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

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Nov 06, 2009  The solution for communicating from a secondary thread to the main thread is to use signal–slot connections across threads. Normally, the signals and slots mechanism operates synchronously, meaning that the slots connected to a signal are invoked immediately when the signal is emitted, using a direct function call. Signal/slot across threads - remove duplicate events in event loop Signal/slot across threads - remove duplicate events in event loop. As signals/slots.

Synopsis¶

Functions¶

• def eventDispatcher ()

• def exec_ ()

• def exit ([retcode=0])

• def isFinished ()

• def isInterruptionRequested ()

• def isRunning ()

• def loopLevel ()

• def priority ()

• def requestInterruption ()

• def setEventDispatcher (eventDispatcher)

• def setPriority (priority)

• def setStackSize (stackSize)

• def stackSize ()

• def wait ([time=ULONG_MAX])

Slots¶

• def quit ()

• def start ([priority=InheritPriority])

• def terminate ()

Static functions¶

• def currentThread ()

• def idealThreadCount ()

• def msleep (arg__1)

• def setTerminationEnabled ([enabled=true])

• def sleep (arg__1)

• def usleep (arg__1)

• def yieldCurrentThread ()

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 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 queuedconnections .

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 livesin the old thread that instantiated it, not in the new thread that calls run() . This means that all of QThread ‘s queued slots and invokedmethods 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 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

QThreadStorageMandelbrot ExampleSemaphores ExampleWait Conditions Example

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

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

Return type

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

Return type

Returns a pointer to the event dispatcher object for the thread. If no event dispatcher exists for the thread, this function returns None .

int

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

retcode – int

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 exec() . The 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 exec() has been called again. If the eventloop in exec() is not running then the next call to exec() will also return immediately.

int

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

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

See also

bool

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.

See also

bool

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

int

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.

arg__1 – long

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 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.

Return type

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

See also

PrioritysetPriority()start()

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

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

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.

See also

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.

eventDispatcher – QAbstractEventDispatcher

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.

See also

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 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).

stackSize – uint

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.

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enabled – bool

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 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 terminate() will terminate the thread normally. If termination has been deferred (i.e. 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

arg__1 – long

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 event() ).

Note

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

uint

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

See also

priority – Priority

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).

Terminates the execution of the thread. The thread may or may not be terminated immediately, depending on the operating system’s scheduling policies. Use wait() after , 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 setTerminationEnabled() . Calling this function while termination is disabled results in the termination being deferred, until termination is re-enabled. See the documentation of setTerminationEnabled() for more information.

arg__1 – long

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 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.

time – long

bool

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

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

© 2018 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.

QThread inherits QObject. It emits signals to indicate that the thread started or finished executing, and provides a few slots as well.

More interesting is that QObjects can be used in multiple threads, emit signals that invoke slots in other threads, and post events to objects that 'live' in other threads. This is possible because each thread is allowed to have its own event loop.

QObject Reentrancy

QObject is reentrant. Most of its non-GUI subclasses, such as QTimer, QTcpSocket, QUdpSocket, QFtp, and QProcess, are also reentrant, making it possible to use these classes from multiple threads simultaneously. Note that these classes are designed to be created and used from within a single thread; creating an object in one thread and calling its functions from another thread is not guaranteed to work. There are three constraints to be aware of:

• The child of a QObject must always be created in the thread where the parent was created. This implies, among other things, that you should never pass the QThread object (this) as the parent of an object created in the thread (since the QThread object itself was created in another thread).
• Event driven objects may only be used in a single thread. Specifically, this applies to the timer mechanism and the network module. For example, you cannot start a timer or connect a socket in a thread that is not the object's thread.
• You must ensure that all objects created in a thread are deleted before you delete the QThread. This can be done easily by creating the objects on the stack in your run() implementation.

Although QObject is reentrant, the GUI classes, notably QWidget and all its subclasses, are not reentrant. They can only be used from the main thread. As noted earlier, QCoreApplication::exec() must also be called from that thread.

In practice, the impossibility of using GUI classes in other threads than the main thread can easily be worked around by putting time-consuming operations in a separate worker thread and displaying the results on screen in the main thread when the worker thread is finished. This is the approach used for implementing the Mandelbrot and the Blocking Fortune Client example.

