Timeout on system command

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

Hi all, I'm developping a Perl script that do some system calls. Sometimes, these commands will block. I'm looking for a way to 'install' a timeout on these calls. I did some test using signals.

$TIMEOUT = 5;
$CMD = $ARGV[0];

eval {  
   $SIG{ALARM} = sub { die "TIMEOUT\n" };
   alarm $TIMEOUT;
   
   system($CMD);
   
   alarm 0;
};
  
# do error processing
[download]

I do receive the timeout message as expected. However, the command that was started is still running in the background. How to 'kill' the command if the timeout is reached ? If using 'fork', how can I get the output of my command ? I also need the output of the command. Any suggestion ? Thanks.

Comment on Timeout on system command Download Code

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Re: Timeout on system command by revdiablo (Prior) on Jul 06, 2004 at 20:22 UTC
I recommend using a piping form of open, and capturing the returned PID. Then, in your signal handler, you can kill the PID, to ensure it goes away. Example: `my $pid; eval { local $SIG{ALRM} = sub { die "TIMEOUT" }; alarm 2; $pid = open my $fh, "sleep 30\|"; alarm 0; } if ($@ eq "TIMEOUT") { print "kill $pid\n"; kill 1, $pid; } elsif ($@) { die $@; }` [download] Take note that your example code had an error -- the key in %SIG should be ALRM and not ALARM. Update: apparently Joost posted essentially the same answer, but 10 minutes earlier than I did. Oh well, I guess that can be seen as validation of some sort. Update: fixed silly syntax error	[reply] [d/l]
Re: Timeout on system command by Joost (Canon) on Jul 06, 2004 at 20:12 UTC
system() doesn't give you the output of the executed command: it sends the output to STDOUT. You can probably* set ALARM, `open P,"prog \|";` read from P and close. There's lots of documentation and examples in perlipc and perlopentut. * there seem to be some issues with the safe signal handlers in recent perls. try it out. Joost. Update: The executed command will probably not end until it tries to write to its STDOUT, which is bound to the P file-handle in the pseudo-code above. You could try using `kill` to kill it immediately. "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l] [select]
Re: Timeout on system command by edan (Curate) on Jul 07, 2004 at 11:00 UTC
You can find some example code I wrote in answer to somebody else who (it turned out after many iterations), had a very similar problem to solve. You'll find the following nodes helpful (I hope): The root node A reply Another reply The solution was about the same as some of the solutions already posted in this thread, but sometimes it helps to see a few different angles on it... Cheers. -- edan	[reply]
Re: Timeout on system command by eyepopslikeamosquito (Archbishop) on Jul 07, 2004 at 09:06 UTC
The general problem of timing out a slow event is discussed in the Perl Cookbook 2nd edition, Recipe 16.21 "Timing Out an Operation" and also in perlfaq8 "How do I timeout a slow event". On Unix, I suggest using `$SIG{ALRM}/alarm` (carefully). On Win2K and above, I recommend the Win32::Job module. See also Timing and timing out Unix commands and Timing Windows commands.	[reply] [d/l]