How do I know that the client has connected to the server?

[Christian Ehrlicher]

This is working fine for me:

#include <QtNetwork>

int main(int argc, char* argv[])
{
    QCoreApplication app(argc, argv);
    
    QTcpServer server;
    if (!server.listen(QHostAddress::Any, 50153))
        return 1;
    QObject::connect(&server, &QTcpServer::newConnection,
                     &server, [&server]() {
        QTcpSocket *socket = server.nextPendingConnection();
        qDebug() << "client connected from" << socket->peerAddress() << ":" << socket->peerPort();
        socket->write("Test\n");
        socket->flush();
        socket->waitForBytesWritten(300);
        socket->close();
    });
    
    QTcpSocket client;
    QObject::connect(&client, &QTcpSocket::readyRead,
                     &client, [&client]() {
        qDebug() << client.readAll();
        QCoreApplication::quit();
    });
    client.connectToHost("localhost", 50153);
    
    return app.exec();
}

[NintyS]

Thank you very much :D

[JonB]

@Christian-Ehrlicher said in How do I know that the client has connected to the server?:

    socket->flush();
    socket->waitForBytesWritten(300);

Does one have to have these two lines prior to socket->close(), or are you just being "safe"?

[Christian Ehrlicher]

The waitForBytesWritten() is needed.

[NintyS]

@Christian-Ehrlicher I know now why. I use a printf() to show the result. Now when I use qDebug I see my result after connection.

[JonB]

@NintyS
printf() is liable to be at least line-buffered, maybe even fully buffered. You would have to follow it with fflush(stdout) to try to see the output immediately. fprintf(stderr, ...) would probably be better. In any case qDebug() is a better choice for debugging.

[KroMignon]

@Christian-Ehrlicher said in How do I know that the client has connected to the server?:

the waitForBytesWritten() is needed.

Why? My understanding was that waitForBytesWritten() is only required if not event queue is available... But without event queue, the connect() won't work.

[Christian Ehrlicher]

@KroMignon Because of the close() directly afterwards. I'm pretty sure close() will call flush() before but it's not documented.

[J.Hilk]

@KroMignon probably because of the immediate call to close on the socket

you could alternatively, potentially nest lambdas

       QTcpSocket *socket = server.nextPendingConnection();
            qDebug() << "client connected from" << socket->peerAddress() << ":" << socket->peerPort();
            QByteArray data{"Test\n"};
            QObject::connect(socket, &QTcpSocket::bytesWritten, socket, [socket, bytesToSend = data.size()](qint64 bytes)->void{
                static qint64 bytesSend{0};
                bytesSend += bytes;
                if(static_cast<qint64>(bytesToSend) == bytesSend)
                    socket->close();
            });
            socket->write(data);
        });

[KroMignon]

@J-Hilk said in How do I know that the client has connected to the server?:

you could alternatively, potentially nest lambdas

       QTcpSocket *socket = server.nextPendingConnection();
            qDebug() << "client connected from" << socket->peerAddress() << ":" << socket->peerPort();
            QByteArray data{"Test\n"};
            QObject::connect(socket, &QTcpSocket::bytesWritten, socket, [socket, bytesToSend = data.size()](qint64 bytes)->void{
                static qint64 bytesSend{0};
                bytesSend += bytes;
                if(static_cast<qint64>(bytesToSend) == bytesSend)
                    socket->close();
            });
            socket->write(data);
        });

Hmm, I would do it like this:

QTcpSocket *socket = server.nextPendingConnection();
qDebug() << "client connected from" << socket->peerAddress() << ":" << socket->peerPort();
QByteArray data{"Test\n"};
QObject::connect(socket, &QTcpSocket::bytesWritten, socket, [socket](qint64 bytes)->void{
    if(socket->bytesToWrite() == 0)
        socket->close();
});
socket->write(data);

[J.Hilk]

@KroMignon potentially, I'm not familiar enough with the inner workings of QAbstractSocket to say for sure, that bytesToWrite is updated before bytesWritten is emitted 🤷‍♂️

[JonB]

@J-Hilk said in How do I know that the client has connected to the server?:

static qint64 bytesSend{0};

You might like to come join us in my question about this over at https://forum.qt.io/topic/123582/static-in-lambda ? :)