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<title>Qt 4.6: The Network Module in Qt 4</title>
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<h1 class="title">The Network Module in Qt 4<br /><span class="subtitle"></span>
</h1>
<p>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.</p>
<ul><li><a href="#general-overview">General Overview</a></li>
<li><a href="#example-code">Example Code</a></li>
<ul><li><a href="#tcp-client">TCP Client</a></li>
<li><a href="#tcp-server">TCP Server</a></li>
<li><a href="#udp-senders-and-receivers">UDP Senders and Receivers</a></li>
</ul>
<li><a href="#comparison-with-qt-3">Comparison with Qt 3</a></li>
</ul>
<a name="general-overview"></a>
<h2>General Overview</h2>
<p>Compared to Qt 3, the network module in Qt 4 brings the following benefits:</p>
<ul>
<li>The Qt 4 network classes have more intuitive names and APIs. For example, <a href="porting4.html#qserversocket">QServerSocket</a> has been renamed <a href="qtcpserver.html">QTcpServer</a>.</li>
<li>The entire network module is <a href="threads-reentrancy.html#reentrant">reentrant</a>, making it possible to use them simultaneously from multiple threads.</li>
<li>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 (<a href="porting4.html#qsocketdevice">QSocketDevice</a> in Qt 3).</li>
<li><a href="qhostaddress.html">QHostAddress</a> and <a href="qhostinfo.html">QHostInfo</a> support internationalized domain names (RFC 3492).</li>
<li><a href="qurl.html">QUrl</a> is more lightweight and fully supports the latest URI specification draft.</li>
<li>UDP broadcasting is now supported.</li>
</ul>
<p>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.</p>
<p>Here's an overview of the TCP and UDP classes:</p>
<ul>
<li><a href="qtcpsocket.html">QTcpSocket</a> encapsulates a TCP socket. It inherits from <a href="qiodevice.html">QIODevice</a>, so you can use <a href="qtextstream.html">QTextStream</a> and <a href="qdatastream.html">QDataStream</a> to read or write data. It is useful for writing both clients and servers.</li>
<li><a href="qtcpserver.html">QTcpServer</a> allows you to listen on a certain port on a server. It emits a <a href="qtcpserver.html#newConnection">newConnection()</a> signal every time a client tries to connect to the server. Once the connection is established, you can talk to the client using <a href="qtcpsocket.html">QTcpSocket</a>.</li>
<li><a href="qudpsocket.html">QUdpSocket</a> is an API for sending and receiving UDP datagrams.</li>
</ul>
<p><a href="qtcpsocket.html">QTcpSocket</a> and <a href="qudpsocket.html">QUdpSocket</a> inherit most of their functionality from <a href="qabstractsocket.html">QAbstractSocket</a>. You can also use <a href="qabstractsocket.html">QAbstractSocket</a> directly as a wrapper around a native socket descriptor.</p>
<p>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.</p>
<p><a href="qftp.html">QFtp</a> and <a href="qnetworkaccessmanager.html">QNetworkAccessManager</a> and its associated classes use <a href="qtcpsocket.html">QTcpSocket</a> internally to implement the FTP and HTTP protocols. The classes work asynchronously and can schedule (i.e., queue) requests.</p>
<p>The network module contains four helper classes: <a href="qhostaddress.html">QHostAddress</a>, <a href="qhostinfo.html">QHostInfo</a>, <a href="qurl.html">QUrl</a>, and <a href="qurlinfo.html">QUrlInfo</a>. <a href="qhostaddress.html">QHostAddress</a> stores an IPv4 or IPv6 address, <a href="qhostinfo.html">QHostInfo</a> resolves host names into addresses, <a href="qurl.html">QUrl</a> stores a URL, and <a href="qurlinfo.html">QUrlInfo</a> stores information about a resource pointed to by a URL, such as the file size and modification date. (Because <a href="qurl.html">QUrl</a> is used by <a href="qtextbrowser.html">QTextBrowser</a>, it is part of the <a href="qtcore.html">QtCore</a> library and not of <a href="qtnetwork.html">QtNetwork</a>.)</p>
<p>See the <a href="qtnetwork.html">QtNetwork</a> module overview for more information.</p>
<a name="example-code"></a>
<h2>Example Code</h2>
<p>All the code snippets presented here are quoted from self-contained, compilable examples located in Qt's <tt>examples/network</tt> directory.</p>
<a name="tcp-client"></a>
<h3>TCP Client</h3>
<p>The first example illustrates how to write a TCP client using <a href="qtcpsocket.html">QTcpSocket</a>. The client talks to a fortune server that provides fortune to the user. Here's how to set up the socket:</p>
<pre> tcpSocket = new QTcpSocket(this);
connect(tcpSocket, SIGNAL(readyRead()), this, SLOT(readFortune()));
connect(tcpSocket, SIGNAL(error(QAbstractSocket::SocketError)),
this, SLOT(displayError(QAbstractSocket::SocketError)));</pre>
<p>When the user requests a new fortune, the client establishes a connection to the server:</p>
<pre> tcpSocket->connectToHost(hostLineEdit->text(),
portLineEdit->text().toInt());</pre>
<p>When the server answers, the following code is executed to read the data from the socket:</p>
