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

Hi. I've got some code that successfully forks off several child processes that do the same thing with different inputs.

for ($i = 1; $i <= 3; $i++) {
  if ($pid=fork) {
    $return = &my_function($i);
    print "Returned: $return\n";
    exit;
  }
}

print "All done.\n";
The problem is that the "All done" is printed firstly, and then the "Returned:" lines are printed. The output looks like this:

All done.
Returned: sub-process 3 is done.
Returned: sub-process 2 is done.
Returned: sub-process 1 is done.
What's the best way to wait for all those subprocesses to finish (don't worry, I have timeout trapping in &my_function) before proceeding with the rest of the code?

I've read the docs on perlipc and fork but can't quite find what I want. Thanks in advance for your help.

Replies are listed 'Best First'.
Re: Forking and waiting
by lhoward (Vicar) on May 29, 2000 at 07:57 UTC
[reply]
      Right, I should have mentioned that I looked at both of those, too.

      I just tried this variation with some success:

      my ($i, $return, $pid, @pids);
for ($i = 1; $i <= 3; $i++) {
  if ($pid=fork) {
    push @pids, $pid;
  } elsif (defined $pid) {
    $return = &my_function($i);
    print "Returned: $return\n";
    exit;
  }
}

foreach my $childpid (@pids) {
  waitpid($childpid, 0);
}

print "All is done!\n";
      Does that seem a reasonable/efficient way to handle this?

      Thanks.

[reply]
        just wait for the CHILD in the parent: 

        my ($i, $return, $pid, @pids);
   for ($i = 1; $i <= 3; $i++) {
       if ($pid=fork) {
           push @pids, $pid;
           waitpid($pid,0);
       } elsif (defined $pid) {
           $return = &my_function($i);
           print "Returned: $return\n";
           exit;
       }
   }
print "All done.\n";

        [download]
[reply]
[d/l]

        Y'know, for ($i = 1; $i <= 3; $i++) {} is a horribly un-Perlish way to write for (1..3) {}

[reply]
[d/l]
[select]
Sometimes there is text held in the output buffer
by Corion (Patriarch) on May 29, 2000 at 13:55 UTC
    I remember that I got once quite confused by this, but sometimes Perl (and DOS :)) buffer output for a complete line or a certain size. Setting autoflush (I think it's $|) for output by using 
      $| = 1;

    [download]
    does this (but I don't know if it applies).
[reply]
[d/l]

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