For all your forking needs..

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

so I was bugging my friend to teach me forking for a long time and he's tried a million times and I just didn't get it.. well, for this project I'm currently working on, it was necessary to fork a few processes and keep a set number of those running at any given time.. so here's a stripped down version of what I came up with based on his code and after reading over the docs for fork() and wait() a few times:

#!/usr/bin/perl

use strict;
my $MAX_PROCESSES=10;

 my $npids=0;
 for(1..15) {
   my $pid;
   print "Forking: $_\n";
   sleep 1;
   $pid=fork();
   if($pid > 0) { #we forked successfully
        $npids++;
        if($npids>=$MAX_PROCESSES)
        {
                my $wait=wait();
                if($wait) { $npids--; }
        }
        elsif(undef $pid) { # we didn't fork successfully
                print "fork error!\n";
        }
        else { #what do we want to do?
                &doit($_);
                exit(0); # free this pid
        }
        exit(0); # we shouldn't get to this point
   }
  }
  # if we have any stragglers, lets wait for them to finish.
  for(1..$npids){
    my $wt=wait();
   if($wt==-1){
    #print "hey $!\n";
    redo;
   }
  }


sub doit {
        my $num = shift;
        sleep 5;
        print "$num DONE\n";
}
[download]

and yes, the code works just as I would expect in the production version, but in this version it does something odd that I was wondering if someone could clarify for me..
if I run the script from the command line, as the first "doit()" "exits" I get thrown back to a command prompt and the script carries on as usual in the background.. I was thinking this could be dangerous if someone accidentally managed to fork again inside that function, cause you can't just ctrl+c it to kill the program or it could spawn an infinite number of child processes you can't kill or something.. should I be using something other than exit to "end a child's life" :)

thanks..
-brad..

Comment on For all your forking needs.. Download Code

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Re: For all your forking needs.. by chromatic (Archbishop) on Oct 13, 2000 at 01:33 UTC
It makes sense to me. I'm not sure what you intended your logic to be, but it's a little confused at the moment. Try this: `if($pid > 0) { #we forked successfully $npids++; if($npids>=$MAX_PROCESSES) { my $wait=wait(); if($wait) { $npids--; } } elsif(undef $pid) { # we didn't fork successfully print "fork error!\n"; } } else { # $pid == 0, we're a kid, so what do we want to do? &doit($_); exit(0); # free this pid }` [download] The first time through your loop, $npids is less than $MAX_PROCESSES, but $pid is defined (both in the parent and in the child), so it takes the third option. The parent (!!) goes to do_it(), sleeps for five seconds, and then exits. Oops. I figure you probably want the children to exit. The one good way of telling the parent from the child is the value of $pid. If it's positive, you're in the parent. If it's zero, you're in a child. If it's undefined, the fork failed. (I'd move the failure code to the fork() statement.) I can't figure out where AgentM got his comment, though. There's nothing in the newest edition of Programming Perl about wait being deprecated, and waitpid takes a different approach.	[reply] [d/l]
RE: Re: For all your forking needs.. by reyjrar (Hermit) on Oct 13, 2000 at 20:12 UTC
heh.. I knew something was wrong, and as it turns out, in the production version, the one with all the real variables and processes, I actually did code the if elsif else correctly, just mangled it when I made that test program! :) thanks for catching that for me and letting me know where I went wrong.. -brad.	[reply]
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