=over =item 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). C is Perl's portable file locking interface, although it locks only entire files, not records. On many platforms (including most versions or clones of Unix), locks established by C are B. Such discretionary locks are more flexible, but offer fewer guarantees. This means that files locked with C may be modified by programs that do not also use C. Windows NT and OS/2 are among the platforms which enforce mandatory locking. See your local documentation for details. 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 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 added to LOCK_SH or LOCK_EX then C will return immediately rather than blocking waiting for the lock (check the return status to see if you got it). To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE before (un)locking 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 (all?) systems implement lockf(3) in terms of fcntl(2) locking, though, so the differing semantics shouldn't bite too many people. Note also that some versions of C cannot lock things over the network; you would need to use the more system-specific C 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 C<-Ud_flock> to the F program when you configure perl. Here's a mailbox appender for BSD systems. use Fcntl ':flock'; # import LOCK_* constants sub lock { flock(MBOX,LOCK_EX); # and, in case someone appended # while we were waiting... seek(MBOX, 0, 2); } sub unlock { flock(MBOX,LOCK_UN); } open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}") or die "Can't open mailbox: $!"; lock(); print MBOX $msg,"\n\n"; unlock(); See also L for other flock() examples. =back