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 }

Public Functions

QThread(QObject *parent = nullptr)
virtual ~QThread()
QAbstractEventDispatcher *eventDispatcher() const
bool isFinished() const
bool isInterruptionRequested() const
bool isRunning() const
int loopLevel() const
QThread::Priority priority() const
void requestInterruption()
void setEventDispatcher(QAbstractEventDispatcher *eventDispatcher)
void setPriority(QThread::Priority priority)
void setStackSize(uint stackSize)
uint stackSize() const
bool wait(QDeadlineTimer deadline = QDeadlineTimer(QDeadlineTimer::Forever))
bool wait(unsigned long time)

Reimplemented Public Functions

virtual bool event(QEvent *event) override

Public Slots

void exit(int returnCode = 0)
void quit()
void start(QThread::Priority priority = InheritPriority)
void terminate()

Signals

void finished()
void started()

Static Public Members

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

Protected Functions

int exec()
virtual void run()

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
    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.

From Qt 4.8 onwards, it is possible to deallocate objects that live in a thread that has just ended, by connecting 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. These functions were made public in Qt 5.0.

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 QTimer.

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 Example, Semaphores Example, and Wait Conditions Example.

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.

Member Function Documentation

[explicit] QThread::QThread(QObject *parent = nullptr)

Constructs a new QThread to manage a new thread. The parent takes ownership of the QThread. The thread does not begin executing until start() is called.

See also start().

[virtual] QThread::~QThread()

Destroys the QThread.

Note that deleting a QThread object will not stop the execution of the thread it manages. Deleting a running QThread (i.e. isFinished() returns false) will result in a program crash. Wait for the finished() signal before deleting the QThread.

Since Qt 6.3, it is allowed to delete a QThread instance created by a call to QThread::create() even if the corresponding thread is still running. In such a case, Qt will post an interruption request to that thread (via requestInterruption()); will ask the thread's event loop (if any) to quit (via quit()); and will block until the thread has finished.

See also create(), isInterruptionRequested(), exec(), and quit().

[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] QThread *QThread::currentThread()

Returns a pointer to a QThread which manages the currently executing thread.

[static] 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().

[override virtual] bool QThread::event(QEvent *event)

Reimplements: QObject::event(QEvent *e).

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().

[protected] int QThread::exec()

Enters the event loop and waits until exit() is called, returning the value that was passed to exit(). The value returned is 0 if exit() is called via quit().

This function is meant to be called from within run(). It is necessary to call this function to start event handling.

Note: This can only be called within the thread itself, i.e. when it is the current thread.

See also quit() and exit().

[slot] void QThread::exit(int returnCode = 0)

Tells the thread's event loop to exit with a return code.

After calling this function, the thread leaves the event loop and returns from the call to QEventLoop::exec(). The QEventLoop::exec() function returns returnCode.

By convention, a returnCode of 0 means success, any non-zero value indicates an error.

Note that unlike the C library function of the same name, this function does return to the caller – it is event processing that stops.

No QEventLoops will be started anymore in this thread until QThread::exec() has been called again. If the eventloop in QThread::exec() is not running then the next call to QThread::exec() will also return immediately.

Note: This function is thread-safe.

See also quit() and QEventLoop.

[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] 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).

bool QThread::isFinished() const

Returns true if the thread is finished; otherwise returns false.

Note: This function is thread-safe.

See also isRunning().

bool QThread::isInterruptionRequested() const

Return true if the task running on this thread should be stopped. An interruption can be requested by requestInterruption().

This function can be used to make long running tasks cleanly interruptible. Never checking or acting on the value returned by this function is safe, however it is advisable do so regularly in long running functions. Take care not to call it too often, to keep the overhead low.

void long_task() {
     forever {
        if ( QThread::currentThread()->isInterruptionRequested() ) {
            return;
        }
    }
}

Note: This can only be called within the thread itself, i.e. when it is the current thread.

See also currentThread() and requestInterruption().

bool QThread::isRunning() const

Returns true if the thread is running; otherwise returns false.

Note: This function is thread-safe.

See also isFinished().

int QThread::loopLevel() const

Returns the current event loop level for the thread.

Note: This can only be called within the thread itself, i.e. when it is the current thread.

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

Forces the current thread to sleep for msecs milliseconds.

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 msecs under heavy load conditions. Some OSes might round msecs up to 10 ms or 15 ms.

See also sleep() and usleep().

QThread::Priority QThread::priority() const

Returns the priority for a running thread. If the thread is not running, this function returns InheritPriority.

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

[slot] void QThread::quit()

Tells the thread's event loop to exit with return code 0 (success). Equivalent to calling QThread::exit(0).

This function does nothing if the thread does not have an event loop.

Note: This function is thread-safe.

See also exit() and QEventLoop.

void QThread::requestInterruption()

Request the interruption of the thread. That request is advisory and it is up to code running on the thread to decide if and how it should act upon such request. This function does not stop any event loop running on the thread and does not terminate it in any way.

Note: This function is thread-safe.

See also isInterruptionRequested().

[virtual protected] void QThread::run()

The starting point for the thread. After calling start(), the newly created thread calls this function. The default implementation simply calls exec().

You can reimplement this function to facilitate advanced thread management. Returning from this method will end the execution of the thread.

See also start() and wait().

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().

void QThread::setStackSize(uint stackSize)

Sets the maximum stack size for the thread to stackSize. If stackSize is greater than zero, the maximum stack size is set to stackSize bytes, otherwise the maximum stack size is automatically determined by the operating system.

Warning: Most operating systems place minimum and maximum limits on thread stack sizes. The thread will fail to start if the stack size is outside these limits.

See also stackSize().

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

Forces the current thread to sleep for secs seconds.

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 secs under heavy load conditions.

See also msleep() and usleep().

uint QThread::stackSize() const

Returns the maximum stack size for the thread (if set with setStackSize()); otherwise returns zero.

See also setStackSize().

[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, before the run() function is called.

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

See also 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)

Forces the current thread to sleep for usecs microseconds.

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 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.

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.

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