Question

I’ve modified the threaded fortune-server from Qt examples.
The client connects to the server and then sends a header to authenticate.

tcpSocket = new QTcpSocket();
tcpSocket->connectToHost(addr, port);
QByteArray block = "someheader";
int x = tcpSocket->write(block);
qDebug() << x;

The client seems OK here and qDebug prints the actual size of block.

On the server side I’ve predefined incomingConnection and I start thread to each new connection.

void Server::incomingConnection(int socketDescriptor) {
    const QString &str = vec[qrand() % vec.size()];
    SpellThread *thread = new SpellThread(socketDescriptor, str);
    connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
    qDebug() << " -- incoming connection";
    thread->start();
}

I’m connecting sock to check is there something to read. (sock here is QTcpServer*)

void SpellThread::run() {
    qDebug() << " -- in spellthread";
    connect(sock, SIGNAL(readyRead()), this, SLOT(checkBytes()));
    //....
    qDebug() << " -- end spellthread";
}

The first problem is that when I’m sending data from the client, readyRead is not fired. (I’ve added debug message in checkBytes)
Messages are:

 -- incoming connection 
 -- in spellthread 
 -- end spellthread

Although the client prints the actual size of header length.

The second problem is that checkBytes currently is very bad-designed. First it checks is header OK and sets a flag, then it gets the size of message and sets another flag and finally it gets the real message. This is very clumsy. I first tried to escape signals and instead use sock->waitForReadyRead(). However it always returns false. (From the docs: “Reimplement this function to provide a blocking API for a custom device. The default implementation does nothing, and returns false.”).

So how to really make a client/server application in Qt with multiple clients and multiple reads/writes? I really want suggestions to improve design of my application and to solve my current two problems.

Answer 1

You can’t use slots or socket signals with a thread without calling QThread::exec() to start an event loop within that thread/the run function.

Since your checkBytes slot belongs to QThread, it wouldn’t be executed by the thread (there is a detailed article about QThreads here)

The closest example that seems to already do what you want is the Network Chat (particularly the two classes Server and Connection).

———-
Edit

If you need to use threads (without any slot), the QTcpSocket object must belongs to the same thread as the one where you call waitForReadyRead. For example, with:

SpellThread::SpellThread(int socketDescriptor, const QString & str) {
    tcpSocket = new QTcpSocket(); // There should be no parent to be able
                                  // to move it to the thread
    tcpSocket->moveToThread(this);
    ...

Or by creating the QTcpSocket object inside the run function so that it automatically belongs to that thread (it was briefly explained in the fortune example).
If you allocate the QTcpSocket dynamically, and because it won’t have a parent, you should also delete it manually.