for ($i=0; $i<$g; $i++) {
  pipe ($rh, $wh)          # both of these are generated in the loop
  if ($pid[$i]=fork()) {   # parent process
    waitpid($pid[$i], 0);  # wait for child
    close ($wh);
    while (<$rh>) {        # gimme the output
      if ($_ =~ m/^Error/) { push (@error, $_); }
      elsif ($_ =~ m^Hits/) { push (@hits, $_); }
      else { push (@data, $_); }  # each is a result from an engine
    }
  }
  else {
    close ($rh);
    open (STDOUT, ">&$wh");
    open (OUT, "| $caller");  # call the handler with arguments
    close (OUT);
    exit(0);
  }
}
[download]

I fear your version does not work in parallel!
I just tested it with this variant:


$g=10;
$|=1;
for ($i=0; $i<$g; $i++) {
  pipe ($rh, $wh);         # both of these are generated in the loop
  if ($pid[$i]=fork()) {   # parent process
    waitpid($pid[$i], 0);  # wait for child
    close ($wh);
    while (<$rh>) {        # gimme the output
      if ($_ =~ m/^Error/) { push (@error, $_); }
      elsif ($_ =~ m/^Hits/) { push (@hits, $_); }
      else { push (@data, $_); }  # each is a result from an engine
    }
  }
  else {
    close ($rh);
    open (STDOUT, ">&$wh");
    $|=1;
    print "I will wait for ",$i+2," seconds\n";
    sleep $i+1;
    print "Good morning. I slept about ",$i+2," seconds\n";
    exit(0);
  }
}

print @data;
[download]

Two problems:
1. It really doesn't work in parallel
2. It doesn't give the output to the parent
It produces this output:

I will wait for 2 seconds
Good morning. I slept about 2 seconds
I will wait for 3 seconds
Good morning. I slept about 3 seconds
I will wait for 4 seconds
Good morning. I slept about 4 seconds
I will wait for 5 seconds
Good morning. I slept about 5 seconds
I will wait for 6 seconds
Good morning. I slept about 6 seconds
I will wait for 7 seconds
Good morning. I slept about 7 seconds
I will wait for 8 seconds
Good morning. I slept about 8 seconds
I will wait for 9 seconds
Good morning. I slept about 9 seconds
I will wait for 10 seconds
Good morning. I slept about 10 seconds
I will wait for 11 seconds
Good morning. I slept about 11 seconds
[download]

But the output should be something like this:

I will wait for 2 seconds
I will wait for 3 seconds
I will wait for 4 seconds
I will wait for 5 seconds
I will wait for 6 seconds
I will wait for 7 seconds
I will wait for 8 seconds
I will wait for 9 seconds
I will wait for 10 seconds
I will wait for 11 seconds
Good morning. I slept about 2 seconds
Good morning. I slept about 3 seconds
Good morning. I slept about 4 seconds
Good morning. I slept about 5 seconds
Good morning. I slept about 6 seconds
Good morning. I slept about 7 seconds
Good morning. I slept about 8 seconds
Good morning. I slept about 9 seconds
Good morning. I slept about 10 seconds
Good morning. I slept about 11 seconds
[download]

which I produced with a variant of my former version:

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

my $children= 10;
my @out;
my $first_child;

$|=1;

my $pid= open($first_child, '-|');
if ($pid) {
    # parent
    @out= <$first_child>;
}
else {
    my(@child, @childpid);
    for (my $i=0; $i<$children; ++$i) {
        $childpid[$i]= open($child[$i], '|-');
        if ($childpid[$i]) {
            # do nothing yet
        }
        else {
            print "I will wait for ",$i+2," seconds\n";
            sleep $i+1;
            print "Good morning. I slept about ",$i+2," seconds\n";
            exit;
        }
    }
    foreach (@child) {
        close $_;
    }
    exit;
}

print @out;
[download]

This is good, but not quite right. The array @out is working nicely.
It seems that when I try to adapt this to my needs, I only get results from one of my search engines.

I tried eliminating one of the exit; statements, but it doesn't quite work right. Eliminating the first one doesn't change anything. Eliminating the second does give me all the outputs, but give me a lot of additional garbage with it, and messes up the output. Here is the code as it stands now.



my @out;
my $first_child;

$|=1;

my $pid= open($first_child, '-|');
if ($pid) {
    # parent
    @out= <$first_child>;
}
else {
    my(@child, @childpid);
    for (my $i=0; $i<$children; ++$i) {
        $childpid[$i]= open($child[$i], '|-');
        if ($childpid[$i]) {
            # do nothing yet
        }
        else {
            $call = <Something I make here>;  # sorry, can't show that
            open (OUT, "| $call");
            close (OUT);
            exit;
        }
    }
    foreach (@child) {
        close $_;
    }
    exit;
}
[download]


$call is a PERL call that is generated by using information gathered elsewhere in the program. It is built just prior to the call to run it, and will be different for each iteration of the for loop.
Then I work with @out in the rest of the code. I'm thinking that the issue is with how I run $call, but I'm not sure how else to do this.

IPC is new to my coding bag of tricks, so I appreciate all the input everyone has given.