QThread Class Reference

The QThread class provides platform-independent threads.

  1. #include <QThread>

Inherits: QObject.

Inherited by:

Detailed Description

The QThread class provides platform-independent threads.

A QThread represents a separate thread of control within the program; it shares data with all the other threads within the process but executes independently in the way that a separate program does on a multitasking operating system. Instead of starting in main(), QThreads begin executing in run(). By default, run() starts the event loop by calling exec() (see below). To create your own threads, subclass QThread and reimplement run(). For example:

  1. class MyThread : public QThread
  2.  {
  3. public:
  4.     void run();
  5. };
  6.  
  7. void MyThread::run()
  8.  {
  9.     QTcpSocket socket;
  10.     // connect QTcpSocket's signals somewhere meaningful
  11.     ...
  12.     socket.connectToHost(hostName, portNumber);
  13.     exec();
  14. }

This will create a QTcpSocket in the thread and then execute the thread's event loop. Use the start() method to begin execution. Execution ends when you return from run(), just as an application does when it leaves main(). 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. Use wait() to block until the thread has finished execution.

Each thread gets its own stack from the operating system. The operating system also determines the default size of the stack. You can use setStackSize() to set a custom stack size.

Each QThread can have its own event loop. You can start the event loop by calling exec(); you can stop it by calling exit() or quit(). Having an event loop in a thread makes it possible to connect signals from other threads to slots in this thread, using a mechanism called queued connections. It also makes it possible to use classes that require the event loop, such as QTimer and QTcpSocket, in the thread. Note, however, that it is not possible to use any widget classes in the thread.

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.

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.

QThread also provides platform independent sleep functions in varying resolutions. Use sleep() for full second resolution, msleep() for millisecond resolution, and usleep() for microsecond resolution.

See also Thread Support in Qt, QThreadStorage, QMutex, QSemaphore, QWaitCondition, Mandelbrot Example, Semaphores Example, and Wait Conditions Example.

Public Types

Toggle detailsenum QThread::

PriorityPriority { IdlePriority , LowestPriority , LowPriority , NormalPriority , HighPriority , HighestPriority , TimeCriticalPriority , InheritPriority 7 ...} { IdlePriority , LowestPriority , LowPriority , NormalPriority , HighPriority , HighestPriority , TimeCriticalPriority , InheritPriority 7 }

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

ConstantValueDescription
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.

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    Public Functions

    Toggle details QThread

    QThreadQThread ( QObject *parent=0 ) ( QObject *parent=0 )

    Constructs a new thread with the given parent. The thread does not begin executing until start() is called.

    See also start().

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    Toggle details QThread

    ~QThread~QThread () ()

    Destroys the thread.

    Note that deleting a QThread object will not stop the execution of the thread it represents. Deleting a running QThread (i.e. isFinished() returns false) will probably result in a program crash. You can wait() on a thread to make sure that it has finished.

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    Toggle details QThread * QThread

    currentThreadcurrentThread () () [static]

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

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    Toggle details Qt::HANDLE QThread

    currentThreadIdcurrentThreadId () () [static]

    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().

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    Toggle details int QThread

    execexec () ()[protected]

    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().

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    Toggle details void QThread

    exitexit ( int returnCode=0 ) ( 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.

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    Toggle details int QThread

    idealThreadCountidealThreadCount () () [static]

    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.

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    Toggle details bool QThread

    isFinishedisFinished () ()const

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

    See also isRunning().

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    Toggle details bool QThread

    isRunningisRunning () ()const

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

    See also isFinished().

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    Toggle details void QThread

    msleepmsleep ( unsigned longmsecs ) ( unsigned longmsecs )[protected]

    Causes the current thread to sleep for msecs milliseconds.

    See also sleep() and usleep().

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    Toggle details Priority QThread

    prioritypriority () ()const

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

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

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    Toggle details void QThread

    runrun () () [virtual protected]

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

    You can reimplemented this function to do other useful work. Returning from this method will end the execution of the thread.

    See also start() and wait().

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    Toggle details void QThread

    setPrioritysetPriority ( Priority priority ) ( 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).

