Yes, you are right. The communication protocol and the predefined communication sequences are central and critical to my application.
Once the communication is not "synchronized" (synchronized in sense of the communication flow) the application will get into a "no communication possible" mode.
Having still some messages in the queue would be catastrophic. I handled this by putting complete Protocol Data Units into the Queue. Thus only complete messages will be send. I am expecting an answer+acknowledge per PDU I have put into the queue. This is how I keep it synchronized. I am aware that this is quite simple and puts the protocol to the center of communication. The application will only close the rs232 in case the expected PDU (answer) has already been received and - the most important thing - the test-sequence is ended.
However, in case I would have some more threads working on the queues my concept would break into pieces.. It would be necessary to check by MyQueue->pending() if a messge is pending to be send. Moreover splitting up Messages into several PDU would make things once more complicated I am not sure how I would handle the stuff in this case. I guess I would assemble the message completely before sending it by rs232?