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QUdpSocket disconnects after data writing

  • Hello everyone! I'm trying to write a client for server (which is not mine) using QUdpSocket. But I have some problems. The main one is that socket goes to unconnected state whenever after any data was written to it.

    Init socket example:

    mUdpSocket = new QUdpSocket(this)
    connect(mUdpSocket, &QUdpSocket::readyRead, this, &SessionManager::onSocketReadyRead);
    connect(mUdpSocket, &QUdpSocket::bytesWritten, [](quint64 b) { qDebug() << "____ UDP BYTES WRITTEN:" << b; });
    connect(mUdpSocket, &QUdpSocket::stateChanged, [this](QAbstractSocket::SocketState socketState) {  qDebug() << "____ UDP SOCKET STATE CHANGED:" << socketState; });
    qDebug() << "____ BIND" << mUdpSocket->bind(QHostAddress::Any);

    Write example:

    void SessionManager::sendMessage(const QByteArray& message)
        if (!message.isEmpty())
            mUdpSocket->writeDatagram(message, QHostAddress(mHost), G_SESSION_PORT);
            // TODO: check written != -1

    Problem occurs after calling writeDatagram. I can see that 76 bytes was written succesfully and was sent to server: signal bytesWritten reports the same amount of bytes written as in sendMessage method. I also can see with Wireshark that package was sent successfully to the server. After that socket goes to unconnected state with no error (QAbstractSocket::UnknownSocketError). At that moment I can see that server tries to send me several packages, but no readyRead signal was emmited from socket. Can someone help me? I dont know what is going wrong.

  • Lifetime Qt Champion

    @ClingerWinger the most likely problem is, that your QUdpSocket is destroyed - directly or indirectly - after writeDatagram().

    Its impossible to guess more from your current code, so if my answer doesnt help you you should upload the complete example so others can try it.


  • Just realised that I forgot what happened at my main.cpp. Object SessionManager was created at lambda, and destroyed there immediately. Sorry for you bother.

  • Lifetime Qt Champion

    @ClingerWinger Glad you figured it out and thanks for the feedback :)

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