error message not working

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

Fellow Monks,

I found a snippet at this node for checking if an instance of a program is running. It seems to work fine, killing a second instance of the program, but I expected the error message "Already executing: $!"; to appear. However, the error message doesn't display. Whats going on? and how can I get the error message to display?

#!/usr/bin/perl

use Fcntl qw(LOCK_EX LOCK_NB);

flock DATA, LOCK_EX | LOCK_NB or die "Already executing: $!";

my $maxprimes=200; #Stop when you've found this many
my $value=1;
my $count=0;

while ($count < $maxprimes){
        $value++;
        my $composite=0;
OUTER: for (my $i=2; $i<$value; $i++){
       for (my $j=$i; $j<$value; $j++){
             if (($j*$i)==$value){
                   $composite=1;
                   last OUTER;
             }
       }
}
if (! $composite){
    $count++;
    print "$value ";
    }
}

# do not remove this __END__
__END__
[download]

Thanks in advance,
Jonathan

Comment on error message not working Download Code

Replies are listed 'Best First'.
Re: error message not working by Arunbear (Prior) on Nov 30, 2004 at 12:09 UTC
On win32, ActivePerl 5.8.4, the 2nd process dies without the error message. I changed `flock DATA, LOCK_EX \| LOCK_NB or die "Already executing: $!";` [download] to `open my $dummy, ">lock" or die $!; flock $dummy, LOCK_EX \| LOCK_NB or die "Already executing: $!";` [download] and then I did get the error message: `Already executing: Domain error at temp2.pl line 7.` [download]	[reply] [d/l] [select]
Re^2: error message not working by bart (Canon) on Nov 30, 2004 at 14:01 UTC
Ah yes. On some operating systems, you can only do a LOCK_EX on a handle that's open for writing. `DATA` is open for reading only. I guess that is why the original fails, and your version works. Update: perldoc -f flock has this to say about it: Note that the emulation built with lockf(3) doesn't provide shared locks, and it requires that FILEHANDLE be open with write intent. These are the semantics that lockf(3) implements. Most (all?) systems implement lockf(3) in terms of fcntl(2) locking, though, so the differing semantics shouldn't bite too many people. That must be the case, here. "Not too many people", right.	[reply]
Re^2: error message not working by Anonymous Monk on Nov 30, 2004 at 12:18 UTC
does this open an actual file? what I was trying to do was open DATA (after __END__) to ensure that the actual script wasn't already being run. I was trying to get Merlyn's snippet from the node I mentioned above to work for me, it does work apparently, but not on windows... Any advise appreciated Jonathan	[reply]
Re^3: error message not working by Arunbear (Prior) on Nov 30, 2004 at 13:32 UTC
It does open an actual file. I know what you are trying to do. I'm only suggesting it as a work-around because opening DATA isn't doing the job on win32. As I showed before, using a real sentinel file does prevent additional instances of the script from running. merlyn suggested opening DATA because the code is simpler that way.	[reply]
Re: error message not working by Happy-the-monk (Canon) on Nov 30, 2004 at 11:57 UTC
`perldoc -f flock` says: `Two potentially non-obvious but traditional "flock" semantics are that it waits indefinitely until the lock is granted, ...` So it works alright it seems, it just waits until it will be allowed to flock the file instead of aborting with that error message. Cheers, Sören	[reply]
Re^2: error message not working by rev_1318 (Chaplain) on Nov 30, 2004 at 12:44 UTC
Actually no, it doesn't, because of the LOCK_NB, flock won't block but returns immediately. Of course, this only works on *nix-like systems, not on windows-like ones, so I'm guessing that's where the problem is.	[reply]
Re: error message not working by fglock (Vicar) on Nov 30, 2004 at 11:57 UTC
It works here (Linux). What's your operating system, Perl version?	[reply]
Re^2: error message not working by Anonymous Monk on Nov 30, 2004 at 12:04 UTC
I'm running on windows 2000, with Activestate perl v5.8.4 So if it works on Linux, how can I get the same thing to work on windows? Thanks, Jonathan	[reply]
Re^3: error message not working by Fletch (Bishop) on Nov 30, 2004 at 12:06 UTC
First find a Linux CD . . . `:)`	[reply]
Re: error message not working by ikegami (Patriarch) on Nov 30, 2004 at 18:29 UTC
Microsoft suggestes that you use the OS's named Semaphores. Win32::Semaphore, which comes with ActiveState, gives you access to those. You could decide whether to use semaphores or flock based on $^O. Be sure to provide a $name argument!!	[reply]