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# QThread Class Reference
[QtCore module]

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

Inherits QObject.

### Types

- enum Priority { IdlePriority, LowestPriority, LowPriority, NormalPriority, ..., InheritPriority }

### Methods

- __init__ (self, QObject parent = None)
- int exec_ (self)
- exit (self, int returnCode = 0)
- bool isFinished (self)
- bool isRunning (self)
- Priority priority (self)
- quit (self)
- run (self)
- setPriority (self, Priority priority)
- setStackSize (self, int stackSize)
- int stackSize (self)
- start (self, Priority priority = QThread.InheritPriority)
- terminate (self)
- bool wait (self, int msecs = ULONG_MAX)

### Static Methods

- QThread currentThread ()
- int currentThreadId ()
- int idealThreadCount ()
- msleep (int)
- setTerminationEnabled (bool enabled = True)
- sleep (int)
- usleep (int)
- yieldCurrentThread ()

### Qt Signals

- void finished ()
- void started ()
- void terminated ()

## Detailed Description

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

A QThread object manages one thread of control within the program. To make code run in a separate thread, simply create a QThread, change the thread affinity of the QObject(s) that contain the code, and start() the new event loop. For example:

     class Worker : public QObject
     {
         Q_OBJECT

     public slots:
         void doWork() {
             ...
         }
     };

     void MyObject.putWorkerInAThread()
     {
         Worker *worker = new Worker;
         QThread *workerThread = new QThread(this);

         connect(workerThread, SIGNAL(started()), worker, SLOT(doWork()));
         connect(workerThread, SIGNAL(finished()), worker, SLOT(deleteLater()));
         worker->moveToThread(workerThread);

         // Starts an event loop, and emits workerThread->started()
         workerThread->start();
     }

The code inside the Worker's slot would then execute in a separate thread. In this example, the QThread triggers the Worker's doWork() slot upon starting, and frees the Worker's memory upon terminating. 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.

Note: If you interact with an object, using any technique other than queued signal/slot connections (e.g. direct function calls), then the usual multithreading precautions need to be taken.

Note: It is not possible to change the thread affinity of GUI objects; they must remain in the main thread.

### 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.

### Subclassing QThread

Subclassing QThread is unnecessary for most purposes, since QThread provides fully-functional thread management capabilities. Nonetheless, QThread can be subclassed if you wish to implement advanced thread management. This is done by adding new member functions to the subclass, and/or by reimplementing run(). QThread's run() function is analogous to an application's main() function -- it is executed when the thread is started, and the thread will end when it returns.

Note: Prior to Qt 4.4, the only way to use QThread for parallel processing was to subclass it and implement the processing code inside run(). This approach is now considered \b {bad practice}; a QThread should only manage a thread, not process data.

If you require event handling and signal/slot connections to work in your thread, and if you reimplement run(), you must explicitly call exec_() at the end of your reimplementation:

     class AdvancedThreadManager : public QThread
     {
     protected:
         void run()
         {
             /* ... other code to initialize thread... */

             // Begin event handling
             exec();
         }
     };

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.

It is important to remember that a QThread object usually lives in the thread where it was created, not in the thread that it manages. This oft-overlooked detail means that a QThread's slots will be executed in the context of its home thread, not in the context of the thread it is managing. For this reason, implementing new slots in a QThread subclass is error-prone and discouraged.

## Type Documentation

### QThread.Priority

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

Constant | Value | Description
--- | --- | ---
`QThread.IdlePriority` | `0` | scheduled only when no other threads are running.
`QThread.LowestPriority` | `1` | scheduled less often than LowPriority.
`QThread.LowPriority` | `2` | scheduled less often than NormalPriority.
`QThread.NormalPriority` | `3` | the default priority of the operating system.
`QThread.HighPriority` | `4` | scheduled more often than NormalPriority.
`QThread.HighestPriority` | `5` | scheduled more often than HighPriority.
`QThread.TimeCriticalPriority` | `6` | scheduled as often as possible.
`QThread.InheritPriority` | `7` | use the same priority as the creating thread. This is the default.

## Method Documentation

### QThread.__init__ (self, QObject parent = None)

The parent argument, if not None, causes self to be owned by Qt instead of PyQt.

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.currentThread ()

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

### int 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().

### int QThread.exec_ (self)

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().

It is necessary to call this function to start event handling.

See also quit() and exit().

### QThread.exit (self, 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.

### 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 (self)

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

See also isRunning().

### bool QThread.isRunning (self)

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

See also isFinished().

### QThread.msleep (int)

Forces the current thread to sleep for msecs milliseconds.

See also sleep() and usleep().

### Priority QThread.priority (self)

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().

### QThread.quit (self)

This method is also a Qt slot with the C++ signature `void 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.

### QThread.run (self)

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().

### QThread.setPriority (self, 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().

### QThread.setStackSize (self, int 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().

### 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().

### QThread.sleep (int)

Forces the current thread to sleep for secs seconds.

See also msleep() and usleep().

### int QThread.stackSize (self)

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

See also setStackSize().

### QThread.start (self, Priority priority = QThread.InheritPriority)

This method is also a Qt slot with the C++ signature `void start(QThread::Priority = QThread.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().

### QThread.terminate (self)

This method is also a Qt slot with the C++ signature `void 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().

### QThread.usleep (int)

Forces the current thread to sleep for usecs microseconds.

See also sleep() and msleep().

### bool QThread.wait (self, int msecs = 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().

### 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.

## Qt Signal Documentation

### void finished ()

This is the default overload of this signal.

This signal is emitted when the thread has finished executing.

See also started() and terminated().

### void started ()

This is the default overload of this signal.

This signal is emitted when the thread starts executing.

See also finished() and terminated().

### void terminated ()

This is the default overload of this signal.

This signal is emitted when the thread is terminated.

See also started() and finished().