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Re^2: Ensuring only one copy of a perl script is running at a time (race)

by tye (Cardinal)
on Dec 19, 2006 at 17:25 UTC ( #590710=note: print w/ replies, xml ) Need Help??


in reply to Re: Ensuring only one copy of a perl script is running at a time
in thread Ensuring only one copy of a perl script is running at a time

It sounds like it is time for you to update your computer science knowledge by learning about race conditions.

I need to ensure only one conucrrent usage [....] works just fine

It works just fine as far as you have noticed so far. It certainly doesn't "ensure" only one concurrent use; it more like usually prevents more than one concurrent use. (:

You code must perform the following steps:

  1. Check current status
  2. if not 1 then exit
  3. Set current status to 2
  4. Do work
  5. Set current status back to 1

And, in a modern computer system, CPU resources are shared, so each process that is serving a CGI request can be interrupted between any of those steps (or in the middle of steps) in order to let some other process do some work for a bit. Two CGI requests coming it at roughly the same time can thus perform those steps in the following order:

One process Other process my $status= CheckStatus(); exit if 1 != $status; my $status= CheckStatus(); exit if 1 != $status; SetStatus( 2 ); SetStatus( 2 ); DoWork(); DoWork(); SetStatus( 1 ); SetStatus( 1 );

Note that they both see the status as "1" and both end up running concurrently. This is why operating systems provide locking mechanisms and why you often need to use such.

- tye        


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Re^3: Ensuring only one copy of a perl script is running at a time (race)
by PockMonk (Beadle) on Dec 19, 2006 at 17:28 UTC
    You will be happy to learn that I do use flock on opening the file so there is no danger of what you describe happening.

    Rather than locking a file for the duration of the script, I simply lock it while changing its status etc. As posted, this allows me greater flexibility to post more info than just in use/not in use by using the file content to write codes to. Thanks anyway!

    Dan

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