<pre> 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;</pre>
<p>The server's answer starts with a <i>size</i> field (which we store in <tt>blockSize</tt>), followed by <i>size</i> bytes of data. If the client hasn't received all the data yet, it waits for the server to send more.</p>
<p>An alternative approach is to use a blocking socket. The code can then be concentrated in one function:</p>
<pre> 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;
}
}
mutex.lock();
QString fortune;
in >> fortune;
emit newFortune(fortune);</pre>
<a name="tcp-server"></a>
<h3>TCP Server</h3>
<p>The following code snippets illustrate how to write a TCP server using <a href="qtcpserver.html">QTcpServer</a> and <a href="qtcpsocket.html">QTcpSocket</a>. Here's how to set up a TCP server:</p>
<pre> 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()));</pre>
<p>When a client tries to connect to the server, the following code in the sendFortune() slot is executed:</p>
<pre> QByteArray block;
QDataStream out(&block, QIODevice::WriteOnly);
out.setVersion(QDataStream::Qt_4_0);
out << (quint16)0;
out << fortunes.at(qrand() % 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();</pre>
<a name="udp-senders-and-receivers"></a>
<h3>UDP Senders and Receivers</h3>
<p>Here's how to broadcast a UDP datagram:</p>
<pre> udpSocket = new QUdpSocket(this);
QByteArray datagram = "Broadcast message " + QByteArray::number(messageNo);
udpSocket->writeDatagram(datagram.data(), datagram.size(),
QHostAddress::Broadcast, 45454);</pre>
<p>Here's how to receive a UDP datagram:</p>
<pre> udpSocket = new QUdpSocket(this);
udpSocket->bind(45454, QUdpSocket::ShareAddress);
connect(udpSocket, SIGNAL(readyRead()),
this, SLOT(processPendingDatagrams()));</pre>
<p>Then in the processPendingDatagrams() slot:</p>
<pre> while (udpSocket->hasPendingDatagrams()) {
QByteArray datagram;
datagram.resize(udpSocket->pendingDatagramSize());
udpSocket->readDatagram(datagram.data(), datagram.size());
statusLabel->setText(tr("Received datagram: \"%1\"")
.arg(datagram.data()));
}</pre>
<a name="comparison-with-qt-3"></a>
<h2>Comparison with Qt 3</h2>
<p>The main difference between Qt 3 and Qt 4 is that the very high level <a href="porting4.html#qnetworkprotocol">QNetworkProtocol</a> and <a href="porting4.html#qurloperator">QUrlOperator</a> 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 <a href="qftp.html">QFtp</a>, and it also provides the <a href="qnetworkaccessmanager.html">QNetworkAccessManager</a>.</p>
<p>The <a href="porting4.html#qsocket">QSocket</a> class in Qt 3 has been renamed <a href="qtcpsocket.html">QTcpSocket</a>. 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.</p>
<p>The <a href="porting4.html#qserversocket">QServerSocket</a> class in Qt 3 has been renamed <a href="qtcpserver.html">QTcpServer</a>. The API has changed quite a bit. While in Qt 3 it was necessary to subclass <a href="porting4.html#qserversocket">QServerSocket</a> and reimplement the newConnection() pure virtual function, <a href="qtcpserver.html">QTcpServer</a> now emits a <a href="qtcpserver.html#newConnection">newConnection()</a> signal that you can connect to a slot.</p>
<p>The <a href="qhostinfo.html">QHostInfo</a> class has been redesigned to use the operating system's getaddrinfo() function instead of implementing the DNS protocol. Internally, <a href="qhostinfo.html">QHostInfo</a> 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.</p>
<p>The <a href="porting4.html#qsocketdevice">QSocketDevice</a> class in Qt 3 is no longer part of the public Qt API. If you used <a href="porting4.html#qsocketdevice">QSocketDevice</a> to send or receive UDP datagrams, use <a href="qudpsocket.html">QUdpSocket</a> instead. If you used <a href="porting4.html#qsocketdevice">QSocketDevice</a> because it supported blocking sockets, use <a href="qtcpsocket.html">QTcpSocket</a> or <a href="qudpsocket.html">QUdpSocket</a> instead and use the blocking functions (<a href="qabstractsocket.html#waitForConnected">waitForConnected()</a>, <a href="qabstractsocket.html#waitForConnected">waitForReadyRead()</a>, etc.). If you used <a href="porting4.html#qsocketdevice">QSocketDevice</a> from a non-GUI thread because it was the only reentrant networking class in Qt 3, use <a href="qtcpsocket.html">QTcpSocket</a>, <a href="qtcpserver.html">QTcpServer</a>, or <a href="qudpsocket.html">QUdpSocket</a> instead.</p>
<p>Internally, Qt 4 has a class called QSocketLayer that provides a cross-platform low-level socket API. It resembles the old <a href="porting4.html#qsocketdevice">QSocketDevice</a> class. We might make it public in a later release if users ask for it.</p>
<p>As an aid to porting to Qt 4, the <a href="qt3support.html">Qt3Support</a> library includes <a href="q3dns.html" class="compat">Q3Dns</a>, <a href="q3serversocket.html" class="compat">Q3ServerSocket</a>, <a href="q3socket.html" class="compat">Q3Socket</a>, and <a href="q3socketdevice.html" class="compat">Q3SocketDevice</a> classes.</p>
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