How to implement a simple tcp connection in Qt?

Question

I tried to modify the Qt network tutorial, and implemented it like:

quint16 blockSize;

void Client::readData()
{
    qDebug() << "Received Data!";
    QByteArray data;

    QDataStream in(tcpSocket);
    in.setVersion(QDataStream::Qt_4_0);

    if (blockSize == 0) {
        if (tcpSocket->bytesAvailable() < (int)sizeof(quint16))
            return;

        in >> blockSize;
    }

    qDebug() << "Received DATA II with blocksize " << blockSize;
    if (tcpSocket->bytesAvailable() < blockSize)
    {
        qDebug() << tcpSocket->bytesAvailable() << ' ' << blockSize;
        return;
    }
    qDebug() << "Received DATA III";
    in >> data;
    qDebug() << data;
    QByteArray dbg = data;   // create a copy to not alter the buffer itself
    dbg.replace('\\', "\\\\"); // escape the backslash itself
    dbg.replace('\0', "\\0");  // get rid of 0 characters
    dbg.replace('\n', "\\n");
    //dbg.replace('"', "\\\"");  // more special characters as you like
    qDebug() << dbg;
    QString data_string(data);
    qDebug() << "Emitting Signal";
    emit Client::gotData(data_string);

}

void Server::sendData(void)
{
        QByteArray block;
        QDataStream out(&block, QIODevice::WriteOnly);
        out.setVersion(QDataStream::Qt_4_0);
        out << (quint16)0;
        out << "Hello World, this is a very long text!";
        out.device()->seek(0);
        out << (quint16)(block.size() - sizeof(quint16));
        qDebug() << "First number is: " << (quint16)(block.size() - sizeof(quint16));
        qDebug() << "Blocksize is: " << block.size() << "with quint size: " << sizeof(quint16);

        qDebug() << "Sending data!";
        //Debug
        QByteArray dbg = block;   // create a copy to not alter the buffer itself
        dbg.replace('\\', "\\\\"); // escape the backslash itself
        dbg.replace('\0', "\\0");  // get rid of 0 characters
        dbg.replace('\n', "\\n");
        //dbg.replace('"', "\\\"");  // more special characters as you like
        qDebug() << dbg;
        connect(clientConnection, SIGNAL(disconnected()), clientConnection, SLOT(deleteLater()));
        clientConnection->write(block);
        //clientConnection->disconnectFromHost();
}

Connecting works fine, but when I call sendData(), I get (from the same function) the transmitted block:

First number is:  43
Blocksize is:  45 with quint size:  2
Sending data!
"\0+\0\0\0'Hello World, this is a very long text!\0"

My first problem is: Where do all the \0 come from? As far as I understand the code, first I'm creating a 0, then I write the text, and them I am going back to the front and write the full size of the block. Is it because of the size of the (quint16)? When entering nothing, I get \0\0 and a size of 0, which is correct. The size of the example above is 43, which corresponds to + in ascii (and this sign is in the block above, too).
My second problem is: My readData()-function does not recognize the block size, (it always returns 450 as block size, which is clearly wrong). Why? Did I miss something?
UPDATE: After changing QByteArray data; to QString data; my problems are gone, no more strange \0 in my code -> should have used the right data type -> Head->Desk