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  Help: BufferUnderflow in my game using NIO UDP  (Read 1094 times)
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Offline eVloier

Innocent Bystander




Java games rock!


« Posted 2004-07-04 16:31:41 »

Hi there, I keep getting "BufferUnderflow" exception in my 2D spaceship multiplayer game. I'm using NIO's UDP, in my network class, all the communication between clients and server for game initialization worked fine, data did get through and processed. however when game starts, whenever server sends client something(eg, other client's command, server game state), client encounters that BufferUnderflow. Following is the code where the problem occurs, any help will be really appreciated.

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//***client's receiving method***
//***this is the method throws BufferUnderflow exception***

public void cReceive(long rStartTime) {
       
    do {

      readBuffer.clear(); //read buffer is initialized in constructor
      SocketAddress sourceAddr = null;

      // read from the channel into our buffer
      try {
        sourceAddr = dChannel.receive(readBuffer);
      }
      catch (Exception ioe) {
        System.out.println(ioe);
      }

      // check for end-of-stream
      if (sourceAddr == null) {
        break;
      }
      else {
     
        readBuffer.flip();
       
        int i = 0;

        //***following line, will throw the underflow exception***

        int thePnum = readBuffer.getInt();

        i++; // i is 1 here
        int value = readBuffer.getInt();
       
        System.out.println("No underflow until got packet num and type.");

        ......
      }

    }
    while ((System.currentTimeMillis()-rStartTime)<10);
}


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//***server's sending code***
  public void broadcastGameState() {
        ByteBuffer gameStateBuffer = ByteBuffer.allocate(3072); //enough for holding 100 objects
                                                  //consider use compression later on
        Vector userGameState=null;
        for (int u=2; u<nextAvailableUserId; u++) {
              if (clientsAddrs[u-1]!=null) {
                    gameStateBuffer.clear();
                    userGameState = gameEngine.getUserGameState(u);
                    if (userGameState == null) {
                          continue;
                    } else {
                          gameStateBuffer.clear();
                          ///put packet num, type, (server id?), frame num
                          gameStateBuffer.putInt(0, -1); //packetNum
                          gameStateBuffer.putInt(4, 19); //type is 19
                          gameStateBuffer.putInt(8, SERVER_USER_ID);
                          gameStateBuffer.putInt(12, currentFramesEnd);
                          for (int k=0; k<userGameState.size(); k++) {
                                int[] theObj = (int[]) userGameState.elementAt(k);
                                for (int j=0; j<7; j++) {
                                      gameStateBuffer.putInt(k*28+j*4+16, theObj[j]);
                                }
                          }
                          gameStateBuffer.flip();
                          try {
                                dChannel.send(gameStateBuffer, clientsAddrs[u-1]);
                                                          } catch (Exception e) {
                                System.out.println("Exception when broadcasting game state: "+e);
                          }
                    }
              }
        }
           
  }
Offline blahblahblahh

JGO Coder


Medals: 1


http://t-machine.org


« Reply #1 - Posted 2004-07-13 14:11:16 »

Seeing as no-one else has come to the rescue, I'll have a go Smiley although I warn you I only rarely deal with UDP issues (most of my stuff is UDP/TCP independent, and so I tend to work with TCP as debugging is a little easier).

OK, first things first. You didn't say whether you're in non-blocking mode. This matters because the UDP read methods act differently depending on mode (subtle but important differences!). I think I can guess, based on what you do next, but it would help to know for sure before I start barking up the wrong tree.

Moving on, the BUE is caused because there were fewer than 4 bytes in the UDP payload. So, the first thing you do is download a packet-sniffer and check EXACTLY how many bytes are in the FIRST UDP packet to arrive.

For instance, it could easily be that the UDP packet is being split before arrival. There are several possible causes for this; do NOT assume UDP packets arrive precisely as sent!

It would also help to know EXACTLY what data was arriving, which the packet sniffer will tell you (a good one understand UDP and display  the payload for you as a text string; good ones are free / open-source).

malloc will be first against the wall when the revolution comes...
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