Stuck in Interation which is protected by a semaphore

Question

For learning purposes, I am implementing UDP with the mechanisms of TCP (so that it guarantees safe transfer).

The Semaphore I am using is binary, so its sem = new Semaphore(1);. I use this semaphore to control the entrance for my sendBuf, which is a List containing all packages which have been send, but not yet confirmed. Since I sometimes remove packages out of it when I get an ACK, I need to make sure I am not iterating with one thread while another thread is deleting something out of it.

The thing which is really bugging me is this:

public void timeoutTask(long seqNum) {
    System.out.println("Timeout for package with SeqNum " + seqNum
            + " happened.");
    timeoutValue *= 2;
    try {
        System.out.println("Acquire? in timeouttask");
        sem.acquire();
        System.out.println("Acquired! in timeouttask");
    } catch (InterruptedException e1) {
        System.out.println("semaphore not acquired");
        e1.printStackTrace();
    }for (FCpacket packet : sendBuf) {

        System.out.println("Iterating!");
        if (packet.getSeqNum() == seqNum) {
            System.out.println("Package for seqNum " + seqNum + " found!");
            reSendData = packet.getSeqNumBytesAndData();
            DatagramPacket reSendPacket = new DatagramPacket(reSendData,
                    reSendData.length, hostaddress, SERVER_PORT);

            try {
                clientSocket.send(reSendPacket);
                System.out.println("Packet with seq " + seqNum
                        + " send again");
                packet.setTimestamp(0);
                startTimer(packet);
                new ReceiveHandler(reSendData, reSendData.length,
                        clientSocket, rcvData, UDP_PACKET_SIZE, this).run();
            } catch (IOException e) {
                System.out.println("Couldn't send package");
                e.printStackTrace();
            }
        }
    }
    sem.release();
    System.out.println("released! in timeouttask");

Console output gives me the following:

Acquire? in timeouttask

Acquired! in timeouttask

Iterating!

Paket for seqNum 1 found!

Packet with seq 1 send again

So it gets the semaphore, starts iterating, it even sends the package, so by now it should Either: Iterate again ("iterating!") OR release the semaphore. None of the above happens, it is just stuck. I have no idea why - any ideas?

What do you mean by " I am implementing UDP with the mechanisms of TCP " ?? — SZD, Nov 22 '16 at 16:20
just what I said in the brackets. UDP does not guarantee that the packets arrive at the destination, TCP does check if they do, and if not sends them again. — InDaPond, Nov 23 '16 at 13:05
It seems that thread is wainting in run() method for incoming packets — Javoslaw, Nov 23 '16 at 14:04
But that is a new thread, why does the old one wait? He creates a new thread and then his work is done — InDaPond, Nov 23 '16 at 14:21
Google how to start a thread, because I think you misinterpreting what run() does. — chimmi, Nov 23 '16 at 16:13
The code between the printing "Packet with seq..." and the next print statement is either not returning, or throwing an uncaught exception. — G. Blake Meike, Nov 26 '16 at 03:05

score 1 · Answer 1 · edited May 23 '17 at 12:13

If ReceiveHandler is a Thread, it should be invoked as

new ReceiveHandler(reSendData, reSendData.length, clientSocket, rcvData, UDP_PACKET_SIZE, this).start();

But if it is a Runnable, it should be invoked as

new Thread(new ReceiveHandler(reSendData, reSendData.length, clientSocket, rcvData, UDP_PACKET_SIZE, this)).start();

run() will not execute the task in a separate Thread.

see: What's the difference between Thread start() and Runnable run()

Stuck in Interation which is protected by a semaphore

1 Answers1