QThread Class

The QThread class provides a platform-independent way to manage threads. More...

Header: #include <QThread>
CMake: find_package(Qt6 REQUIRED COMPONENTS Core)
target_link_libraries(mytarget PRIVATE Qt6::Core)
qmake: QT += core
Inherits: QObject

Public Types

enum Priority { IdlePriority, LowestPriority, LowPriority, NormalPriority, HighPriority, …, InheritPriority }
(since 6.9) enum class QualityOfService { Auto, High, Eco }

Public Functions

QAbstractEventDispatcher *eventDispatcher() const
void setEventDispatcher(QAbstractEventDispatcher *eventDispatcher)
void setPriority(QThread::Priority priority)
bool wait(QDeadlineTimer deadline = QDeadlineTimer(QDeadlineTimer::Forever))
bool wait(unsigned long time)

Public Slots

void start(QThread::Priority priority = InheritPriority)
void terminate()

Signals

void finished()
void started()

Static Public Members

QThread *create(Function &&f, Args &&... args)
Qt::HANDLE currentThreadId()
int idealThreadCount()
void msleep(unsigned long msecs)
(since 6.6) void sleep(std::chrono::nanoseconds nsecs)
void sleep(unsigned long secs)
void usleep(unsigned long usecs)
void yieldCurrentThread()

Static Protected Members

void setTerminationEnabled(bool enabled = true)

Detailed Description

A QThread object manages one thread of control within the program. QThreads begin executing in run(). By default, run() starts the event loop by calling exec() and runs a Qt event loop inside the thread.

You can use worker objects by moving them to the thread using QObject::moveToThread().

 class Worker : public QObject
 {
     Q_OBJECT

 public slots:
     void doWork(const QString &parameter) {
         QString result;
         /* ... here is the expensive or blocking operation ... */
         emit resultReady(result);
     }

 signals:
     void resultReady(const QString &result);
 };

 class Controller : public QObject
 {
     Q_OBJECT
     QThread workerThread;
 public:
     Controller() {
         Worker *worker = new Worker;
         worker->moveToThread(&workerThread);
         connect(&workerThread, &QThread::finished, worker, &QObject::deleteLater);
         connect(this, &Controller::operate, worker, &Worker::doWork);
         connect(worker, &Worker::resultReady, this, &Controller::handleResults);
         workerThread.start();
     }
     ~Controller() {
         workerThread.quit();
         workerThread.wait();
     }
 public slots:
     void handleResults(const QString &);
 signals:
     void operate(const QString &);
 };

The code inside the Worker's slot would then execute in a separate thread. However, you are free to connect the Worker's slots to any signal, from any object, in any thread. It is safe to connect signals and slots across different threads, thanks to a mechanism called queued connections.

Another way to make code run in a separate thread, is to subclass QThread and reimplement run(). For example:

 class WorkerThread : public QThread
 {
     Q_OBJECT
 public:
     explicit WorkerThread(QObject *parent = nullptr) : QThread(parent) { }
 protected:
     void run() override {
         QString result;
         /* ... here is the expensive or blocking operation ... */
         emit resultReady(result);
     }
 signals:
     void resultReady(const QString &s);
 };

 void MyObject::startWorkInAThread()
 {
     WorkerThread *workerThread = new WorkerThread(this);
     connect(workerThread, &WorkerThread::resultReady, this, &MyObject::handleResults);
     connect(workerThread, &WorkerThread::finished, workerThread, &QObject::deleteLater);
     workerThread->start();
 }

In that example, the thread will exit after the run function has returned. There will not be any event loop running in the thread unless you call exec().

It is important to remember that a QThread instance lives in the old thread that instantiated it, not in the new thread that calls run(). This means that all of QThread's queued slots and invoked methods will execute in the old thread. Thus, a developer who wishes to invoke slots in the new thread must use the worker-object approach; new slots should not be implemented directly into a subclassed QThread.

Unlike queued slots or invoked methods, methods called directly on the QThread object will execute in the thread that calls the method. When subclassing QThread, keep in mind that the constructor executes in the old thread while run() executes in the new thread. If a member variable is accessed from both functions, then the variable is accessed from two different threads. Check that it is safe to do so.

Note: Care must be taken when interacting with objects across different threads. As a general rule, functions can only be called from the thread that created the QThread object itself (e.g. setPriority()), unless the documentation says otherwise. See Synchronizing Threads for details.

Managing Threads

QThread will notify you via a signal when the thread is started() and finished(), or you can use isFinished() and isRunning() to query the state of the thread.

You can stop the thread by calling exit() or quit(). In extreme cases, you may want to forcibly terminate() an executing thread. However, doing so is dangerous and discouraged. Please read the documentation for terminate() and setTerminationEnabled() for detailed information.

You often want to deallocate objects that live in a thread when a thread ends. To do this, connect the finished() signal to QObject::deleteLater().

Use wait() to block the calling thread, until the other thread has finished execution (or until a specified time has passed).

QThread also provides static, platform independent sleep functions: sleep(), msleep(), and usleep() allow full second, millisecond, and microsecond resolution respectively.

