problem with QLocalSocket sending continues data to QLocalServer

Christian Ehrlicher

@Venkateswaran Why? It's a stream with no further protocol. It can even happen that you get a readyRead signal for every single byte you sent.

JonB

@Venkateswaran
And it absolutely is not! :)

readyRead fires when there is at least 1 byte available.
readAll reads all that happens to be there when it is called. Anything ranging from 0 bytes to every byte sent!
readyRead won't fire again till you've done a readAll.

That's it. No one-to-one. Your job to buffer received data at receiver, or split it up, if that's what you want to do.

Venkateswaran

@JonB Thanks for clarifying this. Now I have another problem, on the server-side, I need to call another function with received test string from the client. I have tested the readAll() and as you said I'm receiving 130 bytes all at once (which is 5 times my test string). So how do I separate on server-side? I'm using QDataStream on client-side to send data and is there an elegant way to decouple the received bytes at server-side with QDataStream? maybe its a really basic question but it would be helpful for me if you show some example.

JonB

@Venkateswaran
Start by: have you read (and understood!) all the detail in https://doc.qt.io/qt-5/qdatastream.html#details ? And do look at subsection https://doc.qt.io/qt-5/qdatastream.html#using-read-transactions, because you may be looking for that.

How is the client sending? ("I'm using QDataStream on client-side to send data" --- with just which calls on what data object types?) If you are using QDataStream at client send you will want QDataStream at server receive. And just btw: when you talk sometimes about "bytes" and sometimes about "string" do you indeed want QDataStream or did you maybe want QTextStream?

Venkateswaran

@JonB also should i need to call m_socket->waitForBytesWritten(); at client side ?

JonB

@Venkateswaran
Nope :) Not unless you want to, it just stops client proceeding to the next line of code till bytes are written. But it makes no difference to the protocol/behaviour. And no effect at server-side.

Venkateswaran

@JonB Thanks for the information. I want to send Boolean, Number, String from the client so DataStream would be the right one.
This is how I'm sending data from the client :

void TestClient::sendDataToServer() {
    QByteArray block;
    QDataStream out(&block, QIODevice::WriteOnly);
    out.setVersion(QDataStream::Qt_5_10);
    QString testString = "test data";
    out << quint32(testString.size());
    out << testString;
    m_socket->write(block);
    m_socket->flush();
}

This is what I tried at the server side to receive data (and it work for this basic example).

void TestServer::onNewData() {
    QLocalSocket* client = qobject_cast<QLocalSocket*>(sender());
    qCritical() << "TestServer::onNewData" << client->bytesAvailable();
    QDataStream in;
    in.setDevice(client);
    in.setVersion(QDataStream::Qt_5_10);
    quint32 blockSize = 0;
    QString test;

    while(client->bytesAvailable() > (int)sizeof(quint32)) {
        in >> blockSize;
        if (client->bytesAvailable() < blockSize || in.atEnd()) return;
        in >> test;
        qDebug() << test << "printing received value";
    }
    qCritical() << "data read by server";
}

But looks like This Answer has a nice example to start with

JonB

@Venkateswaran
Note that the link you reference (by @VRonin) uses the transactions I suggested earlier: https://doc.qt.io/qt-5/qdatastream.html#using-read-transactions. No bytesAvailable() or buffering. Up to you.

VRonin

@JonB said in problem with QLocalSocket sending continues data to QLocalServer:

readyRead won't fire again till you've done a readAll.

This is not correct. From https://doc.qt.io/qt-5/qabstractsocket.html

The readyRead() signal is emitted every time a new chunk of data has arrived.

So it doesn't care whether you read the data or not

I suggest having a look at this article it uses QTcpSocket but the code is exactly the same for QLocalSocket. I'd focus on the ChatClient::onReadyRead method and its explanation below

JonB

@VRonin said in problem with QLocalSocket sending continues data to QLocalServer:

This is not correct.

Damn, and sorry! I thought that it had said that, but not. I may have confused with

readyRead() is not emitted recursively; if you reenter the event loop or call waitForReadyRead() inside a slot connected to the readyRead() signal, the signal will not be reemitted (although waitForReadyRead() may still return true).