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30 \title Network Programming API
31 \brief Classes for Network Programming
37 \page network-programming.html
38 \title Network Programming
40 \brief An Introduction to Network Programming with Qt
42 The QtNetwork module offers classes that allow you to write TCP/IP clients
43 and servers. It offers lower-level classes such as QTcpSocket,
44 QTcpServer and QUdpSocket that represent low level network concepts,
45 and high level classes such as QNetworkRequest, QNetworkReply and
46 QNetworkAccessManager to perform network operations using common protocols.
47 It also offers classes such as QNetworkConfiguration,
48 QNetworkConfigurationManager and QNetworkSession that implement bearer
53 \section1 Qt's Classes for Network Programming
55 The following classes provide support for network programming in Qt.
57 \annotatedlist network
59 \section1 High Level Network Operations for HTTP and FTP
61 The Network Access API is a collection of classes for performing
62 common network operations. The API provides an abstraction layer
63 over the specific operations and protocols used (for example,
64 getting and posting data over HTTP), and only exposes classes,
65 functions, and signals for general or high level concepts.
67 Network requests are represented by the QNetworkRequest class,
68 which also acts as a general container for information associated
69 with a request, such as any header information and the encryption
70 used. The URL specified when a request object is constructed
71 determines the protocol used for a request.
72 Currently HTTP, FTP and local file URLs are supported for uploading
75 The coordination of network operations is performed by the
76 QNetworkAccessManager class. Once a request has been created,
77 this class is used to dispatch it and emit signals to report on
78 its progress. The manager also coordinates the use of
79 \l{QNetworkCookieJar}{cookies} to store data on the client,
80 authentication requests, and the use of proxies.
82 Replies to network requests are represented by the QNetworkReply
83 class; these are created by QNetworkAccessManager when a request
84 is dispatched. The signals provided by QNetworkReply can be used
85 to monitor each reply individually, or developers may choose to
86 use the manager's signals for this purpose instead and discard
87 references to replies. Since QNetworkReply is a subclass of
88 QIODevice, replies can be handled synchronously or asynchronously;
89 i.e., as blocking or non-blocking operations.
91 Each application or library can create one or more instances of
92 QNetworkAccessManager to handle network communication.
94 \section1 Using TCP with QTcpSocket and QTcpServer
96 TCP (Transmission Control Protocol) is a low-level network
97 protocol used by most Internet protocols, including HTTP and FTP,
98 for data transfer. It is a reliable, stream-oriented,
99 connection-oriented transport protocol. It is particularly well
100 suited to the continuous transmission of data.
102 \image tcpstream.png A TCP Stream
104 The QTcpSocket class provides an interface for TCP. You can use
105 QTcpSocket to implement standard network protocols such as POP3,
106 SMTP, and NNTP, as well as custom protocols.
108 A TCP connection must be established to a remote host and port
109 before any data transfer can begin. Once the connection has been
110 established, the IP address and port of the peer are available
111 through QTcpSocket::peerAddress() and QTcpSocket::peerPort(). At
112 any time, the peer can close the connection, and data transfer
113 will then stop immediately.
115 QTcpSocket works asynchronously and emits signals to report status
116 changes and errors, just like QNetworkAccessManager. It
117 relies on the event loop to detect incoming data and to
118 automatically flush outgoing data. You can write data to the
119 socket using QTcpSocket::write(), and read data using
120 QTcpSocket::read(). QTcpSocket represents two independent streams
121 of data: one for reading and one for writing.
123 Since QTcpSocket inherits QIODevice, you can use it with
124 QTextStream and QDataStream. When reading from a QTcpSocket, you
125 must make sure that enough data is available by calling
126 QTcpSocket::bytesAvailable() beforehand.
128 If you need to handle incoming TCP connections (e.g., in a server
129 application), use the QTcpServer class. Call QTcpServer::listen()
130 to set up the server, and connect to the
131 QTcpServer::newConnection() signal, which is emitted once for
132 every client that connects. In your slot, call
133 QTcpServer::nextPendingConnection() to accept the connection and
134 use the returned QTcpSocket to communicate with the client.
136 Although most of its functions work asynchronously, it's possible
137 to use QTcpSocket synchronously (i.e., blocking). To get blocking
138 behavior, call QTcpSocket's waitFor...() functions; these suspend
139 the calling thread until a signal has been emitted. For example,
140 after calling the non-blocking QTcpSocket::connectToHost()
141 function, call QTcpSocket::waitForConnected() to block the thread
142 until the \l{QTcpSocket::connected()}{connected()} signal has
145 Synchronous sockets often lead to code with a simpler flow of
146 control. The main disadvantage of the waitFor...() approach is
147 that events won't be processed while a waitFor...() function is
148 blocking. If used in the GUI thread, this might freeze the
149 application's user interface. For this reason, we recommend that
150 you use synchronous sockets only in non-GUI threads. When used
151 synchronously, QTcpSocket doesn't require an event loop.
153 The \l{network/fortuneclient}{Fortune Client} and
154 \l{network/fortuneserver}{Fortune Server} examples show how to use
155 QTcpSocket and QTcpServer to write TCP client-server
156 applications. See also \l{network/blockingfortuneclient}{Blocking
157 Fortune Client} for an example on how to use a synchronous
158 QTcpSocket in a separate thread (without using an event loop),
159 and \l{network/threadedfortuneserver}{Threaded Fortune Server}
160 for an example of a multithreaded TCP server with one thread per
163 \section1 Using UDP with QUdpSocket
165 UDP (User Datagram Protocol) is a lightweight, unreliable,
166 datagram-oriented, connectionless protocol. It can be used when
167 reliability isn't important. For example, a server that reports
168 the time of day could choose UDP. If a datagram with the time of
169 day is lost, the client can simply make another request.