Per-Thread Event Loop

Each thread can have its own event loop. The initial thread starts its event loops using QCoreApplication::exec(); other threads can start an event loop using QThread::exec(). Like QCoreApplication, QThread provides an exit(int) function and a quit() slot.

An event loop in a thread makes it possible for the thread to use certain non-GUI Qt classes that require the presence of an event loop (such as QTimer, QTcpSocket, and QProcess). It also makes it possible to connect signals from any threads to slots of a specific thread. This is explained in more detail in the Signals and Slots Across Threads section below.

A QObject instance is said to live in the thread in which it is created. Events to that object are dispatched by that thread's event loop. The thread in which a QObject lives is available using QObject::thread().

Note that for QObjects that are created before QApplication, QObject::thread() returns zero. This means that the main thread will only handle posted events for these objects; other event processing is not done at all for objects with no thread. Use the QObject::moveToThread() function to change the thread affinity for an object and its children (the object cannot be moved if it has a parent).

Calling delete on a QObject from a thread other than the one that owns the object (or accessing the object in other ways) is unsafe, unless you guarantee that the object isn't processing events at that moment. Use QObject::deleteLater() instead, and a DeferredDelete event will be posted, which the event loop of the object's thread will eventually pick up. By default, the thread that owns a QObject is the thread that creates the QObject, but not after QObject::moveToThread() has been called.

If no event loop is running, events won't be delivered to the object. For example, if you create a QTimer object in a thread but never call exec(), the QTimer will never emit its timeout() signal. Calling deleteLater() won't work either. (These restrictions apply to the main thread as well.)

You can manually post events to any object in any thread at any time using the thread-safe function QCoreApplication::postEvent(). The events will automatically be dispatched by the event loop of the thread where the object was created.

Event filters are supported in all threads, with the restriction that the monitoring object must live in the same thread as the monitored object. Similarly, QCoreApplication::sendEvent() (unlike postEvent()) can only be used to dispatch events to objects living in the thread from which the function is called.

Accessing QObject Subclasses from Other Threads

QObject and all of its subclasses are not thread-safe. This includes the entire event delivery system. It is important to keep in mind that the event loop may be delivering events to your QObject subclass while you are accessing the object from another thread.

If you are calling a function on an QObject subclass that doesn't live in the current thread and the object might receive events, you must protect all access to your QObject subclass's internal data with a mutex; otherwise, you may experience crashes or other undesired behavior.

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Like other objects, QThread objects live in the thread where the object was created -- not in the thread that is created when QThread::run() is called. It is generally unsafe to provide slots in your QThread subclass, unless you protect the member variables with a mutex.

On the other hand, you can safely emit signals from your QThread::run() implementation, because signal emission is thread-safe.

Signals and Slots Across Threads

Qt supports these signal-slot connection types:

• Auto Connection (default) If the signal is emitted in the thread which the receiving object has affinity then the behavior is the same as the Direct Connection. Otherwise, the behavior is the same as the Queued Connection.'
• Direct Connection The slot is invoked immediately, when the signal is emitted. The slot is executed in the emitter's thread, which is not necessarily the receiver's thread.
• Queued Connection The slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed in the receiver's thread.
• Blocking Queued Connection The slot is invoked as for the Queued Connection, except the current thread blocks until the slot returns.

  Note: Using this type to connect objects in the same thread will cause deadlock.

• Unique Connection The behavior is the same as the Auto Connection, but the connection is made only if it does not duplicate an existing connection. i.e., if the same signal is already connected to the same slot for the same pair of objects, then the connection is not made and connect() returns false.

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The connection type can be specified by passing an additional argument to connect(). Be aware that using direct connections when the sender and receiver live in different threads is unsafe if an event loop is running in the receiver's thread, for the same reason that calling any function on an object living in another thread is unsafe.

QObject::connect() itself is thread-safe.

The Mandelbrot example uses a queued connection to communicate between a worker thread and the main thread. To avoid freezing the main thread's event loop (and, as a consequence, the application's user interface), all the Mandelbrot fractal computation is done in a separate worker thread. The thread emits a signal when it is done rendering the fractal.

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Similarly, the Blocking Fortune Client example uses a separate thread for communicating with a TCP server asynchronously.

© 2016 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.