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The Network Module in Qt 4

The network module in Qt 4 provides some new features, such as support for internationalized domain names, better IPv6 support, and better performance. And since Qt 4 allows us to break binary compatibility with previous releases, we took this opportunity to improve the class names and API to make them more intuitive to use.

General Overview

Compared to Qt 3, the network module in Qt 4 brings the following benefits:

  • The Qt 4 network classes have more intuitive names and APIs. For example, QServerSocket has been renamed QTcpServer.
  • The entire network module is reentrant, making it possible to use them simultaneously from multiple threads.
  • It is now possible to send and receive UDP datagrams and to use synchronous (i.e., blocking) sockets without having to use a low-level API (QSocketDevice in Qt 3).
  • QHostAddress and QHostInfo support internationalized domain names (RFC 3492).
  • QUrl is more lightweight and fully supports the latest URI specification draft.
  • UDP broadcasting is now supported.

The Qt 4 network module provides fundamental classes for writing TCP and UDP applications, as well as higher-level classes that implement the client side of the HTTP and FTP protocols.

Here's an overview of the TCP and UDP classes:

  • QTcpSocket encapsulates a TCP socket. It inherits from QIODevice, so you can use QTextStream and QDataStream to read or write data. It is useful for writing both clients and servers.
  • QTcpServer allows you to listen on a certain port on a server. It emits a newConnection() signal every time a client tries to connect to the server. Once the connection is established, you can talk to the client using QTcpSocket.
  • QUdpSocket is an API for sending and receiving UDP datagrams.

QTcpSocket and QUdpSocket inherit most of their functionality from QAbstractSocket. You can also use QAbstractSocket directly as a wrapper around a native socket descriptor.

By default, the socket classes work asynchronously (i.e., they are non-blocking), emitting signals to notify when data has arrived or when the peer has closed the connection. In multithreaded applications and in non-GUI applications, you also have the opportunity of using blocking (synchronous) functions on the socket, which often results in a more straightforward style of programming, with the networking logic concentrated in one or two functions instead of spread across multiple slots.

QFtp and QHttp use QTcpSocket internally to implement the FTP and HTTP protocols. Both classes work asynchronously and can schedule (i.e., queue) requests.

The network module contains four helper classes: QHostAddress, QHostInfo, QUrl, and QUrlInfo. QHostAddress stores an IPv4 or IPv6 address, QHostInfo resolves host names into addresses, QUrl stores a URL, and QUrlInfo stores information about a resource pointed to by a URL, such as the file size and modification date. (Because QUrl is used by QTextBrowser, it is part of the QtCore library and not of QtNetwork.)

See the QtNetwork module overview for more information.

Example Code

All the code snippets presented here are quoted from self-contained, compilable examples located in Qt's examples/network directory.

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:


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

     QDataStream in(tcpSocket);

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

         in >> blockSize;

     if (tcpSocket->bytesAvailable() < blockSize)

     QString nextFortune;
     in >> nextFortune;

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

     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());

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

         quint16 blockSize;
         QDataStream in(&socket);
         in >> blockSize;

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

         QMutexLocker locker(&mutex);

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

TCP Server

The following code snippets illustrate how to write a TCP server using QTcpServer and QTcpSocket. Here's how to set up a TCP server:

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

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

When a client tries to connect to the server, the following code in the sendFortune() slot is executed:

     QByteArray block;
     QDataStream out(&block, QIODevice::WriteOnly);
     out << (quint16)0;
     out << % fortunes.size());
     out << (quint16)(block.size() - sizeof(quint16));

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


UDP Senders and Receivers

Here's how to broadcast a UDP datagram:

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

Here's how to receive a UDP datagram:

     udpSocket = new QUdpSocket(this);

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

Then in the processPendingDatagrams() slot:

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

Comparison with Qt 3

The main difference between Qt 3 and Qt 4 is that the very high level QNetworkProtocol and QUrlOperator abstraction has been eliminated. These classes attempted the impossible (unify FTP and HTTP under one roof), and unsurprisingly failed at that. Qt 4 still provides QFtp and QHttp classes, but only with the more mature API that appeared in Qt 3.1.

The QSocket class in Qt 3 has been renamed QTcpSocket. The new class is reentrant and supports blocking. It's also easier to handle closing than with Qt 3, where you had to connect to both the QSocket::connectionClosed() and the QSocket::delayedCloseFinished() signals.

The QServerSocket class in Qt 3 has been renamed QTcpServer. The API has changed quite a bit. While in Qt 3 it was necessary to subclass QServerSocket and reimplement the newConnection() pure virtual function, QTcpServer now emits a newConnection() signal that you can connect to a slot.

The QHostInfo class has been redesigned to use the operating system's getaddrinfo() function instead of implementing the DNS protocol. Internally, QHostInfo simply starts a thread and calls getaddrinfo() in that thread. This wasn't possible in Qt 3 because getaddrinfo() is a blocking call and Qt 3 could be configured without multithreading support.

The QSocketDevice class in Qt 3 is no longer part of the public Qt API. If you used QSocketDevice to send or receive UDP datagrams, use QUdpSocket instead. If you used QSocketDevice because it supported blocking sockets, use QTcpSocket or QUdpSocket instead and use the blocking functions (waitForConnected(), waitForReadyRead(), etc.). If you used QSocketDevice from a non-GUI thread because it was the only reentrant networking class in Qt 3, use QTcpSocket, QTcpServer, or QUdpSocket instead.

Internally, Qt 4 has a class called QSocketLayer that provides a cross-platform low-level socket API. It resembles the old QSocketDevice class. We might make it public in a later release if users ask for it.

As an aid to porting to Qt 4, the Qt3Support library includes Q3Dns, Q3ServerSocket, Q3Socket, and Q3SocketDevice classes.

[Previous: The Qt 4 Database GUI Layer] [Home] [Next: The Qt 4 Style API]


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