171 \image udppackets.png UDP Packets
173 The QUdpSocket class allows you to send and receive UDP
174 datagrams. It inherits QAbstractSocket, and it therefore shares
175 most of QTcpSocket's interface. The main difference is that
176 QUdpSocket transfers data as datagrams instead of as a continuous
177 stream of data. In short, a datagram is a data packet of limited
178 size (normally smaller than 512 bytes), containing the IP address
179 and port of the datagram's sender and receiver in addition to the
180 data being transferred.
182 QUdpSocket supports IPv4 broadcasting. Broadcasting is often used
183 to implement network discovery protocols, such as finding which
184 host on the network has the most free hard disk space. One host
185 broadcasts a datagram to the network that all other hosts
186 receive. Each host that receives a request then sends a reply
187 back to the sender with its current amount of free disk space.
188 The originator waits until it has received replies from all
189 hosts, and can then choose the server with most free space to
190 store data. To broadcast a datagram, simply send it to the
191 special address QHostAddress::Broadcast (255.255.255.255), or
192 to your local network's broadcast address.
194 QUdpSocket::bind() prepares the socket for accepting incoming
195 datagrams, much like QTcpServer::listen() for TCP servers.
196 Whenever one or more datagrams arrive, QUdpSocket emits the
197 \l{QUdpSocket::readyRead()}{readyRead()} signal. Call
198 QUdpSocket::readDatagram() to read the datagram.
200 The \l{network/broadcastsender}{Broadcast Sender} and
201 \l{network/broadcastreceiver}{Broadcast Receiver} examples show how to
202 write a UDP sender and a UDP receiver using Qt.
204 QUdpSocket also supports multicasting. The
205 \l{network/multicastsender}{Multicast Sender} and
206 \l{network/multicastreceiver}{Multicast Receiver} examples show how to use
207 write UDP multicast clients.
209 \section1 Resolving Host Names using QHostInfo
211 Before establishing a network connection, QTcpSocket and
212 QUdpSocket perform a name lookup, translating the host name
213 you're connecting to into an IP address. This operation is
214 usually performed using the DNS (Domain Name Service) protocol.
216 QHostInfo provides a static function that lets you perform such a
217 lookup yourself. By calling QHostInfo::lookupHost() with a host
218 name, a QObject pointer, and a slot signature, QHostInfo will
219 perform the name lookup and invoke the given slot when the
220 results are ready. The actual lookup is done in a separate
221 thread, making use of the operating system's own methods for
222 performing name lookups.
224 QHostInfo also provides a static function called
225 QHostInfo::fromName() that takes the host name as argument and
226 returns the results. In this case, the name lookup is performed
227 in the same thread as the caller. This overload is useful for
228 non-GUI applications or for doing name lookups in a separate,
229 non-GUI thread. (Calling this function in a GUI thread may cause
230 your user interface to freeze while the function blocks as
231 it performs the lookup.)
233 \section1 Support for Network Proxies
235 Network communication with Qt can be performed through proxies,
236 which direct or filter network traffic between local and remote
239 Individual proxies are represented by the QNetworkProxy class,
240 which is used to describe and configure the connection to a proxy.
241 Proxy types which operate on different levels of network communication
242 are supported, with SOCKS 5 support allowing proxying of network
243 traffic at a low level, and HTTP and FTP proxying working at the
244 protocol level. See QNetworkProxy::ProxyType for more information.
246 Proxying can be enabled on a per-socket basis or for all network
247 communication in an application. A newly opened socket can be
248 made to use a proxy by calling its QAbstractSocket::setProxy()
249 function before it is connected. Application-wide proxying can
250 be enabled for all subsequent socket connections through the use
251 of the QNetworkProxy::setApplicationProxy() function.
253 Proxy factories are used to create policies for proxy use.
254 QNetworkProxyFactory supplies proxies based on queries for specific
255 proxy types. The queries themselves are encoded in QNetworkProxyQuery
256 objects which enable proxies to be selected based on key criteria,
257 such as the purpose of the proxy (TCP, UDP, TCP server, URL request),
258 local port, remote host and port, and the protocol in use (HTTP, FTP,
261 QNetworkProxyFactory::proxyForQuery() is used to query the factory
262 directly. An application-wide policy for proxying can be implemented
263 by passing a factory to QNetworkProxyFactory::setApplicationProxyFactory()
264 and a custom proxying policy can be created by subclassing
265 QNetworkProxyFactory; see the class documentation for details.
267 \section1 Bearer Management Support
269 Bearer Management controls the connectivity state of the device such that
270 the application can start or stop network interfaces and roam
271 transparently between access points.
273 The QNetworkConfigurationManager class manages the list of network
274 configurations known to the device. A network configuration describes the
275 set of parameters used to start a network interface and is represented by
276 the QNetworkConfiguration class.
278 A network interface is started by openning a QNetworkSession based on a
279 given network configuration. In most situations creating a network session
280 based on the platform specified default network configuration is
281 appropriate. The default network configuration is returned by the
282 QNetworkConfigurationManager::defaultConfiguration() function.
284 On some platforms it is a platform requirement that the application open a
285 network session before any network operations can be performed. This can be
286 tested by the presents of the
287 QNetworkConfigurationManager::NetworkSessionRequired flag in the value
288 returned by the QNetworkConfigurationManager::capabilities() function.
290 \sa {Bearer Management}