FAQ 5.18: How can I lock a file?

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5.18: How can I lock a file?

    Perl's builtin flock() function (see perlfunc for details) will call
    flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004
    and later), and lockf(3) if neither of the two previous system calls
    exists. On some systems, it may even use a different form of native
    locking. Here are some gotchas with Perl's flock():

    1   Produces a fatal error if none of the three system calls (or their
        close equivalent) exists.

    2   lockf(3) does not provide shared locking, and requires that the
        filehandle be open for writing (or appending, or read/writing).

    3   Some versions of flock() can't lock files over a network (e.g. on
        NFS file systems), so you'd need to force the use of fcntl(2) when
        you build Perl. But even this is dubious at best. See the flock
        entry of perlfunc and the INSTALL file in the source distribution
        for information on building Perl to do this.

        Two potentially non-obvious but traditional flock semantics are that
        it waits indefinitely until the lock is granted, and that its locks
        are *merely advisory*. Such discretionary locks are more flexible,
        but offer fewer guarantees. This means that files locked with
        flock() may be modified by programs that do not also use flock().
        Cars that stop for red lights get on well with each other, but not
        with cars that don't stop for red lights. See the perlport manpage,
        your port's specific documentation, or your system-specific local
        manpages for details. It's best to assume traditional behavior if
        you're writing portable programs. (If you're not, you should as
        always feel perfectly free to write for your own system's
        idiosyncrasies (sometimes called "features"). Slavish adherence to
        portability concerns shouldn't get in the way of your getting your
        job done.)

        For more information on file locking, see also "File Locking" in
        perlopentut if you have it (new for 5.6).


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