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The QtNetwork module provides TCP/IP networking and higher layer network protocols to other modules. For example, QtWebkit and QtDeclarative.

The code is divided into submodules roughly corresponding to network layers.

Platform specific code is located in _platform.cpp files, with _generic.cpp or _tcp.cpp being a basic functionality version for other platforms without a detailed port.

Breakdown by submodule


The kernel contains classes which are used by multiple other layers, so don't fit well in one place. It is a mix of "kernel" and "tools" classes, but there aren't so many to need to further subdivide yet.

Low level

  • qhostaddress - data type for an IP address
  • qnetworkinterface - data type and code for enumerating interfaces (i.e. what shows up in the ifconfig/ipconfig command)


  • qhostinfo - used to resolve a host name to an IP address
  • qdnslookup - used for querying DNS records (e.g. SVC, MX..)


  • qauthenticator - data type, also contains code for calculating authentication responses. Used by http (access) and proxies (socket)
  • qnetworkproxy - data type, also contains code for enumerating proxies from the operating system. (see also socket layer for proxy handlers)
  • qurlinfo - QFileInfo for urls… used by QFtp


The socket subdirectory contains classes related to sockets. This is mainly TCP/IP related, but QLocalSocket is implemented on top of TCP only as a fallback. Note that QLocalSocket is not related to QAbstractSocket despite having a similar API.

Sockets are implemented using socket engines. Virtual functions are called on QAbstractSocketEngine, which are implemented by QNativeSocketEngine or a proxy.

local sockets

  • qlocalsocket - implements a local loopback socket
  • qlocalserver - implements a server socket to accept incoming connections
  • qlocal*_unix - AF_UNIX socket implementation
  • qlocal*_win - windows named pipe implementation
  • qlocal*tcp - TCP loopback implementation

IP sockets

  • qabstractsocket - base class for QTcpSocket and QUdpSocket, contains most of the implementation for both
  • qtcpsocket - TCP specific code
  • qudpsocket - UDP specific code
  • qtcpserver - server socket to accept incoming connections

socket engines

  • qabstractsocketengine - base class (interface)
  • qnativesocketengine - contains cross platform code for a real socket
  • qnativesocketengine(platform) - contains platform specific implementation using native API
  • qhttpsocketengine - implements http proxy using the CONNECT method (caching methods are dealt with in the http code in the access layer)


Implements the secure sockets layer. See the article Hacking on Qt's SSL Support for detail


Implements QNetworkAccessManager and related classes. This is the most complicated area of QtNetwork.

QNetworkAccessManager creates a class from the QNetworkReplyImpl family, and returns it to the application as a QNetworkReply There is a factory function to do this, plugins are not supported though. The generic QNetworkReplyImpl has a pointer to a backend which actually does the work. The non generic versions are optimised/specialised

HTTP is performed in a worker thread, to avoid disruption to the main UI thread (e.g. loading from network while scrolling)

Generally, a QAbstractSocket pointer is used to refer to a socket, and instantiated with either QTcpSocket or QSslSocket depending on the url scheme.

You can also take a look at this blogpost about the Qt HTTP stack.

QNetworkAccessManager front end

  • qnetworkaccessmanager
  • qnetworkrequest
  • qnetworkreply

QNetworkReply implementations

  • qnetworkreplyimpl - generic
  • qnetworkreplydataimpl - handles the data scheme
  • qnetworkreplyfileimpl - handles the file scheme
  • qnetworkreplyhttpimpl - handles the http(s) scheme
  • qnetworkaccesscachebackend - backend for loading from cache
  • qnetworkaccessdebugpipebackend - backend for unit testing
  • qnetworkaccessfilebackend - backend for file scheme (overlap/duplication?)
  • qnetworkaccessftpbackend - backend for ftp scheme


  • qftp - contains implementation for FTP
  • qnetworkaccessftpbackend - backend for ftp scheme


  • qhttpmultipart - container for multipart mime types when uploading
  • qhttpthreaddelegate - worker thread task, handles the inter thread communication. Owns the QHttpNetworkConnection
  • qhttpnetworkconnection - a logical connection to a server, owns multiple channels on which it sends queued requests.
  • qhttpnetworkconnectionchannel - a TCP socket connection to a server. May be reused if pipelineing is enabled.
  • qhttpnetworkheader - creation and parsing of http headers. Inherited by QNetworkReply and QNetworkRequest
  • qnetworkreplyhttpimpl - handles the http(s) scheme, creates the thread delegate.

cache and cookies

  • qabstractnetworkcache - base class for a url:object cache
  • qnetworkdiskcache - reference implementation for a cache backed by regular files in the filesystem
  • qnetworkcookie - data type for a cookie and associated metadata (e.g. security information)
  • qnetworkcookiejar - basic container for cookies, will be reimplemented by web browser or other application that needs persistant cookies

general backend

  • qnetworkaccesscache - used to hold open tcp connections that can be reused later
  • qnetworkaccessauthenticationmanager - cache of authentication credentials, tried first before emitting authenticationRequired signal


The bearer API is concerned with enumerating network bearers and controlling their lifetime. It is mainly important on mobile, battery powered devices Much of the implementation is in plugins (src/plugins/bearer/(platform)) There is some overlap with QNetworkInterface, but that is a passive api. The fallback bearer implementation works by polling QNetworkInterface, but the real implementations use OS specific event driven APIs.

An important concept is the "service network", which is an ordered list of network configurations (e.g. "home WLAN", "office WLAN", "3G data"); and the "default configuration", which is the user's preferred choice according to global configuration in the OS. The default configuration normally is a service network where the OS supports this.

The bearer API is used elsewhere in the network module, e.g. QNetworkAccessManager starts the default configuration unless it has been configured differently. QNetworkProxy::systemProxyForQuery can use a network configuration to make sure the correct proxy is used in multihoming situationss.

  • qnetworkconfigmanager - enumeration of network configurations, notification of configuration changes
  • qnetworkconfiguration - data type for a network configuration
  • qnetworksession - used to open/start a network configuration, and keep active for the object lifetime
  • qsharednetworksession - A thread local singleton used by QNetworkAccessManager
  • qbearerengine - interface to the backend
  • qbearerplugin - plugin loading

Areas for improvement

  • QAuthenticator (in the backend - the public interface is fine)
  • QFtp (withdrawn as a public API in Qt5)
  • QNetworkConfigurationManager (first construction blocks the thread for a noticable time)
  • sockets in sockets (QSslSocket, QHttpSocketEngine, QSocks5SocketEngine) each have a QAbstractSocket, leading to multiple levels of buffering and potential for racy shutdown
  • loading ssl certificates is slow and blocking (except linux which uses openssl demand loading, which only supports files)
  • we should support plugins for URL scheme handlers (e.g. ldap:) so that other Qt modules or applications themselves can get new types of url using QNetworkAccessManager.
  • see also JIRA