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

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    Toggle details void QThread

    setStackSizesetStackSize ( uint stackSize ) ( 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().

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    Toggle details void QThread

    setTerminationEnabledsetTerminationEnabled ( bool enabled=true ) ( bool enabled=true )[protected]

    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().

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    Toggle details void QThread

    sleepsleep ( unsigned longsecs ) ( unsigned longsecs )[protected]

    Forces the current thread to sleep for secs seconds.

    See also msleep() and usleep().

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    Toggle details uint QThread

    stackSizestackSize () ()const

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

    See also setStackSize().

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    Toggle details void QThread

    usleepusleep ( unsigned longusecs ) ( unsigned longusecs )[protected]

    Causes the current thread to sleep for usecs microseconds.

    See also sleep() and msleep().

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    Toggle details bool QThread

    waitwait ( unsigned longtime=ULONG_MAX ) ( unsigned longtime=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().

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    Toggle details void QThread

    yieldCurrentThreadyieldCurrentThread () () [static]

    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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    Toggle details bool QThread

    finishedfinished () ()const Qt3-support

    Use isFinished() instead.

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    Toggle details bool QThread

    runningrunning () ()const Qt3-support

    Use isRunning() instead.

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    Signals

    Toggle details void QThread

    finishedfinished () () [signal]

    This signal is emitted when the thread has finished executing.

    See also started() and terminated().

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    Toggle details void QThread

    startedstarted () () [signal]

    This signal is emitted when the thread starts executing.

    See also finished() and terminated().

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    Toggle details void QThread

    terminatedterminated () () [signal]

    This signal is emitted when the thread is terminated.

    See also started() and finished().

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    Public Slots

    Toggle details void QThread

    quitquit () () [slot]

    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.

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    Toggle details void QThread

    startstart ( Priority priority=InheritPriority ) ( Priority priority=InheritPriority ) [slot]

    Begins execution of the thread by calling run(), which should be reimplemented in a QThread subclass to contain your code. 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().

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    Toggle details void QThread

    terminateterminate () () [slot]

    Terminates the execution of the thread. The thread may or may not be terminated immediately, depending on the operating systems scheduling policies. Use QThread::wait() after terminate() for synchronous termination.

    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().

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    Notes provided by the Qt Community
    Best Practice
    • 5

    Votes: 58

    Coverage: Qt library 4.7, 4.8

    Picture of Andre Andre

    Robot Herder
    30 notes

    Nokia Certified Qt Developer

    Subclassing no longer recommended way of using QThread

    The documentation above still only gives a sample of using QThread by subclassing it. That is no longer needed, and actually no longer the recommended standard way of using QThread. QThread must be thought of as a class to manage a thread, but not as the actual code or object that runs in that thread. To use QThread to manage a thread, subclassing is hardly ever needed. Instead, create a worker object that derives from QObject, and implement a slot on that worker to perform the work you need doing.

    Consider the following pattern as your default way to use QThread:

    1. class Worker : public QObject
    2. {
    3.     Q_OBJECT
    4.  
    5. public slots:
    6.     void doWork() {
    7.         /* ... */
    8.     }
    9. };
    10.  
    11. /* ... */
    12. QThread *thread = new QThread;
    13. Worker *worker = new Worker;
    14. //obj is a pointer to a QObject that will trigger the work to start. It could just be this
    15. connect(obj, SIGNAL(startWork()), worker, SLOT(doWork()));
    16. worker->moveToThread(thread);
    17. thread->start();
    18. //obj will need to emit startWork() to get the work going.

    Alternatively, you could do:

    1. //based on the same Worker class as above:
    2.  
    3. /* ... */
    4. QThread *thread = new QThread;
    5. Worker *worker = new Worker;
    6. worker->moveToThread(thread);
    7. thread->start();
    8. QMetaObject::invokeMethod(worker, "doWork", Qt::QueuedConnection);

    Many more hints and details, including the pitfalls of using QThread in combination with signals and slots, can be found this must read wiki article [developer.qt.nokia.com].

    [Revisions]