Check for another running instance of the same program

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

We have a Perl utility (let's call it utility.pl) that is supposed to avoid starting if another instance of itself is already running. The way we've implemented this requirement is to check for the existence of a semaphore file, semaphore.txt, in a particular directory. If the semaphore file exists, the utility program dies with a message saying

Either another instance of utility.pl is already running, or the file semaphore.txt needs to be deleted.

If the semaphore file doesn't exist, the utility first writes it, then does its work (which can take minutes), then unlinks it. Is this the best way to do this, or can somebody suggest a better way?

Update

Thank you, LanX, for the link to the thread from 2006. What I meant by "better" was "not theoretically constituting a race condition". Also not leaving its semaphore file lying around when it crashes, which has happened. The solution that opens and locks $0 is my favorite so far, but I'm on Windows, so the second running instance dies silently without divulging my error.

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Re: Check for another running instance of the same program by LanX (Saint) on May 13, 2013 at 22:15 UTC
Did you see Ensuring only one copy of a perl script is running at a time? It discusses various approaches (including "semaphores") and pro and cons (including the risk of race conditions) Additionally it links to further discussions. Cheers Rolf ( addicted to the Perl Programming Language)	[reply]
Re: Check for another running instance of the same program by dd-b (Monk) on May 13, 2013 at 22:34 UTC
You can safely automate the deletion of the semaphore file by writing the PID to it, and having later programs check if the PID exists as a running process; if not it can delete the file and proceed. Consider your requirements carefully; for example, does it apply to one system, or all systems that can access a particular NFS mount, or all systems on a built-in list with IP connectivity, or what exactly. These lead to very different solutions. Do you want to consider expanding it to "no more than n" copies running at once, or is the simple one at a time good?	[reply]
Re: Check for another running instance of the same program by vinoth.ree (Monsignor) on May 14, 2013 at 03:18 UTC
LanX ++ There are many ways to ensure single instance of perl program is running. PID files are the traditional way to do it. we can also hold a lock on a file,for example the program itself. This small piece of code will do the trick: `use Fcntl ':flock'; open my $self, '<', $0 or die "Couldn't open self: $!"; flock $self, LOCK_EX \| LOCK_NB or croak "This script is already runnin +g";` [download] One advantage over PID files is that files automatically get unlocked when the program exits. It is much easier to implement in a reliable way. *All is well*	[reply] [d/l]
Re: Check for another running instance of the same program by ww (Archbishop) on May 13, 2013 at 22:16 UTC
It's one plausible way... among many. Most of them have occasioned some discussion here. Super_Search and search engines are your friends. Advising you that x or y or z is the "best" way without a lot more details on your case would be "iffy," at best. If you didn't program your executable by toggling in binary, it wasn't really programming!	[reply]
Re: Check for another running instance of the same program by Anonymous Monk on May 14, 2013 at 05:01 UTC
I would use File::HomeDir with something else, maybe File::Lockfile ( lock $my_data/appname/.lockfile )	[reply]
Re: Check for another running instance of the same program by scorpio17 (Canon) on May 14, 2013 at 14:03 UTC
I had to solve a similar problem once myself: RFC: A new module to help avoid running multiple instances of the same script (via cron, for example) Be sure to read the entire thread - my original solution gets improved upon as other monks offer comments and suggestions.	[reply]
Re: Check for another running instance of the same program by sundialsvc4 (Abbot) on May 14, 2013 at 02:25 UTC
`/me nods ...` Semaphore-files are easy and quick, especially with regard to an activity that “takes minutes to perform.” Is there a particular reason why you might be second-guessing this decision now?


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