I am not able to join a thread when another child process is running which was forked using open.

[ashishyadavlko1]

I want to do following things from main thread/process: 1. Communicate to another process using pipe. 2. Create threads to do certain task. 3. Wait for all the threads to complete. Following is the pseudo code I am trying:

 use threads;
 use IO::Handle;
 sub dummy {
      print "\n!!!!". $$;
      return 0;
 }

 open($handle, "| cat -v") || die "Unable to open connection to BT Dri
+ver: $!\n";

 $handle->autoflush(1);

 #close $handle; If I uncomment this, threads can be joined. But I don
+'t want to terminate this child process.

 $thr2 = threads->create(\&dummy);
 sleep 2;
 print "\n$thr2";
 foreach $thr (threads->list(threads::joinable))
 {
      print "\nIam here";
      print "\n!!!". $thr;
      $thr->join();
 }

Code gets stuck when I try to join the thread even though it is joinable. Am I doing something fundamentally wrong here? I am using Perl 5.10.0

[choroba]

Crossposted at StackOverflow. It is more polite to inform people about the fact so they do not have to spend time on a problem already solved somewhere else.

[Anonymous Monk]

I think maybe, based on things I've read

One problem is you're using 5.10.0, which had some thread issues. Newer is better.

Another problem is pipes and threads may not be reliable in general, at least probably not on windows, has to do with blocking reads.

If you're using threads, forget about pipes, communicate through an array.

If you still want pipe communication, try forks (unless you're on windows), its threads api implemented using fork.

See Non-blocking Reads from Pipe Filehandle, any module with streaming pipe implementation for use with threads and other [responses on threads by BrowserUk

[BrowserUk]

Hm. Your code, with a couple of minor tweaks works fine for me:

#! perl -slw
use strict;
use threads;
use IO::Handle;

sub dummy {
    print "\n!!!!". $$;
    return 0;
}

open( my $handle, "| cat -v") || die "Unable to open connection to BT 
+Driver: $!\n";

$handle->autoflush(1);

#close $handle; If I uncomment this, threads can be joined. But I don'
+t want to terminate this child process.

my $thr2 = threads->create(\&dummy);

sleep 2;

print "\n$thr2";
for my $thr (threads->list(threads::joinable)) {
    print "\nIam here";
    print "\n!!!". $thr;
    print $thr->join();
}

__END__
C:\test>junk72

!!!!288

threads=SCALAR(0x37b820)

Iam here

!!!threads=SCALAR(0x367440)
0

Try it. Then try and work out why.

[thargas]

from perlthrtut:

Thinking of mixing "fork()" and threads? Please lie down and wait until the feeling passes. Be aware that the semantics of "fork()" vary between platforms. For example, some UNIX systems copy all the current threads into the child process, while others only copy the thread that called "fork()". You have been warned!

[zentara]

want to do following things from main thread/process: 1. Communicate to another process using pipe. 2. Create threads to do certain task. 3. Wait for all the threads to complete.

You might find some ideas in Using Select and pipes to communicate with threads

[sundialsvc4]

Here is what I recommend:

1. First, get your thoughts in order.   Devise what you want the end result to be, and avoid getting distracted by how to get to those results.
2. Now, as for “how to get to those results,” start hauling out the big-guns like Parallel::ForkManager.

What I am really trying to say here is that, “the real important thing about the Perl language is:   http://search.cpan.org.   Where, at this particular moment in time, you will find 75424 Uploads, 24,169 Distributions; 103,218 Modules; 9,487 Uploaders, literally about every conceivable subject or task including rocket-science.   Therefore, no matter what it is that you are doing, you do a lot less “original programming” and a lot more “picking and choosing.”   And so, that is what I think you should be doing here.

When you install almost any of those packages onto your system, it subjects itself to hundreds or even thousands of self-tests.   It has been subjected (via the so-called CPANTS testing-service) to test-runs on a myriad of actual environments.   Links to this, and to bug and issue reports, are all there on the same page.   So, you have just been handed a magic bridge that runs over many stinking swamps of wasted time.

[BrowserUk]

And my recommendation to you is to take up knitting and avoid eating the yellow snow.