$main_folder = '/u/preport/preport';
$lockfile = "$main_folder.lock";
$ticket = "$main_folder.ticket";

sub get_ticket
{
    my($ticket_no);
    my($wait);

    # Wait for semaphore:
    
    $wait = "true";
    
    while ($wait)
    {
        # Wait for semaphore to be released (if set):
    
        while (-f $lockfile) { sleep(5); }
    
        # When semaphore is gone, create own one:
    
        unless (open(LOCKFILE, ">>$lockfile"))
        {
            unlink($lockfile);
            warn "$self: can't write semaphore '$lockfile': $!\n";
            return(undef);
        }
        print LOCKFILE "$$\n";
        close(LOCKFILE);
    
        # Check wether semaphore is our's or was created
        # in the meantime by another program:
    
        unless (open(LOCKFILE, "<$lockfile"))
        {
            unlink($lockfile);
            warn "$self: can't read semaphore '$lockfile': $!\n";
            return(undef);
        }
        $_ = <LOCKFILE>;
        chop;
        close(LOCKFILE);
    
        # Loop if semaphore isn't our's:
    
        $wait = ($_ != $$);
        if ($wait)
        {
            warn "Semaphore access conflict detected!\n";
            warn "Waiting for other process to terminate.\n";
        }
    }
    
    # Get next ticket number:
    
    $ticket_no = '';
    
    unless (open(TICKET, "<$ticket"))
    {
        unlink($lockfile);
        warn "$self: can't read '$ticket': $!\n";
        return(undef);
    }
    
    while (<TICKET>)
    {
        chop;
        if (/^\d+$/) { $ticket_no = $_; }
    }
    
    close(TICKET);
    
    if ($ticket_no eq '')
    {
        unlink($lockfile);
        warn "$self: no ticket number in '$ticket'!\n";
        return(undef);
    }
    
    $ticket_no++;
    
    if ($ticket_no > 9999)
    {
        $ticket_no = '0000';
    }
    
    unless (open(TICKET, ">$ticket"))
    {
        unlink($lockfile);
        warn "$self: can't write '$ticket': $!\n";
        return(undef);
    }
    
    print TICKET "$ticket_no\n";
    
    close(TICKET);
    
    # Release semaphore:
    
    unlink($lockfile);
    
    return($ticket_no);
}