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Quint16 zero crash

  • I'm reading in a block size for network code, using a quint16 to store the block size. I start the blockSize off as 0, and it's read in when the readMessage() slot is called from the socket's readyRead() signal. The program instantly crashes as soon as readMessage() is called, before the first line even runs. However, if I initialize blockSize as 1 instead of 0, there is no crash at all. The blockSize is never read in if I do this, though, so there are problems. This is an incredibly stupid bug and I don't know what the problem is.

    class cPlayer

    friend class MainServer;

    int id;
    QTcpSocket* socket;
    quint16 blockSize;
    cPlayer() { blockSize = 0; }

    Here's the code where the connections are setup. The message that is sent goes through, the client responds, and then the server crashes. The first line of readMessage is (qDebug() << "Message Received") and it doesn't happen.
    void MainServer::handleNewConnection()
    while(server->hasPendingConnections()) {
    cPlayer* newPlayer = new cPlayer;
    newPlayer->id = numPlayers++;
    newPlayer->socket = server->nextPendingConnection();
    connect(newPlayer->socket, SIGNAL(disconnected()), this, SLOT(handleDisconnection()));
    QSignalMapper* sMap = new QSignalMapper(this);
    sMap->setMapping(newPlayer->socket, newPlayer->id);
    connect(newPlayer->socket, SIGNAL(readyRead()), sMap, SLOT(map()));
    connect(sMap, SIGNAL(mapped(int)), this, SLOT(readMessage(int)));
    appendPlainText(QString("[%1]%2 connected - ID %3\n").arg(QTime::currentTime().toString()).arg(newPlayer->socket->peerAddress().toString()).arg(numPlayers-1));
    sendMessage(newPlayer->id, 0);

  • Lifetime Qt Champion

    Could you post a bit more code ? We don't know how sendMessage works nor readMessage and there might lie your problem.

    On an unrelated note, one thing that you could avoid is recreating sMap each time, you don't need one mapper per connection.

  • In line 7:
    [code]newPlayer->socket = server->nextPendingConnection();[/code]

    What happen, when nextPendingConnection()==NULL ?

  • Interestingly, you mention the quint16, but you never use it in any code (except for setting it to zero).

    I suggest using a debugger or something like valgrind to find out where the crash comes from.

  • Here is the sendMessage code:

    void MainServer::readMessage(int id)
    appendPlainText("This line is never being hit");
    cPlayer* Player = playerAt(id);
    QDataStream in(Player->socket);
    if(Player->blockSize == 0) {
    if(Player->socket->bytesAvailable() < (int)sizeof(quint16))
    in >> Player->blockSize;
    appendPlainText(QString("Block Size:%1").arg(Player->blockSize));
    if(Player->socket->bytesAvailable() < (quint16)Player->blockSize)
    quint16 msgid;
    in >> msgid;
    appendPlainText(QString("Received Message %1\n").arg(msgid));
    switch(msgid) {
    case 0:
    sendMessage(id, 1);
    Player->blockSize = 0;

  • Figured it out, dumb mistake. I was sending a map that didn't exist. I assumed readMessage was failing to run because I wasn't seeing my appended text, but it was just because the crash was happening before Qt's update event.

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