Why QTcpSocket received wrong data?

Question

I'm writing a simple network application. Client sending to server message server printing it in QTextEdit and responding to client . I'm using QTcpServer and QTcpSocket. There is a problem I can't solve. Receiving data is quint16 + QTime + QString which is sending as QByteArrey. I use quint16 for receiving size of data blocks. And for some reason when client send to server

next block size: 16 (quint16 value)
block size: 18 

Server get:

next block size: 30073  (quint16 value)
block size: 18 

As you can see, for some reason server getting from QDataStrem wrong variable value and it is always 30073. I don't understand why?

void Widget::slotSendToServer()
{
    logTextEdit->append("slotSendToServer()");
    QByteArray arrBlock;
    QDataStream serverSendStream(&arrBlock, QIODevice::ReadWrite);
    QString messageStr = messageLineEdit->text();

    serverSendStream << quint16(0) << QTime::currentTime()
                     << messageStr;

    serverSendStream.device()->seek(0);
    serverSendStream << (quint16)(arrBlock.size() - sizeof(quint16));

    qDebug() << "next_block_size:"
             <<(quint16)(arrBlock.size() - sizeof(quint16)) 
             << endl
             << "full size of Byte arrey:" << arrBlock.size();

    tcpSocket->write(arrBlock);
    messageLineEdit->clear();
}



void Widget::slotReadClient()
{
    logTextEdit->append("slotReadClient()");
    QTcpSocket *tcpSocket = (QTcpSocket*)sender();
    QDataStream clientReadStream(tcpSocket);

    while(true)
    {
        if (!next_block_size)
        {
            if (tcpSocket->bytesAvailable() < sizeof(quint16))
            {
                break;
            }
            clientReadStream >> next_block_size;
        }

        if (tcpSocket->bytesAvailable() < next_block_size)
        {
            break;
        }
        QTime   time;
        QString messageTextStr;
        clientReadStream >> time >> messageTextStr;

        QString messageCompleteStr =
                time.toString() + " " + "Client has sent - " 
                                + messageTextStr;
        logTextEdit->append("Message received: ");
        logTextEdit->append(messageCompleteStr);

        next_block_size = 0;

        sendToClient(tcpSocket,
                     "Server Response: Received \"" 
                      + messageTextStr + "\"");
    }
}

Answer 1

You should ensure that the variable next_block_size is initialized to 0 each time the socket is connected.

If you don't reuse the same QTcpSocket object, this can be done in your Widget class constructor, or if you do, in a slot connected to signal connected().