QThread Class Reference

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

 #include <QThread>

Inherits: QObject.

Inherited by:

Public Types

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

Public Functions

QThread ( QObject * parent = 0 )
~QThread ()
void exit ( int returnCode = 0 )
bool isFinished () const
bool isRunning () const
Priority priority () const
void setPriority ( Priority priority )
void setStackSize ( uint stackSize )
uint stackSize () const
bool wait ( unsigned long time = ULONG_MAX )
  • 29 public functions inherited from QObject

Public Slots

void quit ()
void start ( Priority priority = InheritPriority )
void terminate ()
  • 1 public slot inherited from QObject

Signals

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

Static Public Members

QThread * currentThread ()
Qt::HANDLE currentThreadId ()
int idealThreadCount ()
void yieldCurrentThread ()
  • 7 static public members inherited from QObject

Protected Functions

int exec ()
virtual void run ()
  • 8 protected functions inherited from QObject

Static Protected Members

void msleep ( unsigned long msecs )
void setTerminationEnabled ( bool enabled = true )
void sleep ( unsigned long secs )
void usleep ( unsigned long usecs )

Additional Inherited Members

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().

 class Worker : public QObject
 {
     Q_OBJECT
     QThread workerThread;

 public slots:
     void doWork(const QString &parameter) {
         // ...
         emit resultReady(result);
     }

 signals:
     void resultReady(const QString &result);
 };

 class Controller : public QObject
 {
     Q_OBJECT
     QThread workerThread;
 public:
     Controller() {
         Worker *worker = new Worker;
         worker->moveToThread(&workerThread);
         connect(workerThread, SIGNAL(finished()), worker, SLOT(deleteLater()));
         connect(this, SIGNAL(operate(QString)), worker, SLOT(doWork(QString)));
         connect(worker, SIGNAL(resultReady(QString)), this, SLOT(handleResults(QString)));
         workerThread.start();
     }
     ~Controller() {
         workerThread.quit();
         workerThread.wait();
     }
 public slots:
     void handleResults(const QString &);
 signals:
     void operate(const QString &);
 };

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:

 class WorkerThread : public QThread
 {
     Q_OBJECT
     void run() {
         QString result;
         /* expensive or blocking operation  */
         emit resultReady(result);
     }
 signals:
     void resultReady(const QString &s);
 };

 void MyObject::startWorkInAThread()
 {
     WorkerThread *workerThread = new WorkerThread(this);
     connect(workerThread, SIGNAL(resultReady(QString)), this, SLOT(handleResults(QString)));
     connect(workerThread, SIGNAL(finished()), workerThread, SLOT(deleteLater()));
     workerThread->start();
 }

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.

ConstantValueDescription
QThread::IdlePriority0scheduled only when no other threads are running.
QThread::LowestPriority1scheduled less often than LowPriority.
QThread::LowPriority2scheduled less often than NormalPriority.
QThread::NormalPriority3the default priority of the operating system.
QThread::HighPriority4scheduled more often than NormalPriority.
QThread::HighestPriority5scheduled more often than HighPriority.
QThread::TimeCriticalPriority6scheduled as often as possible.
QThread::InheritPriority7use the same priority as the creating thread. This is the default.

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.

QThread * QThread::currentThread () [static]

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

Qt::HANDLE QThread::currentThreadId () [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().

int QThread::exec () [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().

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.

void QThread::finished () [signal]

This signal is emitted when the thread has finished executing.

See also started() and terminated().

int QThread::idealThreadCount () [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.

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().

void QThread::msleep ( unsigned long msecs ) [static protected]

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().

void QThread::quit () [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.

void QThread::run () [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 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::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().

void QThread::setTerminationEnabled ( bool enabled = true ) [static 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().

void QThread::sleep ( unsigned long secs ) [static protected]

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().

void QThread::start ( Priority priority = InheritPriority ) [slot]

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().

void QThread::started () [signal]

This signal is emitted when the thread starts executing.

See also finished() and terminated().

void QThread::terminate () [slot]

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().

void QThread::terminated () [signal]

This signal is emitted when the thread is terminated.

See also started() and finished().

void QThread::usleep ( unsigned long usecs ) [static protected]

Forces the current thread to sleep for usecs microseconds.

See also sleep() and msleep().

bool QThread::wait ( unsigned long 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().

void QThread::yieldCurrentThread () [static]

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

Notes provided by the Qt Community
Best Practice
  • 5

Votes: 62

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]