FAQ 5.21 Why can't I just open(FH, ">file.lock")?

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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 .


5.21: 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(LCK, "> 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(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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Re: FAQ 5.21 Why can't I just open(FH, ">file.lock")?

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Am I reading this wrong or is there a solution?

Bill H

Re: FAQ 5.21 Why can't I just open(FH, ">file.lock")?

On Sun, 02 Nov 2008 04:16:01 -0800, Bill H wrote:
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Yes; see perlfaq 5.  This FAQ has been quoted in isolation, but it is in
fact not independent; it's part of a sequence of FAQs that form a
conversation about locking.

Peter Scott / /

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