w 1 $host 2>NUL` =~ /Received = 0/ ? 0 : 1;
}

my %pid_to_host;
my %host_result;

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;
  warn "reaping $pid for $host\n";
  $host_result{$host} = $? ? 0 : 1;
  1;
}

my @hosts = map "10.0.1.$_", "025".."026";

for (@hosts) {
  wait_for_a_kid() if keys %pid_to_host > 10;
  if (my $pid = fork) {
    ## parent does...
    $pid_to_host{$pid} = $_;
    warn "$pid is processing $_\n";
  } else { # child does
    ## child does...
    exit !ping_a_host($_);
  }
}

## final reap:
1 while wait_for_a_kid();

for (sort keys %host_result) {
  print "$_ is ", ($host_result{$_} ? "good" : "bad"), "\n";
}
[download]

The code runs, however there are two problems I've noticed. 1) The final print statement never prints, regardless of the system state. 2) If the IP is not up, the process never finishes; it outputs:

C:\Scripts\Parallel>ping_parallel.pl
-3676 is processing 10.0.1.025
-3072 is processing 10.0.1.026
[download]

and then sits there. Any ideas what may be wrong?

Comment on Windows fork problem or my ignorance? Select or Download Code

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Re: Windows fork problem or my ignorance? by ikegami (Patriarch) on Mar 21, 2008 at 12:02 UTC
Some quick debugging shows that the print loop is reached, but that the hash is empty. `return 0 if $pid < 0;` wasn't adjusted for the OS. Windows doesn't support forking. When you call `fork` a thread is actually created. For those pseudo-processes, `$$` is negative, so `wait` can return a negative number on success. Use `return 0 if $pid == -1;` instead.	[reply] [d/l] [select]
Re^2: Windows fork problem or my ignorance? by romandas (Pilgrim) on Mar 21, 2008 at 12:59 UTC
Thanks! That solved some of the problem, at least the non-printing portion of it. However, when I run it against the range 10.0.1.25..30, I get this: C:\Scripts\Parallel>ping_parallel.pl -4012 is processing 10.0.1.025 -3148 is processing 10.0.1.026 reaping -4012 for 10.0.1.025 reaping -3148 for 10.0.1.026 10.0.1.025 is good 10.0.1.026 is good ## This one finished C:\Scripts\Parallel>ping_parallel.pl -2808 is processing 10.0.1.025 -3424 is processing 10.0.1.026 -2532 is processing 10.0.1.027 -3360 is processing 10.0.1.028 -696 is processing 10.0.1.029 -4036 is processing 10.0.1.030 reaping -2808 for 10.0.1.025 reaping -3424 for 10.0.1.026 reaping -2532 for 10.0.1.027 reaping -3360 for 10.0.1.028 reaping -696 for 10.0.1.029 [download] and sits here. I know that .27 - .30 are not up. Any idea why it sits there?	[reply] [d/l]
Re^3: Windows fork problem or my ignorance? by ikegami (Patriarch) on Mar 21, 2008 at 22:56 UTC
I can reproduce the problem. `wait` never returns. Looks like a bug in Perl. Fortunately, it appears to be fixed in 5.10.0. Read more... (1285 Bytes) Maybe it had to do with `` using `wait` as well. On unix, that wouldn't be a problem since the two calls to `wait` happen in different processes. The pseudo-processes `fork` creates on Windows are just threads, so the two calls to `wait` happen in the same process.	[reply] [d/l] [select]
Re: Windows fork problem or my ignorance? by NetWallah (Canon) on Mar 21, 2008 at 17:07 UTC
Perhaps you could recycle some of my old code that does Multi-threaded ping of a list of hosts (Tested successfully on Windows). If you just want to get the Threads part, please view this node. "As you get older three things happen. The first is your memory goes, and I can't remember the other two... " - Sir Norman Wisdom	[reply]
Re: Windows fork problem or my ignorance? (sorting IP4) by ikegami (Patriarch) on Mar 21, 2008 at 23:15 UTC
A better sort: `sub pack_ip4 { pack 'C4', split /\./, @_ ? $_[0] : $_ } for ( map substr($_, 4), sort map pack_ip4($_).$_, keys %host_result ) { ... }` [download] That way, it'll sort the IPs numerically (.30 will be after .29 instead of after .3) without having to resort to adding zeroes.	[reply] [d/l]