FLOCK
flock FILEHANDLE,OPERATION

Calls flock(2) on FILEHANDLE. See the flock (2) manpage for definition of OPERATION. Returns TRUE for success, FALSE on failure. Will produce a fatal error if used on a machine that doesn't implement flock(2). Here's a mailbox appender for BSD systems.

$LOCK_SH = 1; $LOCK_EX = 2; $LOCK_NB = 4; $LOCK_UN = 8; sub lock { flock(MBOX,$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();

Note that flock() can't lock things over the network. You need to do locking with fcntl() for that.

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