Thread Support in Qt

Qt provides thread support in the form of platform-independent threading classes, a thread-safe way of posting events, and signal-slot connections across threads. This makes it easy to develop portable multithreaded Qt applications and take advantage of multiprocessor machines. Multithreaded programming is also a useful paradigm for performing time-consuming operations without freezing the user interface of an application.

Earlier versions of Qt offered an option to build the library without thread support. Since Qt 4.0, threads are always enabled.

Topics:

• Recommended Reading
• The Threading Classes
• Starting Threads with QThread
• Synchronizing Threads
• Reentrancy and Thread-Safety
• Threads and QObjects
• Concurrent Programming
• Thread-Support in Qt Modules

Recommended Reading

This document is intended for an audience that has knowledge of, and experience with, multithreaded applications. If you are new to threading see our Recommended Reading list:

• Threads Primer: A Guide to Multithreaded Programming
• Thread Time: The Multithreaded Programming Guide
• Pthreads Programming: A POSIX Standard for Better Multiprocessing
• Win32 Multithreaded Programming

The Threading Classes

These classes are relevant to threaded applications.

Asynchronous Run	The <QtConcurrentRun> header provides a way to run a function in a separate thread.
Concurrent Filter and Filter-Reduce	The <QtConcurrentFilter> header provides concurrent Filter and Filter-Reduce.
Concurrent Map and Map-Reduce	The <QtConcurrentMap> header provides concurrent Map and MapReduce.
QAtomicInt	Platform-independent atomic operations on integers
QAtomicPointer	Template class that provides platform-independent atomic operations on pointers
QFuture	Represents the result of an asynchronous computation
QFutureSynchronizer	Convenience class that simplifies QFuture synchronization
QFutureWatcher	Allows monitoring a QFuture using signals and slots
QMutex	Access serialization between threads
QMutexLocker	Convenience class that simplifies locking and unlocking mutexes
QReadLocker	Convenience class that simplifies locking and unlocking read-write locks for read access
QReadWriteLock	Read-write locking
QRunnable	The base class for all runnable objects
QSemaphore	General counting semaphore
QThread	Platform-independent threads
QThreadPool	Manages a collection of QThreads
QThreadStorage	Per-thread data storage
QWaitCondition	Condition variable for synchronizing threads
QWriteLocker	Convenience class that simplifies locking and unlocking read-write locks for write access
QtConcurrent	High-level APIs that make it possible to write multi-threaded programs without using low-level threading primitives

Note: Qt's threading classes are implemented with native threading APIs; e.g., Win32 and pthreads. Therefore, they can be used with threads of the same native API.