Note: wait() and the sleep() functions should be unnecessary in general, since Qt is an event-driven framework. Instead of wait(), consider listening for the finished() signal. Instead of the sleep() functions, consider using QChronoTimer.

The static functions currentThreadId() and currentThread() return identifiers for the currently executing thread. The former returns a platform specific ID for the thread; the latter returns a QThread pointer.

To choose the name that your thread will be given (as identified by the command ps -L on Linux, for example), you can call setObjectName() before starting the thread. If you don't call setObjectName(), the name given to your thread will be the class name of the runtime type of your thread object (for example, "RenderThread" in the case of the Mandelbrot example, as that is the name of the QThread subclass). Note that this is currently not available with release builds on Windows.

See also Thread Support in Qt, QThreadStorage, Synchronizing Threads, Mandelbrot, Producer and Consumer using Semaphores, and Producer and Consumer using Wait Conditions.

Member Type Documentation

enum QThread::Priority

This enum type indicates how the operating system should schedule newly created threads.

ConstantValueDescription
QThread::IdlePriority0scheduled only when no other threads are running.
QThread::LowestPriority1scheduled less often than LowPriority.
QThread::LowPriority2scheduled less often than NormalPriority.
QThread::NormalPriority3the default priority of the operating system.
QThread::HighPriority4scheduled more often than NormalPriority.
QThread::HighestPriority5scheduled more often than HighPriority.
QThread::TimeCriticalPriority6scheduled as often as possible.
QThread::InheritPriority7use the same priority as the creating thread. This is the default.

[since 6.9] enum class QThread::QualityOfService

This enum describes the quality of service level of a thread, and provides the scheduler with information about the kind of work that the thread performs. On platforms with different CPU profiles, or with the ability to clock certain cores of a CPU down, this allows the scheduler to select or configure a CPU core with suitable performance and energy characteristics for the thread.

ConstantValueDescription
QThread::QualityOfService::Auto0The default value, leaving it to the scheduler to decide which CPU core to run the thread on.
QThread::QualityOfService::High1The scheduler should run this thread to a high-performance CPU core.
QThread::QualityOfService::Eco2The scheduler should run this thread to an energy-efficient CPU core.

This enum was introduced in Qt 6.9.

See also Priority, serviceLevel(), and QThreadPool::serviceLevel().

Member Function Documentation

[static] template <typename Function, typename... Args> QThread *QThread::create(Function &&f, Args &&... args)

Creates a new QThread object that will execute the function f with the arguments args.

The new thread is not started – it must be started by an explicit call to start(). This allows you to connect to its signals, move QObjects to the thread, choose the new thread's priority and so on. The function f will be called in the new thread.

Returns the newly created QThread instance.

Note: the caller acquires ownership of the returned QThread instance.

Warning: do not call start() on the returned QThread instance more than once; doing so will result in undefined behavior.

See also start().

[static noexcept] Qt::HANDLE QThread::currentThreadId()

Returns the thread handle of the currently executing thread.

Warning: The handle returned by this function is used for internal purposes and should not be used in any application code.

Note: On Windows, this function returns the DWORD (Windows-Thread ID) returned by the Win32 function GetCurrentThreadId(), not the pseudo-HANDLE (Windows-Thread HANDLE) returned by the Win32 function GetCurrentThread().

QAbstractEventDispatcher *QThread::eventDispatcher() const

Returns a pointer to the event dispatcher object for the thread. If no event dispatcher exists for the thread, this function returns nullptr.

See also setEventDispatcher().

[private signal] void QThread::finished()

This signal is emitted from the associated thread right before it finishes executing.

When this signal is emitted, the event loop has already stopped running. No more events will be processed in the thread, except for deferred deletion events. This signal can be connected to QObject::deleteLater(), to free objects in that thread.

Note: If the associated thread was terminated using terminate(), it is undefined from which thread this signal is emitted.

Note: This is a private signal. It can be used in signal connections but cannot be emitted by the user.

See also started().

[static noexcept] int QThread::idealThreadCount()

Returns the ideal number of threads that this process can run in parallel. This is done by querying the number of logical processors available to this process (if supported by this OS) or the total number of logical processors in the system. This function returns 1 if neither value could be determined.

Note: On operating systems that support setting a thread's affinity to a subset of all logical processors, the value returned by this function may change between threads and over time.

Note: On operating systems that support CPU hotplugging and hot-unplugging, the value returned by this function may also change over time (and note that CPUs can be turned on and off by software, without a physical, hardware change).

[static] void QThread::msleep(unsigned long msecs)

This is an overloaded function, equivalent to calling:

 QThread::sleep(std::chrono::milliseconds{msecs});

Note: This function does not guarantee accuracy. The application may sleep longer than msecs under heavy load conditions. Some OSes might round msecs up to 10 ms or 15 ms.

See also sleep() and usleep().

void QThread::setEventDispatcher(QAbstractEventDispatcher *eventDispatcher)

Sets the event dispatcher for the thread to eventDispatcher. This is only possible as long as there is no event dispatcher installed for the thread yet.

