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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.