How to timeout a while loop?

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

Hello my esteemed monks,

I need to set a timeout for an operation I perform inside a while loop, but I´m having trouble understanding the dinamic of the alarm() function. Can you guys help me out? Here´s the (wrong!) code:

$SIG{ALRM} = sub { print "...timeout"; next READING; };

my $number = 1;

READING: while ( $number < 10 ) {

   eval {

      alarm(3);

      print $number;
   
      if ( $number == 3 ) { sleep 4; } # This is to simulate the delay
+ the routines may generate...
   
      $number++;
   
      print "\n";
      
   }

}

if ($@) { # this I copied from the Camel book, but not sure about what
+ it means and if it is really necessary in this case... (I don´t want
+ to 'die'; i just want to go to the 'next' iteration.)
    if ($@ =~ /timeout/) {
                            # timed out; do what you will here
    } else {
        alarm(0);           # clear the still-pending alarm
        die;                # propagate unexpected exception
    } 
}
[download]

Thanks a lot fellows,

Andre_br

Comment on How to timeout a while loop? Download Code

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Re: How to timeout a while loop? by suaveant (Parson) on Apr 25, 2005 at 20:13 UTC
You want a `die 'timeout';` in your `$SIG{ALRM}` that will break you out at the end of your eval (put a ; at the end of that eval) then put your code catch code... like so `$SIG{ALRM} = sub {die 'timeout'}; while ( $number < 10 ) { eval { alarm(3); print $number; if ( $number == 3 ) { sleep 4; } # This is to simulate the delay + the routines may generate... $number++; print "\n"; }; alarm(0); if($@ eq 'timeout') { next; } else { die $@; } }` [download] Not tested, but should get you pointed in the right direction. Alarm 0 is important to unset the alarm. - Ant - Some of my best work - (1 2 3)	[reply] [d/l] [select]
Re^2: How to timeout a while loop? by Andre_br (Pilgrim) on Apr 25, 2005 at 20:36 UTC
Hey Suaveant, Great! Thanks a lot; in fact i needed to change some details for it to work, like putting the sub that listens the $@ outside the while, but now it is just perfect. Here´s the final code, for the monk's record: `my $number = 0; $SIG{ALRM} = sub {die 'timeout'}; while ( $number < 10 ) { eval { $number++; alarm(3); if ( $number == 3 ) { sleep 4; } # This is to simulate the delay + the routines may generate... print $number; print "\n"; }; alarm(0); } if($@ eq 'timeout') { next; } else { die $@; }` [download] Thanks a lot, my friend Andre_br	[reply] [d/l]
Re^3: How to timeout a while loop? by suaveant (Parson) on Apr 25, 2005 at 20:47 UTC
Umm.. if you leave the if at the bottom outside the while the next is useless, and any $@ values will cause the loop to continue, including non-timeout values. You could possibly also miss valid die messages. To be really useful you should probably check the $@ at the end of the eval after the alarm(0); - Ant - Some of my best work - (1 2 3)	[reply]