Uninitialized value in numeric eq (==) after 65 iteration of a loop using fork?!?

doowah2004 has asked for the wisdom of the Perl Monks concerning the following question:

Hi All,

I have been farting and fooling around with fork, and I was trying to see the time difference in having child processes create more child processes to complete a task vs the parent creating all the children. so here is what I did:

#!/usr/bin/perl -w
use strict;
use Time::HiRes qw(usleep ualarm gettimeofday tv_interval);

open (FILE, ">testfork.txt");
my $runs = 33;
my $start = [gettimeofday];

while ($runs) {
        my $pid = fork;

        # Parent
        if ($pid) { $runs--; }

        # Child
        if ($pid == 0) {
                my $pid = fork;
                if ($pid == 0) {
                    my $pid = fork;
                    if ($pid == 0) {
                        print FILE "$runs\n";
                        exit;
                    }
                    print FILE "$runs\n";
                    exit;
                }
                print FILE "$runs\n";
                exit;
        }

}
my $elapse = tv_interval ($start);
close (FILE);

print "Elapsed time = $elapse\n";
exit;
[download]

This should create 99 lines in the output file and it does. I then change the code to:

#!/usr/bin/perl -w
use strict;
use Time::HiRes qw(usleep ualarm gettimeofday tv_interval);

open (FILE, ">testfork.txt");
my $runs = 99;
my $start = [gettimeofday];

while ($runs) {
        my $pid = fork;

        # Parent
        if ($pid) { $runs--; }

        # Child
        if ($pid == 0) {
#                 my $pid = fork;
#                 if ($pid == 0) {
#                     my $pid = fork;
#                     if ($pid == 0) {
#                         print FILE "$runs\n";
#                         exit;
#                     }
#                     print FILE "$runs\n";
#                     exit;
#                 }
                print FILE "$runs\n";
                exit;
        }

}
my $elapse = tv_interval ($start);
close (FILE);

print "Elapsed time = $elapse\n";
exit;
[download]

This should also create 99 lines in the output file, but consistently after 65 lines I get the following:

Use of uninitialized value in numeric eq (==) at testfork1.pl line 16.

I can not see why this is happening. Does this happen to anyone else running the code? If can anyone enlighten me on why?

Thank you all in advance,

Cameron

Comment on Uninitialized value in numeric eq (==) after 65 iteration of a loop using fork?!? Select or Download Code

Replies are listed 'Best First'.
Re: Uninitialized value in numeric eq (==) after 65 iteration of a loop using fork?!? by ccn (Vicar) on Sep 13, 2004 at 14:24 UTC
perldoc -f fork It returns the child pid to the parent process, 0 to the child process, or "undef" if the fork is unsuccessful. When you compare undefined `$pid` with any number you get warning You can make something like this: `die "Can't fork: $!" unless defined $pid;`	[reply] [d/l] [select]
Re: Uninitialized value in numeric eq (==) after 65 iteration of a loop using fork?!? by zejames (Hermit) on Sep 13, 2004 at 14:30 UTC
Your code is working fine here (ActivePerl 5.8 and Windows XP). I have to agree with ccn : there may be a system limit preventing you from create 99 child processes at once. Could be give us more information about your system limit ? Kind regards -- zejames	[reply]
Re^2: Uninitialized value in numeric eq (==) after 65 iteration of a loop using fork?!? by doowah2004 (Monk) on Sep 13, 2004 at 14:45 UTC
Apparently that is the problem, I updated the code with die and I get: Can't fork: Resource temporarily unavailable at testfork1.pl line 14. There must be some sort of limit. My system is ActivePerl 5.8.3, Win XP SP2. I am unsure how to check or change the ability of my system to create child processes, but it looks like that might be the problem. It is interesting that I did not run into this with the code that had children creating children? zejames, out of curiousity, what times did you get when running the code? Thanks all for your quick and correct insight to my prob. Cameron	[reply]
Re^3: Uninitialized value in numeric eq (==) after 65 iteration of a loop using fork?!? by zejames (Hermit) on Sep 13, 2004 at 15:03 UTC
In fact, your code did not work on my machine either : I ran it under Perl builder, which provided me no error except that it did not terminate it. Running it on the comment line was more interesting and gave the same result as your. Interesting to see a different behaviour when using Perl Builder or not. So I've got no figure for you ;) -- zejames	[reply]
Re: Uninitialized value in numeric eq (==) after 65 iteration of a loop using fork?!? by BrowserUk (Patriarch) on Sep 13, 2004 at 20:40 UTC
There is (or appears to be, though I haven't found it documented anywhere) a 64 (concurrent) child limit to the win32 fork emulation. This is probably due to the use of `WaitForMultipleObjects(Ex)` to implement wait and/or waitpid which has a maximum number of "objects" it can wait on `WinNT.h:#define MAXIMUM_WAIT_OBJECTS 64 // Maximum number of wait objects` This limit could be lifted, as it has been with threads (win32 forks are threads underneath). Examine what is said, not who speaks. "Efficiency is intelligent laziness." -David Dunham "Think for yourself!" - Abigail "Memory, processor, disk in that order on the hardware side. Algorithm, algorithm, algorithm on the code side." - tachyon	[reply] [d/l] [select]
Re: Uninitialized value in numeric eq (==) after 65 iteration of a loop using fork?!? by TheEnigma (Pilgrim) on Sep 13, 2004 at 16:25 UTC
I've never used fork before, so I tried your code to see if I could learn something. I got the same errors as the rest of you, failing after 65 lines (printing 99 to 35). But if I add either of the following before the child if statement `if(!defined $pid){ die "$runs: $!"; } OR if(!defined $pid){ print FILE "undefined at $runs\n"; $runs--; next; }` [download] it only prints 64 times (99 to 36). Actually, that's what I would expect. Why do we get an error on the `if ($pid == 0) {` line, but then it still goes and prints one more time? I also find it interesting that when I use my second if statement above, it prints down to 37, then all the undefined lines; and then it prints 36. Is there something significant about that? TheEnigma	[reply] [d/l] [select]


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