An event dispatcher is automatically created for the main thread when QCoreApplication is instantiated and on start() for auxiliary threads.

This method takes ownership of the object.

See also eventDispatcher().

void QThread::setPriority(QThread::Priority priority)

This function sets the priority for a running thread. If the thread is not running, this function does nothing and returns immediately. Use start() to start a thread with a specific priority.

The priority argument can be any value in the QThread::Priority enum except for InheritPriority.

The effect of the priority parameter is dependent on the operating system's scheduling policy. In particular, the priority will be ignored on systems that do not support thread priorities (such as on Linux, see http://linux.die.net/man/2/sched_setscheduler for more details).

See also Priority, priority(), and start().

[static protected] void QThread::setTerminationEnabled(bool enabled = true)

Enables or disables termination of the current thread based on the enabled parameter. The thread must have been started by QThread.

When enabled is false, termination is disabled. Future calls to QThread::terminate() will return immediately without effect. Instead, the termination is deferred until termination is enabled.

When enabled is true, termination is enabled. Future calls to QThread::terminate() will terminate the thread normally. If termination has been deferred (i.e. QThread::terminate() was called with termination disabled), this function will terminate the calling thread immediately. Note that this function will not return in this case.

See also terminate().

[static, since 6.6] void QThread::sleep(std::chrono::nanoseconds nsecs)

Forces the current thread to sleep for nsecs.

Avoid using this function if you need to wait for a given condition to change. Instead, connect a slot to the signal that indicates the change or use an event handler (see QObject::event()).

Note: This function does not guarantee accuracy. The application may sleep longer than nsecs under heavy load conditions.

This function was introduced in Qt 6.6.

[static] void QThread::sleep(unsigned long secs)

Forces the current thread to sleep for secs seconds.

This is an overloaded function, equivalent to calling:

 QThread::sleep(std::chrono::seconds{secs});

See also msleep() and usleep().

[slot] void QThread::start(QThread::Priority priority = InheritPriority)

Begins execution of the thread by calling run(). The operating system will schedule the thread according to the priority parameter. If the thread is already running, this function does nothing.

The effect of the priority parameter is dependent on the operating system's scheduling policy. In particular, the priority will be ignored on systems that do not support thread priorities (such as on Linux, see the sched_setscheduler documentation for more details).

See also run() and terminate().

[private signal] void QThread::started()

This signal is emitted from the associated thread when it starts executing, so any slots connected to it may be called via queued invocation. Whilst the event may have been posted before run() is called, any cross-thread delivery of the signal may still be pending.

Note: This is a private signal. It can be used in signal connections but cannot be emitted by the user.

See also run() and finished().

[slot] void QThread::terminate()

Terminates the execution of the thread. The thread may or may not be terminated immediately, depending on the operating system's scheduling policies. Use QThread::wait() after terminate(), to be sure.

When the thread is terminated, all threads waiting for the thread to finish will be woken up.

Warning: This function is dangerous and its use is discouraged. The thread can be terminated at any point in its code path. Threads can be terminated while modifying data. There is no chance for the thread to clean up after itself, unlock any held mutexes, etc. In short, use this function only if absolutely necessary.

Termination can be explicitly enabled or disabled by calling QThread::setTerminationEnabled(). Calling this function while termination is disabled results in the termination being deferred, until termination is re-enabled. See the documentation of QThread::setTerminationEnabled() for more information.

Note: This function is thread-safe.

See also setTerminationEnabled().

[static] void QThread::usleep(unsigned long usecs)

This is an overloaded function, equivalent to calling:

 QThread::sleep(std::chrono::microseconds{secs});

Note: This function does not guarantee accuracy. The application may sleep longer than usecs under heavy load conditions. Some OSes might round usecs up to 10 ms or 15 ms; on Windows, it will be rounded up to a multiple of 1 ms.

See also sleep() and msleep().

bool QThread::wait(QDeadlineTimer deadline = QDeadlineTimer(QDeadlineTimer::Forever))

Blocks the thread until either of these conditions is met:

  • The thread associated with this QThread object has finished execution (i.e. when it returns from run()). This function will return true if the thread has finished. It also returns true if the thread has not been started yet.
  • The deadline is reached. This function will return false if the deadline is reached.

A deadline timer set to QDeadlineTimer::Forever (the default) will never time out: in this case, the function only returns when the thread returns from run() or if the thread has not yet started.

This provides similar functionality to the POSIX pthread_join() function.

Note: On some operating systems, this function may return true while the operating system thread is still running and may be executing clean-up code such as C++11 thread_local destructors. Operating systems where this function only returns true after the OS thread has fully exited include Linux, Windows, and Apple operating systems.

See also sleep() and terminate().

bool QThread::wait(unsigned long time)

This is an overloaded function.

time is the time to wait in milliseconds. If time is ULONG_MAX, then the wait will never timeout.

[static] void QThread::yieldCurrentThread()

Yields execution of the current thread to another runnable thread, if any. Note that the operating system decides to which thread to switch.