The Network Module in Qt 4

TCP Client

The first example illustrates how to write a TCP client using QTcpSocket. The client talks to a fortune server that provides fortune to the user. Here's how to set up the socket:

        tcpSocket = new QTcpSocket(this);

        connect(tcpSocket, SIGNAL(readyRead()), this, SLOT(readFortune()));
        connect(tcpSocket, SIGNAL(error(QAbstractSocket::SocketError)), this, SLOT(displayError(QAbstractSocket::SocketError)));

When the user requests a new fortune, the client establishes a connection to the server:

        tcpSocket->connectToHost(hostLineEdit->text(),
                                 portLineEdit->text().toInt());

When the server answers, the following code is executed to read the data from the socket:

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

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

            in >> blockSize;
        }

        if (tcpSocket->bytesAvailable() < blockSize)
            return;

        QString nextFortune;
        in >> nextFortune;

        if (nextFortune == currentFortune) {
            QTimer::singleShot(0, this, SLOT(requestNewFortune()));
            return;
        }

        currentFortune = nextFortune;

The server's answer starts with a size field (which we store in blockSize), followed by size bytes of data. If the client hasn't received all the data yet, it waits for the server to send more.

An alternative approach is to use a blocking socket. The code can then be concentrated in one function:

            const int Timeout = 5 * 1000;

            QTcpSocket socket;
            socket.connectToHost(serverName, serverPort);

            if (!socket.waitForConnected(Timeout)) {
                emit error(socket.error(), socket.errorString());
                return;
            }

            while (socket.bytesAvailable() < (int)sizeof(quint16)) {
                if (!socket.waitForReadyRead(Timeout)) {
                    emit error(socket.error(), socket.errorString());
                    return;
                }
            }

            quint16 blockSize;
            QDataStream in(&socket);
            in.setVersion(QDataStream::Qt_4_0);
            in >> blockSize;

            while (socket.bytesAvailable() < blockSize) {
                if (!socket.waitForReadyRead(Timeout)) {
                    emit error(socket.error(), socket.errorString());
                    return;
                }
            }

            QMutexLocker locker(&mutex);

            QString fortune;
            in >> fortune;
            emit newFortune(fortune);

        tcpServer = new QTcpServer(this);
        if (!tcpServer->listen()) {
            QMessageBox::critical(this, tr("Fortune Server"),
                                  tr("Unable to start the server: %1.")
                                  .arg(tcpServer->errorString()));
            close();
            return;
        }

        connect(tcpServer, SIGNAL(newConnection()), this, SLOT(sendFortune()));

        QByteArray block;
        QDataStream out(&block, QIODevice::WriteOnly);
        out.setVersion(QDataStream::Qt_4_0);
        out << (quint16)0;
        out << fortunes.at(rand() % fortunes.size());
        out.device()->seek(0);
        out << (quint16)(block.size() - sizeof(quint16));

        QTcpSocket *clientConnection = tcpServer->nextPendingConnection();
        connect(clientConnection, SIGNAL(disconnected()),
                clientConnection, SLOT(deleteLater()));

        clientConnection->write(block);
        clientConnection->disconnectFromHost();

        udpSocket = new QUdpSocket(this);
        QByteArray datagram = "Broadcast message " + QByteArray::number(messageNo);
        udpSocket->writeDatagram(datagram.data(), datagram.size(),
                                 QHostAddress::Broadcast, 45454);

        udpSocket = new QUdpSocket(this);
        udpSocket->bind(45454);

        connect(udpSocket, SIGNAL(readyRead()),
                this, SLOT(processPendingDatagrams()));

        while (udpSocket->hasPendingDatagrams()) {
            QByteArray datagram;
            datagram.resize(udpSocket->pendingDatagramSize());
            udpSocket->readDatagram(datagram.data(), datagram.size());
            statusLabel->setText(tr("Received datagram: \"%1\"")
                                 .arg(datagram.data()));
        }