Networking and Connectivity

Qt provides classes for both high-level and low-level network communication, classes for web integration, and classes for interprocess communication.

For high-level network traffic, Qt Network provides an abstraction layer over the operations used, showing only high-level classes and functions. Qt Network can also handle lower level protocols like TCP and UDP. Classes like QTcpSocket and QUdpSocket enable the developer to send and receive messages using the TCP or UDP protocol.Name resolution is done with QHostInfo. QHostInfo is called before creating a network connection with QTcpSocket or QUdpSocket. Filtering and redistributing network traffic via proxies can handled by the QNetWorkProxy class.

Qt also offers functionalities for inter-process communication (IPC). The class QProcess is used to start external programs. Qt D-Bus provides support for D-Bus, an interprocess communication and remoteprocedure calling mechanism. It mostly communicates via a central server application, called a bus. However, it is also possible to let applications communicate directly with each other. QSharedMemory provides access to a shared memory segment by multiple threads and processes. It is however also possible for a single process to exclusively lock that shared memory.

Network Communication

Qt supports a wide range of network communication, with a focus on HTTP, TCP, and UDP.

At the HTTP level, the Qt Network module offers the network access API, which consists mainly of QNetworkRequest, QNetworkAccessManager, and QNetworkReply. Put simply, the QNetworkRequest resembles an HTTP request, which gets passed to the QNetworkAccessManager to send the request on the wire; this class returns a QNetworkReply, which enables parsing the HTTP reply. The network access API uses the socket classes mentioned below (for TCP and SSL) internally.

For communication at the socket level, QTcpSocket, QUdpSocket and QSslSocket should be used. These classes offer a synchronous API through the waitFor* methods as well as an asynchronous API; if possible (i.e. if the event loop is running), the asynchronous API should always be preferred. Qt also offers QTcpServer to enable the server-side part of a TCP communication. Please note that there is no HTTP server class in Qt.

For secure communication via SSL, Qt Network offers a wide range of classes alongside the central QSslSocket, e.g. QSslCertificate, QSslConfiguration and QSslError. The only supported backend for SSL in Qt is OpenSSL, which needs to be installed separately.

Qt also offers HTTP support in QML by either using HTTP explicitly via XmlHttpRequest, or by transparently using HTTP URLs in e.g. Image sources.

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Inter-Process Communication

Qt provides several ways to implement Inter-Process Communication (IPC) in Qt applications.

The cross-platform Qt Network module provides classes that make network programming portable and easy. It offers high-level classes (e.g., QNetworkAccessManager, QFtp) that communicate using specific application-level protocols, and lower-level classes (e.g., QTcpSocket, QTcpServer, QSslSocket) for implementing protocols.

The cross-platform shared memory class, QSharedMemory, provides access to the operating system's shared memory implementation. It allows safe access to shared memory segments by multiple threads and processes. Additionally, QSystemSemaphore can be used to control access to resources shared by the system, as well as to communicate between processes.

The Qt D-Bus module is a cross-platform library you can use to implement IPC using the D-Bus protocol. It extends Qt's signals and slots mechanism to the IPC level, allowing a signal emitted by one process to be connected to a slot in another process.

The cross-platform class QProcess can be used to start external programs as child processes, and to communicate with them. It provides an API for monitoring and controlling the state of the child process. QProcess gives access to the input/output channels of child process by inheriting from QIODevice.

The QLocalSocket class provides a local socket. On Windows this is a named pipe and on Unix this is a local domain socket. The QLocalServer class provides a local socket based server. This class makes it possible to accept incoming local socket connections.

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