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// Copyright (C) 2026 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
\page ipc-overview.html
\title Inter-process communication in Qt
\brief Overview of inter-process communication in Qt applications.
Qt provides several ways to implement inter-process communication
(IPC) in applications. The IPC functionality in Qt includes:
\list
\li Synchronizing processes and sharing resources
\li Exchanging byte-level data locally or over a network
\li Sharing replicated Qt objects between processes or systems
\li Passing structured messages or remote procedure calls (RPC)
\li Using M2M industry standard protocols for IoT and industrial systems
\endlist
Many of the IPC mechanisms are for networked devices and use network protocols
such as TCP, UDP, and HTTP. The \l{Networking and Connectivity} overview covers
the networking topic in more detail.
Qt supports structured data formats such as JSON, XML, Protobuf, and CBOR. For
more information about structured data, see the
\l{Data processing and I/O} overview pages.
\section1 Synchronizing processes and sharing resources
For processes that share local resources or systems that synchronize actions
and data, you can use system semaphores and shared memory. These
functionalities are part of the \l{Qt Core} module.
Qt provides classes for inter-process synchronization, including
\l{QSystemSemaphore}. Similar to \l{QSemaphore},
it is accessible by multiple processes on the same system. It is globally
identified by a \e key, which in Qt is represented by the
\l{QNativeIpcKey} class. Additionally, depending on the OS, Qt may support
multiple different backends for sharing memory.
For a thorough explanation, see the following pages:
\list
\li \l{Synchronization Primitives}
\li \l{Shared Memory}
\li \l{Native IPC Keys}
\endlist
\section2 Shared memory
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 controls access to resources shared by the system and
enables communication between processes.
\section1 Creating child processes
The \l{QProcess} class can start external programs as child processes and
communicate with them. It provides an API for monitoring and
controlling the state of the child process. QProcess gives access to the input
and output channels of the child process by inheriting from QIODevice.
\section1 Sharing Qt objects among remote processes or systems
The \l{Qt Remote Objects} module provides a remote object mechanism to enable
devices and processes to share Qt objects. It uses a \e Source object that
is synchronized with \e Replica objects using an intermediary \e Node. These
Replicas are essentially proxies that mirror the Source object's properties,
signals, and slots so that remote processes can interact with the object
without direct knowledge of the network transport.
\section1 Sending messages with Remote Procedure Calls (RPC)
Remote Procedure Call (RPC) is a high-level mechanism that abstracts the
underlying network communication. With RPC, a client can call functions that
execute on a remote server as if they were local function calls. Multiple
languages provide support for RPC technologies, simplifying implementation in
complex systems that use multiple programming languages.
The \l{Qt GRPC} module implements Remote Procedure Calls (RPC) using the gRPC
protocol. gRPC is a widely adopted framework for sending messages in serialized
format, most commonly using HTTP/2 for efficient network communication.
gRPC is well-suited for communication in modular microservices and low-latency
applications such as chat and streaming platforms.
\section1 Using M2M industry standard protocols
Machine-to-Machine (M2M) industry protocols are specific to systems that are
under strict resource constraints such as sensors and automation systems. Often
they are specific to the industry and use cases such as manufacturing,
healthcare, and environmental monitoring. These systems are often low power and
have limited processing capabilities.
The \l{Qt MQTT} module provides MQTT functionality to Qt applications. MQTT is
a protocol that uses the publish and subscribe pattern to send information
within a system. In an MQTT system, a \e broker manages the sending and
receiving to \e clients. Clients can be publishers, subscribers, or both. MQTT
is effective in low-bandwidth and high-latency environments such as industrial
and manufacturing systems.
The \l{Qt CoAP} module provides Constrained Application Protocol (CoAP) to Qt
applications. CoAP is a protocol that uses UDP and RESTful principles to send
information within constrained systems. CoAP is especially designed to take
advantage of compact datagram messages and low overhead.
The \l{Qt OPC UA} module provides OPC UA to Qt applications. It uses a client
and server IPC mechanism for industrial systems. OPC UA is for complex systems
and has built-in security measures.
You can implement hybrid systems with MQTT, CoAP, and OPC UA. For example, you
can implement the cloud communication in MQTT, the main system with OPC UA, and
connect the individual sensors and devices using CoAP.
One protocol may be widely adopted in a specific industry and its development
ecosystem. The system architecture and the messages are also formed
differently. For example, MQTT mainly uses TCP while CoAP uses UDP.
Consequently, MQTT can use TLS and CoAP can use DTLS for secure transfers.
\section1 Using the D-Bus protocol
The \l{Qt D-Bus} module is a Unix-only library you can use to implement IPC
using the D-Bus protocol. It extends Qt's \l{Signals and Slots} mechanism to
the IPC level, allowing a signal emitted by one process to be connected to a
slot in another process.
\section1 Using sessions in applications
On Linux/X11, Windows, and macOS, Qt provides support for session management.
Sessions allow events to be propagated to processes, for example, to notify
when a shutdown occurs. The processes and applications can then perform any
necessary operations such as saving open documents. For more information, see
the \l{Session Management} page.
\section1 IPC security considerations
Because data is transferred among processes or systems, you need to consider
privacy and security issues. Attackers can intercept data within networks and
foreign processes can monitor data within a system.
These are some security measures that you can implement:
\list
\li Use encryption when sending or receiving data.
See \l{Secure Sockets Layer (SSL) Classes}.
\li Use strong authentication and authorization mechanisms.
See \l{Qt Network Authorization}.
\li Validate and process untrusted data.
See \l{Handling Untrusted Data}.
\li Set rate limits and monitor traffic to prevent misuse.
\li Incorporate salting or nonce techniques to protect against
monitoring and replay attacks.
\li Update your environment to fix security vulnerabilities.
\endlist
The \l{Security in Qt} page discusses Qt's security policies and measures. Qt
modules may implement different protocols and therefore may have additional
security considerations.
*/