sub passeer
{
    my($semaphore) = @_; # this is a filename!
    my($wait) = 1;

    # Wait for semaphore:
    
    WAIT:
    while ($wait)
    {
        # Wait for semaphore to be released (if set):
    
        while (-f $semaphore) { sleep(5); }
    
        # When semaphore is gone, create own one:
    
        unless (open(SEMAPHORE, ">$semaphore"))
        {
            unlink($semaphore);
            warn "can't write semaphore '$semaphore': $!\n";
            next WAIT;
        }
        print SEMAPHORE "$$\n";
        close(SEMAPHORE);
    
        # Check wether semaphore is our's or was created
        # in the meantime by another process:
    
        unless (open(SEMAPHORE, "<$semaphore"))
        {
            unlink($semaphore);
            warn "can't read semaphore '$semaphore': $!\n";
            next WAIT;
        }
        $_ = <SEMAPHORE>;
        chop;
        close(SEMAPHORE);
    
        # Loop if semaphore isn't our's:
    
        $wait = ($_ != $$);

#       if ($wait)
#       {
#           warn "Semaphore access conflict detected!\n";
#           warn "Waiting for other process to terminate.\n";
#       }

    }
}

sub verlaat
{
    my($semaphore) = @_;

    # Release semaphore:
    
    unlink($semaphore);
}