use POSIX qw(pause);
$SIG(ALRM) = ...
...
alarm $seconds;
...

pause; #wait for a signal without
       #using CPU time
[download]

Perfect! Thanks.

John

Here is the code

use strict;

sub Call_with_Timeout{
   my ($TIMEOUT, $CODEREF) = @_;

   eval {
      local $SIG{ALRM} = sub { die "alarm time out" };
      alarm $TIMEOUT;
      $CODEREF->(); # Whatever needs to be done
      alarm 0;
      1;  # return value from eval on normalcy
  } or return -1; # Timeout happened
  return 1; # OK
}

 my $t=Call_with_Timeout(3, sub{ sleep 3, print "$_.\n" for 1..2});
   print "Timed out=$t;\n";
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When I tried using sleep in the loop, it bombed out after the first alarm -- I got one print from the handler, and then the program exited. However, I just tried again with this:

#!/usr/bin/perl

use Time::HiRes qw ( setitimer ITIMER_REAL time );
my $count;

$SIG{ALRM} = sub { print time, "\n" };
setitimer(ITIMER_REAL, 1, 1);

while (1) {
        sleep 1;
        }

and it worked fine. If this is reliable, it does the job for me. But I wonder why the first time it only worked for the first iteration...

Thanks!

Ignore the "my $count" -- that's left over from an earlier experiment. Apart from the while loop, this is based on an example in the Time::HiRes documentation.

alarm is useful to interupt other processing at a given time or regular intervals, but if you have nothing else to do but wait for the intervening period, then you are using the wrong tool. Forget alarm and use sleep.

Do something every 10 seconds:

my $deadline = time() + 10;
while( 1 ) {
    sleep 1 while time() < $deadline;
    $deadline = time() +10;
    ## Do the something
}
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Using sleep (or usleep from Time::HiRes) is how I've done it in the past, but the problem is accumulating time inaccuracy do to delays in processing.

For a bit more context, I'm taking measurements from electronic test equipment using the GPIB bus. I want to take those readings, say, every 10 seconds. The time it takes to tell the instrument to send a reading, for the instrument to process that request, and then return the result, is somewhat variable. So it's very hard to calibrate the sleep time to keep accuracy.

What I want to do is trigger the measurement cycle every X seconds regardless of how long (less than X, obviously) the measurement process takes.

In one previous iteration, I used HiRes gettime calls before and after the read/write cycle to determine how long the processing took, and used that to adjust the sleep time, but that seemed an awfully awkward way to go. I was hoping that a recurring alarm (which the HiRes setItimer function allows) would be a more elegant way to go.

Thanks! Hope this helps clarify.

What I want to do is trigger the measurement cycle every X seconds regardless of how long (less than X, obviously) the measurement process takes.

That's why I used a while loop sleeping for 1 second each time to accumulate the 10 seconds, rather than a 10 second sleep:

my $deadline = time() + 10;
while( 1 ) {
    sleep 1 while time() < $deadline;
    $deadline = time() +10;
    ## Do the something
}
[download]

This way, no matter how long "Do something" takes, so long as it's less that 10 seconds, the next reading will be initiated on time. To within one second.

Eg. If it takes 3 seconds to process, the while loop will iterate 7 times. If it takes 7 seconds, the loop will iterate 3 times only.

If you need to get more accurate, then use Time::HiRes and sleep for 1/10 of a second in the while loop. It will be 10 times more accurate and still consume almost imeasurable cpu.

Need more accurate still? Then sleep for 1/100th of a second. It will still consume very little cpu and be another order of magnitude more accurate.

Beyond that, you start getting into the realms of how long it takes to read the clock, affecting your accuracy, but if you need accuracy beyond 1/1000th of a second, you should probably be using C or assembler.

---

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Look into the setitimer function in the POSIX module. It will give you a stream of signals at repeated intervals. This might be a real slick solution combined with the pause suggestion made by traveler.