FAQ 5.22 Why can't I just open(FH, ">file.lock")?

Discussion in 'Perl Misc' started by PerlFAQ Server, Mar 11, 2011.

  1. This is an excerpt from the latest version perlfaq5.pod, which
    comes with the standard Perl distribution. These postings aim to
    reduce the number of repeated questions as well as allow the community
    to review and update the answers. The latest version of the complete
    perlfaq is at http://faq.perl.org .

    --------------------------------------------------------------------

    5.22: Why can't I just open(FH, ">file.lock")?

    A common bit of code NOT TO USE is this:

    sleep(3) while -e 'file.lock'; # PLEASE DO NOT USE
    open my $lock, '>', 'file.lock'; # THIS BROKEN CODE

    This is a classic race condition: you take two steps to do something
    which must be done in one. That's why computer hardware provides an
    atomic test-and-set instruction. In theory, this "ought" to work:

    sysopen my $fh, "file.lock", O_WRONLY|O_EXCL|O_CREAT
    or die "can't open file.lock: $!";

    except that lamentably, file creation (and deletion) is not atomic over
    NFS, so this won't work (at least, not every time) over the net. Various
    schemes involving link() have been suggested, but these tend to involve
    busy-wait, which is also less than desirable.



    --------------------------------------------------------------------

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    are not necessarily experts in every domain where Perl might show up,
    so please include as much information as possible and relevant in any
    corrections. The perlfaq-workers also don't have access to every
    operating system or platform, so please include relevant details for
    corrections to examples that do not work on particular platforms.
    Working code is greatly appreciated.

    If you'd like to help maintain the perlfaq, see the details in
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    PerlFAQ Server, Mar 11, 2011
    #1
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