flock(3) - perldoc - phpman

Che Dong

⚡ TLDR: flock (tldr-pages)

Manage file locks from shell scripts.

Run a command with a file lock as soon as the lock is available
flock {{path/to/lock.lock}} {{command}}
Run a command with a file lock, or exit if the lock is currently being held (with exit code 1)
flock {{-n|--nonblock}} {{path/to/lock.lock}} {{command}}
Run a command with a file lock, or exit with a specific error code if the lock is currently being held
flock {{-n|--nonblock}} {{-E|--conflict-exit-code}} {{123}} {{path/to/lock.lock}} {{command}}
Run a command with a file lock, waiting up to 10 seconds for the lock to be available before giving up
flock {{-w|--timeout}} 10 {{path/to/lock.lock}} {{command}}
Backup a bunch of files, waiting for the previous `tar` command to finish if it's still running elsewhere and holding the same lock file (can be used in a `cron` job that runs often)
flock {{path/to/backup.lock}} {{tar -cvf path/to/backup.tar path/to/data/}}

    flock FILEHANDLE,OPERATION
            Calls flock(2), or an emulation of it, on FILEHANDLE. Returns
            true for success, false on failure. Produces a fatal error if
            used on a machine that doesn't implement flock(2), fcntl(2)
            locking, or lockf(3). "flock" is Perl's portable file-locking
            interface, although it locks entire files only, not records.

            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
            programs that do not also use "flock" may modify files locked
            with "flock". See perlport, your port's specific documentation,
            and your system-specific local manpages for details. It's best
            to assume traditional behavior if you're writing portable
            programs. (But 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.)

            OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly
            combined with LOCK_NB. These constants are traditionally valued
            1, 2, 8 and 4, but you can use the symbolic names if you import
            them from the Fcntl module, either individually, or as a group
            using the ":flock" tag. LOCK_SH requests a shared lock, LOCK_EX
            requests an exclusive lock, and LOCK_UN releases a previously
            requested lock. If LOCK_NB is bitwise-or'ed with LOCK_SH or
            LOCK_EX, then "flock" returns immediately rather than blocking
            waiting for the lock; check the return status to see if you got
            it.

            To avoid the possibility of miscoordination, Perl now flushes
            FILEHANDLE before locking or unlocking it.

            Note that the emulation built with lockf(3) doesn't provide
            shared locks, and it requires that FILEHANDLE be open with write
            intent. These are the semantics that lockf(3) implements. Most
            if not all systems implement lockf(3) in terms of fcntl(2)
            locking, though, so the differing semantics shouldn't bite too
            many people.

            Note that the fcntl(2) emulation of flock(3) requires that
            FILEHANDLE be open with read intent to use LOCK_SH and requires
            that it be open with write intent to use LOCK_EX.

            Note also that some versions of "flock" cannot lock things over
            the network; you would need to use the more system-specific
            "fcntl" for that. If you like you can force Perl to ignore your
            system's flock(2) function, and so provide its own
            fcntl(2)-based emulation, by passing the switch "-Ud_flock" to
            the Configure program when you configure and build a new Perl.

            Here's a mailbox appender for BSD systems.

                # import LOCK_* and SEEK_END constants
                use Fcntl qw(:flock SEEK_END);

                sub lock {
                    my ($fh) = @_;
                    flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
                    # and, in case we're running on a very old UNIX
                    # variant without the modern O_APPEND semantics...
                    seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
                }

                sub unlock {
                    my ($fh) = @_;
                    flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
                }

                open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
                    or die "Can't open mailbox: $!";

                lock($mbox);
                print $mbox $msg,"\n\n";
                unlock($mbox);

            On systems that support a real flock(2), locks are inherited
            across "fork" calls, whereas those that must resort to the more
            capricious fcntl(2) function lose their locks, making it
            seriously harder to write servers.

            See also DB_File for other "flock" examples.

            Portability issues: "flock" in perlport.