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
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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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Should this be updated to use the more modern form of open()?

    open($LCK, '>', 'file.lock');

And likewise for the sysopen call, which should probably use a scalar
object rather than a bareword ($FH rather than FH)..

Keith Thompson (The_Other_Keith)  <
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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

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Yep. I'll make the fix.

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