$|=1;
[download]

That did the trick. I hadn't come across autoflush before (I guess because I'd never done this sort of thing before with perl). Progress bar is now working.

Thank you Perl Monks, you clever people you :-)

Dan

Thank you for the reply. I'm not saying its definitely not a buffering problem but I don't think it is since it doesn't get printed EVER, even when the other thread is joined and prints its results. I tried adding a newline character to the end of the string, no difference.

Additionally, when the main thread exits it warns "A thread exited while 2 threads were running" as if the progress bar thread was still merrily churning away doing something (frustratingly, not printing a progress bar) when the main thread exited.

Anything else I should be checking?
Thanks
Dan

The problem with the lack of output is almost definitely a buffering issue. Try making the request using wget or with telnetand see exactly what is being returned.

This code demonstrates that the logic works, and it outputs a dot every 10 seconds, but only forces the user to wait a maximum of one second more than the process would have taken anyway:

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

# set a shared value of TRUE for running the progress bar
my $keepRunningProgressBar : shared = 1;

# Set the progress bar running indefinitely
my $progressBar = threads->create(
    sub { 
        my $i = 0;
        while ($keepRunningProgressBar == 1) {
            if ($i++ % 10 == 0) {
                print ".\n";
            }
            sleep(1);
        }
    }
); 

# Now do stuff in the work thread
my $threadForSpidering = threads->new(
    sub {
        my $i = 0; 
        while ($i++<5) {
            print "Working\n"; 
            sleep 1;
    }});

#The work thread finished
$threadForSpidering->join;

#And tells the progress bar to stop running
$keepRunningProgressBar = 0;
$progressBar->join;

print "\nGOT HERE\n";
[download]

Thank you. You are of course right about the timing. That's very simple, I should have seen that. No warnings now when the main thread exits. Thanks!

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

print "Content-type: text/html\n\n";
print "Starting work<br>\n";

$|++;

# set a shared value of TRUE for running the progress bar
my $keepRunningProgressBar : shared = 1;


# Set the progress bar running indefinitely
my $progressBar = threads->create( \&dot_thread )->detach;

# Now do stuff in the work thread
my $threadForSpidering = threads->new( \&workthread );

#The work thread finished
#$threadForSpidering->join; #you have 3 threads, not just one
#main, dot_thread, and workthread
#And tells the progress bar to stop running
$keepRunningProgressBar = 0;

#<>;  #without waiting here you get the thread exited error
foreach my $thread ( threads->list ) {
   $thread->join;
}

print "done<br>.\n";
exit;
##########################################################

sub dot_thread {
   $|++;
   while ( $keepRunningProgressBar == 1 ) {
      print ".\n";
      sleep( 1 );
   }
}

sub workthread {
   for ( 1 .. 10 ) {
      print "\tworking\n";
      sleep 1;
   }
}
[download]

Other people have already mentioned setting $| = 1 to stop perl buffering the output.

You also need to stop your web server buffering it. Ensure that your script’s filename starts nph-. (NPH stands for non-parsed headers.) You will also need to ensure that the http headers your script writes are terminated by a CRLF, as the web server will no longer do that for you.

This is a Perl-language CGI script, yes. Its actually for one of several bots I'm writing to help out with wikipedia, if anyone's interested :-)

The buffering was causing a problem - it wasn't something I'd come across before so I didn't really know where to look for the cause of the problem - since it was the first time I'd used threads I assumed that I must be doing something wrong with the threads code it seems my threads code was actually ok.

Thanks for the response!