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How do I lock a file?

by vroom (Pope)
on Jan 08, 2000 at 08:45 UTC ( #1881=perlquestion: print w/replies, xml ) Need Help??

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

How do I lock a file?

Originally posted as a Categorized Question.

Replies are listed 'Best First'.
Re: How do I lock a file?
by ender (Novice) on Mar 23, 2000 at 00:33 UTC
    Well, you can use flock():
    #!/usr/bin/perl
    
    use strict;
    use Fcntl ':flock'; # import LOCK_* constants
    
    open(MYFILE, ">testing");
    flock(MYFILE, LOCK_EX);
    print "file locked\n"; sleep(100);
    flock(MYFILE, LOCK_UN);
    print "file unlocked\n";
    close MYFILE;
    
    That will lock it as far as other processes that use flock() are concerned. Ie: if you run that, (and it locks and goes to sleep for 100 seconds) and then run another instance of it while the first is sleeping, it will wait for the other to unlock the file. However, if you run one instance of that and "echo lala > testing" it will ignore the lock and just write the file.

    I don't know enough to say wether using fcntl() might yeild better results when dealing with other processes that don't try to lock the file before accessing it.

    I'd say look at the perldocs on flock() and fcntl() or do some kind of cooperative locking. Ie: touch a lockfile and check it before opening the real file.

      Don't do cooperative locking -- it's not atomic. Suppose you had two processes each contending for a single file. The first file might touch the lock file and then immediately give up control to the second program (hey, it's a race condition and it really happens!). The second program is unable to open the file, even though the first is not using it, because the first has marked the lock file -- or it could try it anyway, grabbing the lock out from underneath it. The behavior is undefined.

      flock is probably your best bet, as it uses the CPU's specific SINGLE operation for obtaining/releasing a lock.

Re: How do I lock a file?
by turnstep (Parson) on Apr 04, 2000 at 09:10 UTC

    Update: The question that this is answering seems to have disappeared. So bear with me if the answer seems a bit strange.


    Perl might even be using fcntl for the flock function: if it can't find a local version of flock that it can use, it will use fcntl instead. Generally, it's flock or nothing: just make sure that all your processes are using flock, and everything will be happy. lockf and fcntl can still be used, but they are heavy-duty solutions, and should only be brought out when really, really needed, and you really, really know what you are doing. The higher level flock is quite sufficient for almost any task

    Also, make sure that you check the return values of 'flock', as well as 'open' and 'close'. Specifically unlocking a file is seldom necessary, as 'close' is guaranteed to unlock it for you.

Re: How do I lock a file?
by danstuken (Novice) on Aug 01, 2007 at 13:23 UTC
    Note:

    if you

    open(FILE, ">testing")
    you will truncate the file before you have it locked, this may not be what you want in a production system! Use
    open(FILE, ">>testing")
    before you get the lock and then truncate() if you want to, er, truncate the file once its locked. Or lock a separate semaphore file who's contents are unimportant.

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