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1. Signal And Slot In Qthread
2. Invoke Slot In Qthread

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Threads in an operating system are a very simple thing. Write a function, maybe bundle it with some data and push it onto a newly created thread. Use a mutex or other method to safely communicate with the thread if necessary. Whether it are Win32, POSIX or other threads, they all basically work the same and are quite fool-proof.

Those who have discovered the joys of the Qt framework may assume that threads in Qt are just like this, and they would be right. However, there are several different ways to use threads in Qt, and it might not be obvious which approach to choose. The article, Multithreading Technologies in Qt, compares the different approaches.

The rest of this article demonstrates one of these methods: QThread + a worker QObject. This method is intended for use cases which involve event-driven programming and signals + slots across threads.

Usage with Worker class

The main thing in this example to keep in mind when using a QThread is that it's not a thread. It's a wrapper around a thread object. This wrapper provides the signals, slots and methods to easily use the thread object within a Qt project. To use it, prepare a QObject subclass with all your desired functionality in it. Then create a new QThread instance, push the QObject onto it using moveToThread(QThread*) of the QObject instance and call start() on the QThread instance. That's all. You set up the proper signal/slot connections to make it quit properly and such, and that's all.

Declare Worker class

For a basic example, check this class declaration for the Worker class:

class Worker : public QObject {

public:

public slots:

signals:

private:

};

We add at least one public slot which will be used to trigger the instance and make it start processing data once the thread has started. Now, let's see what the implementation for this basic class looks like.

Worker::Worker() { // Constructor

}

Worker::~Worker() { // Destructor

}

void Worker::process() { // Process. Start processing data.

}

While this Worker class doesn't do anything special, it nevertheless contains all the required elements. It starts processing when its main function, in this case process(), is called and when it is done it emits the signal finished() which will then be used to trigger the shutdown of the QThread instance it is contained in.

By the way, one extremely important thing to note here is that you should NEVER allocate heap objects (using new) in the constructor of the QObject class as this allocation is then performed on the main thread and not on the new QThread instance, meaning that the newly created object is then owned by the main thread and not the QThread instance. This will make your code fail to work. Instead, allocate such resources in the main function slot such as process() in this case as when that is called the object will be on the new thread instance and thus it will own the resource.

Create a new Worker instance

Now, let's see how to use this new construction by creating a new Worker instance and putting it on a QThread instance:

QThread* thread = new QThread;Worker* worker = new Worker();worker->moveToThread(thread);connect(worker, SIGNAL (error(QString)), this, SLOT (errorString(QString)));connect(thread, SIGNAL (started()), worker, SLOT (process()));connect(worker, SIGNAL (finished()), thread, SLOT (quit()));connect(worker, SIGNAL (finished()), worker, SLOT (deleteLater()));connect(thread, SIGNAL (finished()), thread, SLOT (deleteLater()));thread->start();

The connect() series here is the most crucial part. The first connect() line hooks up the error message signal from the worker to an error processing function in the main thread. The second connects the thread's started() signal to the processing() slot in the worker, causing it to start.

Then the clean-up: when the worker instance emits finished(), as we did in the example, it will signal the thread to quit, i.e. shut down. We then mark the worker instance using the same finished() signal for deletion. Finally, to prevent nasty crashes because the thread hasn't fully shut down yet when it is deleted, we connect the finished() of the thread (not the worker!) to its own deleteLater() slot. This will cause the thread to be deleted only after it has fully shut down.

External Links

1. Maya Posch's blog, http://mayaposch.wordpress.com/2011/11/01/how-to-really-truly-use-qthreads-the-full-explanation/
2. Qt Blog on subclassing QThread is wrong, [1]
3. Woboq Blog on subclassing QThread is not always wrong, [2]

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

Public Types

Public Functions

• 29 public functions inherited from QObject

Public Slots

• 1 public slot inherited from QObject

Signals

• 1 signal inherited from QObject

Static Public Members

• 7 static public members inherited from QObject

Protected Functions

• 8 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 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.

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(), finished(), and terminated(), 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).

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.

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 QTimer.

{Mandelbrot Example}, {Semaphores Example}, {Wait Conditions Example}

See also Thread Support in Qt, QThreadStorage, and Synchronizing Threads.

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 = 0)

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().

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 probably result in a program crash. Wait for the finished() signal before deleting the QThread.

[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().

[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(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.

See also quit() and QEventLoop.

[signal] void QThread::finished()

This signal is emitted when the thread has finished executing.

Note:Signal finished is overloaded in this class. To connect to this one using the function pointer syntax, you must specify the signal type in a static cast, as shown in this example:

See also started() and terminated().

[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::isRunning() const

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

See also isFinished().

[static protected] void QThread::msleep(unsignedlong msecs)

Forces the current thread to sleep for msecs milliseconds.

See also sleep() and usleep().

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.

[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().

Signal And Slot In Qthread

void QThread::setPriority(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).

This function was introduced in Qt 4.1.

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.

Invoke Slot In 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 protected] void QThread::sleep(unsignedlong secs)

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(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 http://linux.die.net/man/2/sched_setscheduler for more details).

See also run() and terminate().

[signal] void QThread::started()

This signal is emitted when the thread starts executing.

See also finished() and terminated().

[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. Listen for the terminated() signal, or 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.

See also setTerminationEnabled().

[signal] void QThread::terminated()

This signal is emitted when the thread is terminated.

See also started() and finished().

[static protected] void QThread::usleep(unsignedlong usecs)

Forces the current thread to sleep for usecs microseconds.

See also sleep() and msleep().

bool QThread::wait(unsignedlong time = 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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