Qthread Slot Quit

• Qthread Yieldcurrentthread
• Qthread Slot

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

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. See also exit and QEventLoop. virtual protected void QThread:: run The starting point for the thread. Run one method within another thread and quit the thread when the method is finished. Qt provides different solutions: Write a function and run it with QtConcurrent::run Derive a class from QRunnable and run it in the global thread pool with QThreadPool::globalInstance-start. It's a bad idea to kill running thread, from design and technical points of view. Usually the thread must own the decision to quit based on 'terminate' flag. For example create new flag 'stop', if quit slot is signaled mark the flag true. In a thread function verify the flag periodically and if it's true - exit thread function. Qthread Slot Quit It’s a good way to test out a new casino without risking any of your own money. Most online casinos (but not all) have a wagering requirement attached to any winnings accumulated from the free spins, that's why it's important to always compare the terms and Qthread Slot Quit conditions.

Public Types

Public Functions

Reimplemented Public Functions

• 32 public functions inherited from QObject

Public Slots

• 1 public slot inherited from QObject

Signals

• 2 signals inherited from QObject

Static Public Members

• 11 static public members inherited from QObject

Protected Functions

• 9 protected functions inherited from QObject

Static Protected Members

Additional Inherited Members

• 1 property inherited from QObject

Detailed Description

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

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().

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:

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. See Synchronizing Threads for details.

Managing Threads

QThread will notifiy 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.

Member Function Documentation

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.

[static] 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.

Note: this function is only available when using C++17.

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

This function was introduced in Qt 5.10.

See also start().

[static] QThread *QThread::create(Function &&f)

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

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.

This function was introduced in Qt 5.10.

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.

Warning: On Windows, the returned value is a pseudo-handle for the current thread. It can't be used for numerical comparison. i.e., this function returns the DWORD (Windows-Thread ID) returned by the Win32 function getCurrentThreadId(), not the HANDLE (Windows-Thread HANDLE) returned by the Win32 function getCurrentThread().

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

Reimplemented from QObject::event().

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 0.

This function was introduced in Qt 5.0.

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.

See also quit() and exit().

void QThread::exit(intreturnCode = 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.

See also quit() and QEventLoop.

[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 can be run on the system. This is done querying the number of processor cores, both real and logical, in the system. This function returns 1 if the number of processor cores could not be detected.

bool QThread::isFinished() const

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

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.

This function was introduced in Qt 5.2.

See also currentThread() and requestInterruption().

bool QThread::isRunning() const

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

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.

This function was introduced in Qt 5.5.

[static] void QThread::msleep(unsignedlongmsecs)

Forces the current thread to sleep for msecs milliseconds.

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.

This function was introduced in Qt 4.1.

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.

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.

This function was introduced in Qt 5.2.

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. That is, before the thread has been started with start() or, in case of the main thread, before QCoreApplication has been instantiated. This method takes ownership of the object.

This function was introduced in Qt 5.0.

See also eventDispatcher().

void QThread::setPriority(QThread::Prioritypriority)

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 InheritPriorty.

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).

This function was introduced in Qt 4.1.

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

void QThread::setStackSize(uintstackSize)

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(boolenabled = 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(unsignedlongsecs)

Forces the current thread to sleep for secs seconds.

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::Prioritypriority = 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().

[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.

Qthread Yieldcurrentthread

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.

See also setTerminationEnabled().

[static] void QThread::usleep(unsignedlongusecs)

Forces the current thread to sleep for usecs microseconds.

See also sleep() and msleep().

bool QThread::wait(unsignedlongtime = ULONG_MAX)

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.
• time milliseconds has elapsed. If time is ULONG_MAX (the default), then the wait will never timeout (the thread must return from run()). This function will return false if the wait timed out.

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

See also sleep() and terminate().

[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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This post is about the use of QThread. It is an answer to a three years old blog post by Brad, my colleague at the time:
You're doing it wrong

In his blog post,Brad explains that he saw many users misusing QThread by sub-classing it, adding someslots to that subclass and doing something like this in the constructor:

They move a thread to itself. As Brad mentions, it is wrong: the QThread is supposed tobe the interface to manage the thread. So it is supposed to be used from the creating thread.

Slots in the QThread object are then not run in that thread and having slots in a subclass of QThreadis a bad practice.

But then Brad continues and discourages any sub-classing of QThread at all.He claims it is against proper object-oriented design.This is where I disagree. Putting code in run() is a valid object-oriented way to extend a QThread:A QThread represents a thread that just starts an event loop, a subclass represents a threadthat is extended to do what's in run().

After Brad's post, some members of the community went on a crusade against sub-classing QThread.The problem is that there are many perfectly valid reasons to subclass QThread.

With Qt 5.0 and Qt 4.8.4, the documentation of QThread was changed so the sample code does not involvesub-classing.Look at the first code sampleof the Qt 4.8 QThread documentation (Update: link to archive.org since the newer documentation is fixed).It has many lines of boiler plate just to run some code in a thread. And the there is evena leak: the QThread is never going to quit and be destroyed.

I was asked on IRC a question from an user who followed that example in orderto run some simple code in a thread. He had a hard time to figure out how to properlydestroy the thread. That is what motivated me to write this blog entry.

If you allow to subclass QThread, this is what you got:

Qthread Slot

This code does no longer leak and is much simpler and has lessoverhead as it does not create useless object.

The Qt threading examplethreadedfortuneserveris an example that uses this pattern to run blocking operations and is much simpler thanthe equivalent using a worker object.

I have submitted a patch to the documentationto not discourage sub-classing QThread anymore.

Rules of thumbs

When to subclass and when not to?

• If you do not really need an event loop in the thread, you should subclass.
• If you need an event loop and handle signals and slots within the thread, you may not need to subclass.

What about using QtConcurrent instead?

QThread is a quite low level and you should better use a higher levelAPI such as QtConcurrent.

Now, QtConcurrent has its own set of problems: It is tied to a single thread pool so it is nota good solution if you want to run blocking operations. It has also some problems in itsimplementation that gives some performance overhead. All of this is fixable.Perhaps even Qt 5.1 will see some improvements.

A good alternative is also the C++11 standard library withstd::threadand std::async which arenow the standard way to run code in a thread. And the good news is that it still works fine with Qt:All other Qt threading primitives can be used with native threads.(Qt will create automatically create a QThread if required).