forking hell

earthworm_jim

Hey there,

anyone have any forking examples in perl? I'm trying to run a perl proggie that forks off a time consuming loop so the user interface isn't affected.

I have the "Programing Perl" outline of a fork, but no sample programs (which is what I'm really after). Post sample code here :

LoLth

Found this on a FAQ site...

I want to start a process ( a rather long-running script) when a button is
pushed and be able to stop it after clicking another button.
Here's the example code:
#!/usr/local/bin/perl

use Tk;

my $main = new MainWindow;
$main->Button(-text=>'run', #make run button
-command => sub {
FORK:{
if($pid=fork){
print "start: $pid\n";
}elsif (defined $pid) {
run_tst();
POSIX::_exit;
}

}
})->pack;


$main->Button(-text=>'cancel', #make cancel button
-command =>sub {
kill 'STOP',$pid;
})->pack;

MainLoop;

sub run_tst{
print "Start:\n";
for($i=0;$i<10000;$i++){ print "$i\n";}
}

################End of code ##########

The code above almost works. I can start the run_tst process by cliking
on the run button, and I can end the process early by clicking on the
cancel button. The problem is that if I allow the run_tst process to run
to its end (count all the way to 10000), I can't stop the run_tst process
again when its started in the future. so basically, I'd like to be able
to start and stop a process many times.

The reply to that was...

Phil Ptkwt Kristin wrote:
>
> I want to start a process ( a rather long-running script) when a button is
> pushed and be able to stop it after clicking another button.
> Here's the example code:
> #!/usr/local/bin/perl
>
> use Tk;

use POSIX; # <<< You need this

>
> my $main = new MainWindow;
> $main->Button(-text=>'run', #make run button
> -command => sub {
> FORK:{
> if($pid=fork){
> print "start: $pid\n";
> }elsif (defined $pid) {
> run_tst();
> POSIX::_exit;

That should be
POSIX::_exit(0);

> }
>
> }
> })->pack;
>
>
> $main->Button(-text=>'cancel', #make cancel button
> -command =>sub {
> kill 'STOP',$pid;
> })->pack;
>
> MainLoop;
>
> sub run_tst{
> print "Start:\n";
> for($i=0;$i<10000;$i++){ print "$i\n";}
> }
>
> ################End of code ##########
>
> The code above almost works. I can start the run_tst process by cliking
> on the run button, and I can end the process early by clicking on the
> cancel button.

STOP signal does _NOT_ end the process, it suspends it until you send
CONT signal (like 'pause' on CD player or VCR). You probably want TERM.

>The problem is that if I allow the run_tst process to run
> to its end (count all the way to 10000), I can't stop the run_tst process
> again when its started in the future.

That is because as you had it child process did not exit and
"took over" MainLoop in some sense. Next "start" actually started a
grandchild and set $pid in child. But next cancel _may_ have
been handled by (original parent) - in short a mess.


Any use?