$sem = IPC::Semaphore->new( 4321, 1, S_IRUSR | S_IWUSR );

if ( $sem ) {
# Semaphore already exists so just open it
    print "Semaphore already exists - just open it\n";
    $sem = IPC::Semaphore->new( 4321, 1, S_IRUSR | S_IWUSR );
} else {
# The semaphore didn't already exit so create it
    print "Create semaphore \n";
    $sem = IPC::Semaphore->new( 4321, 1, IPC_CREAT | S_IRUSR | S_IWUSR );
    print "Semaphore created\n";
    $sem->setval(0,1);
    print "Semaphore initialised\n";
}

print "Locking other threads\n";

$sem->op(0, -1, SEM_UNDO);