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, or 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
Perl.
Here's a mailbox appender for BSD systems.
- use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
- sub lock {
- my ($fh) = @_;
- flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
- # and, in case someone appended while we were waiting...
- seek($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.