Hi, I was toying with the idea of using IO::Select to watch filehandles printed to by threads, as a way of getting feedback directly to the main thread. Alas, I was tripped up by the fact that to get a fileno, which a thread needs for this trick, a disk file must actually be opened. I was looking for a way to avoid disk files. Pipes to the rescue. This script demonstrates how to run an IO::Select object in the main thread, collecting output from threads printing into individual pipes. There is a small trick involved with IO::Pipe, where you need to pass the pipe object off to the threads, before calling the reader method. IO::Pipe objects change once reader() or writer() are called on them.

So here is a basic example. I just compute primes in ranges in the threads, but the results are all fed directly back to the main thread, without any shared variables involved. I use 1 shared var to monitor how many detached threads are running, as the various methods for monitoring like threads->is_running or threads->list() are useless on detached threads.

I don't have a true multi-processor system, so I can't say whether this is faster than just running one big sequential file, but it is a useful techique for monitoring the output of threads. 

#!/usr/bin/perl
use warnings; 
use strict;
use threads;
use threads::shared;
use IO::Select;
use IO::Pipe;

my @ranges = ( [1,100000],[1000001,200000],[200001,300000],
               [300001,400000],[400001,500000] );

my $sel = new IO::Select();

# thread launching

my $count:shared; # a simple shared counter to detect
$count = 0;       # when all detached threads have exited

foreach (@ranges){

    my $pipe = IO::Pipe->new();
    my $start = $_->[0];
    my $end = $_->[1];
#    print "$start $end $pipe\n";

  threads->create( \&thread, $start, $end, $pipe )->detach;
  
  $count++;

  # only call reader after pipe has been passed to thread  
  $sel->add( $pipe->reader() );
}

# watching thread output
print "Watching\n\n";

while(1){     
     
     # use a timeout appropriate for the task
     # as it will trigger the exit test, measured in seconds
     foreach my $h ($sel->can_read(1) ){
         my $buf;
         if ( (sysread($h, $buf, 4096) > 0 ) ){ 
           print "Main says: $buf\n"; 
         }
     if ($count == 0){ print "Done\n"; exit}
     }
}


sub thread{

my( $start, $finish, $pipe ) = @_;

my $wh = $pipe->writer;
$wh->autoflush(1);

print $wh  "thread# ",threads->tid()," -> $start, $finish, $pipe \n"  
+;

foreach my $num ($start .. $finish){
  if ( is_prime($num) ){ 
         print $wh "thread #",threads->tid()," ->$num\n";
        }   
}

sub is_prime {  # from a Merlyn column
 my $guess = shift;
 for (my $divisor = 2; $divisor * $divisor <= $guess; $divisor++) {
   return unless $guess % $divisor;
   }
 return 1;
}

print $wh  "\t\t\t\t\t\t thread# ",threads->tid()," -> finishing \n"  
+;

sleep 1;  # let pipe flush buffers

$count--;

}

__END__

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In reply to Using Select and pipes to communicate with threads by zentara

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