Monks,

I have need of your guidance, for the following problem baffles my impure mind.

The following code works insofar as the Dumper output contains what I expect, but there is no parallel processing -- the children seem to operate sequentially rather than simultaneously.

If I execute the code in the second listing, I can't capture the output, but the children fork and work in parallel.

What am I doing wrong?

Thanks

Listing 1. 
#!/usr/bin/perl -w
use strict;
use warnings;
use Data::Dumper;
use IO::Handle;


my @servers = qw/server1 server2 server3/;
chomp @servers;

my %results = ();
my @kids = ();
for my $host (@servers) {
    pipe(CHILD_RDR,PARENT_WTR);
    PARENT_WTR->autoflush(1);
    if (my $pid = fork) {
        close PARENT_WTR;
        push @kids,$pid;
        $results{$host} = receive_message(\*CHILD_RDR);
        close CHILD_RDR;
    } else {
        die "Cannot fork: $!\n" unless defined $pid;
        close CHILD_RDR;
        my $cmd = q{/usr/bin/ssh } . $host . q{ /bin/date 2>/dev/null}
+;
        my $result = qx{$cmd};
        chomp $result;
        send_message(\*PARENT_WTR,$result);
        close PARENT_WTR;
        exit;
    }
}

for (@kids) {
    waitpid($_,0);
}

print Dumper %results;

sub send_message {
    my $handle = shift;
    my $msg = shift;
    print $handle $msg;
    return;
}

sub receive_message {
    my $handle = shift;
    my $msg;
    chomp($msg = <$handle>);
    return $msg;
}

[download]
Listing 2. 
#!/usr/bin/perl -w
use strict;
use warnings;
use Data::Dumper;
use IO::Handle;


my @servers = qw/server1 server2 server3/;
chomp @servers;

my @kids = ();
for my $host (@servers) {
    if (my $pid = fork) {
        push @kids,$pid;
    } else {
        die "Cannot fork: $!\n" unless defined $pid;
        my $cmd = q{/usr/bin/ssh } . $host . q{ /bin/date 2>/dev/null}
+; 
        my $result = qx{$cmd};
        chomp $result;
        print "$host $result\n";
        exit;
    }
}

for (@kids) {
    waitpid($_,0);
}

[download]

In reply to Forking children operate sequentially?! by darwinscusp

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