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

Hi,

I'm clearly doing something stupid but I can't see what it is. I've broken the test code down to a small case that exhibits the odd behaviour. Can anyone see/explain what I'm doing wrong here? (Apart from using fork() on winnt at all, that is.)

#!C:/Activeperl/bin/perl.exe 
use strict;
use warnings;

open (IN,  "fork2.ini") or die("Can't open input file $!\n");

my $pid=0;
my $x=0;
while (my $computer=<IN>) {

       chomp($computer);
       $x++;   
       
    $pid = fork();
    if ($pid) { 
        print STDERR "Parent $$ got PID $pid from fork($$ $computer)\n
+"; 
        sleep(1);
    }else { 
          print STDERR "\t$$ $computer\n";
          exit(0);         
    }

       if($x > 4) {
          do { $pid=wait() } while ($pid != -1);
          $x=0;
       }
}

do { $pid=wait() } while ($pid != -1);
print "wait() ed until $pid was returned.\n";
   
close(IN);

__DATA__
Computer1
Computer2
Computer3

[download]

The file "fork2.ini" contains the asme as the __DATA__ part... another weirdness is that if I don't use <IN> and use <DATA> instead it prints out the source code!

The output I get from running this is:

C:\logchecks>fork3
Parent 433 got PID -605 from fork(433 test1)
        -605 test1
Parent 433 got PID -461 from fork(433 test2)
        -461 test2
Parent 433 got PID -533 from fork(433 test3)
        -533 test3
Parent 433 got PID -105 from fork(433 test2)
        -105 test2
Parent 433 got PID -521 from fork(433 test3)
        -521 test3
Parent 433 got PID -670 from fork(433 test3)
        -670 test3
wait() ed until -1 was returned.
Done

As you can see, it's printing values from the files out more than once. (Which I don't think it should.)

Seeking enlightenment... Mark.

UPDATE: if I change a couple of things it works... 

my (@Computers=(<IN>); #Do this
#while (my $computer=<IN>) {           #change this
while (my $computer=pop @Computers) {  #to this

[download]

I still don't understand what was going wrong in the original, however.

Replies are listed 'Best First'.
Re: Win32 fork() "problem"
by dawn (Novice) on Mar 26, 2004 at 11:49 UTC
    I've never used them, but a perl book I read recently suggests Win32::Spawn or Win32::Process modules if Fork is being difficult (Fork is introduced experimentally on Win32 systems in Perl 5.6.1). The latter is more robust, more advanced but harder.
[reply]

      Hi, dawn

      I've looked at those modules already and will probably end up utilising Win32::Job on an XP box at some point. This has the benefit of easily being able to set a timeout for a command using $job->run($cmd,$timeout) which is a huge improvement for Win32 platforms.

      I was really hoping I had made a glaring logic error (rather than fork() just playing up) in my use of fork...

      If I can't get a simple case like this to work (does this work on another platform?) I'll have to resort to a more brute force method of divvying up work. :-(

[reply]
[d/l]
        FYI, your code works fine on Linux. Here is the output: 
        Parent 962 got PID 963 from fork(962 Computer1)
        963 Computer1

+Parent 962 got PID 964 from fork(962 Computer2)
        964 Computer2

+       965 Computer3
Parent 962 got PID 965 from fork(962 Computer3)

+Parent 962 got PID 966 from fork(962 )
        966 

+wait() ed until -1 was returned.

        [download]
        Another module that you might look into is IPC::Run. I know that it has at least some Win32 functionality.
[reply]
[d/l]
Re: Win32 fork() "problem"
by runrig (Abbot) on Mar 27, 2004 at 09:33 UTC
    I get similar behavior on Win2000. The fork appears to mess with the file handle:
    #!/usr/bin/perl

use strict;
use warnings;

my $x=0;
while (my $computer=<DATA>) {
       print "computer: [$computer]";
       chomp($computer);
       $x++;   
       
    my $pid = fork();
    if ($pid) { 
        print "Parent $$ got PID $pid from fork($$ $computer)\n"; 
        sleep(1);
    }else { 
          print "\t$$ $computer\n";
          exit(0);         
    }

       if($x > 4) {
          do { $pid=wait() } while ($pid != -1);
          $x=0;
       }
}

my $pid;
do { $pid=wait() } while ($pid != -1);
print "wait() ed until $pid was returned.\n";
   
close(DATA);

__DATA__
Computer1
Computer2
Computer3

# Output
computer: [Computer1
]Parent 1782987 got PID -1853375 from fork(1782987 Computer1)
        -1853375 Computer1
computer: [
]Parent 1782987 got PID -1852747 from fork(1782987 )
        -1852747
computer: [Computer2
]Parent 1782987 got PID -1805143 from fork(1782987 Computer2)
        -1805143 Computer2
computer: [
]Parent 1782987 got PID -1804643 from fork(1782987 )
        -1804643
computer: [Computer3
]Parent 1782987 got PID -1898099 from fork(1782987 Computer3)
        -1898099 Computer3
wait() ed until -1 was returned.

    [download]
    The problem goes away if you slurp the input first, like:
    for my $computer (<DATA>) {
...

    [download]
[reply]
[d/l]
[select]
Re: Win32 fork() "problem"
by terris (Initiate) on Mar 28, 2004 at 16:14 UTC
    You're lucky. In my script, fork() causes perl.exe to exhibit bizarre behavior and crash. ActivePerl 809.
[reply]

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