FAQ 5.20 Why can't I just open(FH, ">file.lock")?

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5.20: 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 subdesirable.


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