Contents

• Public Types
• Public Functions
• Public Slots
• Signals
• Static Public Members
• Protected Functions
• Static Protected Members
• Detailed Description
• Managing threads

Detailed Description

TheQThread class provides a platform-independent way to manage threads.

AQThread object manages one thread of control within the program. QThreads begin executing inrun(). By default,run() starts the event loop by callingexec() and runs a Qt event loop inside the thread.

You can use worker objects by moving them to the thread usingQObject::moveToThread().

class Worker :publicQObject{    Q_OBJECTQThread workerThread;publicslots:void doWork(constQString&parameter) {// ...emit resultReady(result);    }signals:void resultReady(constQString&result);};class Controller :publicQObject{    Q_OBJECTQThread 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();    }publicslots:void handleResults(constQString&);signals:void operate(constQString&);};

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 calledqueued connections.

Another way to make code run in a separate thread, is to subclassQThread and reimplementrun(). For example:

class WorkerThread :publicQThread{    Q_OBJECTvoid run() {QString result;/* expensive or blocking operation  */emit resultReady(result);    }signals:void resultReady(constQString&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 callexec().

It is important to remember that aQThread instancelives in the old thread that instantiated it, not in the new thread that callsrun(). This means that all ofQThread'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 subclassedQThread.

When subclassingQThread, keep in mind that the constructor executes in the old thread whilerun() 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. SeeSynchronizing Threads for details.

Managing threads

QThread will notifiy you via a signal when the thread isstarted(),finished(), andterminated(), or you can useisFinished() andisRunning() to query the state of the thread.

You can stop the thread by callingexit() orquit(). In extreme cases, you may want to forciblyterminate() an executing thread. However, doing so is dangerous and discouraged. Please read the documentation forterminate() andsetTerminationEnabled() 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 thefinished() signal toQObject::deleteLater().

Usewait() to block the calling thread, until the other thread has finished execution (or until a specified time has passed).

The static functionscurrentThreadId() andcurrentThread() return identifiers for the currently executing thread. The former returns a platform specific ID for the thread; the latter returns aQThread pointer.

To choose the name that your thread will be given (as identified by the commandps -L on Linux, for example), you can callsetObjectName() before starting the thread. If you don't callsetObjectName(), 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 theMandelbrot Example, as that is the name of theQThread subclass). Note that this is currently not available with release builds on Windows.

QThread also provides static, platform independent sleep functions:sleep(),msleep(), andusleep() allow full second, millisecond, and microsecond resolution respectively.

Note:wait() and thesleep() functions should be unnecessary in general, since Qt is an event-driven framework. Instead ofwait(), consider listening for thefinished() signal. Instead of thesleep() functions, consider usingQTimer.

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

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 newQThread to manage a new thread. Theparent takes ownership of theQThread. The thread does not begin executing untilstart() is called.

See alsostart().

QThread::~QThread()

Destroys theQThread.

Note that deleting aQThread object will not stop the execution of the thread it manages. Deleting a runningQThread (i.e.isFinished() returns false) will probably result in a program crash. Wait for thefinished() signal before deleting theQThread.

[static]QThread * QThread::currentThread()

Returns a pointer to aQThread 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 untilexit() is called, returning the value that was passed toexit(). The value returned is 0 ifexit() is called viaquit().

This function is meant to be called from withinrun(). It is necessary to call this function to start event handling.

See alsoquit() andexit().

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 toQEventLoop::exec(). TheQEventLoop::exec() function returnsreturnCode.

By convention, areturnCode of 0 means success, any non-zero value indicates an error.

Note that unlike the C library function of the same name, this functiondoes return to the caller -- it is event processing that stops.

No QEventLoops will be started anymore in this thread untilQThread::exec() has been called again. If the eventloop inQThread::exec() is not running then the next call toQThread::exec() will also return immediately.

See alsoquit() andQEventLoop.

[signal]void QThread::finished()

This signal is emitted when the thread has finished executing.

Note:Signalfinished 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:

connect(thread,static_cast<void(QThread::*)()>(&QThread::finished),[=](){/* ... */ });

See alsostarted() andterminated().

[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 alsoisRunning().

bool QThread::isRunning() const

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

See alsoisFinished().

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

Forces the current thread to sleep formsecs milliseconds.

See alsosleep() andusleep().

Priority QThread::priority() const

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

This function was introduced in Qt 4.1.

See alsoPriority,setPriority(), andstart().

[slot]void QThread::quit()

Tells the thread's event loop to exit with return code 0 (success). Equivalent to callingQThread::exit(0).

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

See alsoexit() andQEventLoop.

[virtual protected]void QThread::run()

The starting point for the thread. After callingstart(), the newly created thread calls this function. The default implementation simply callsexec().

You can reimplement this function to facilitate advanced thread management. Returning from this method will end the execution of the thread.

See alsostart() andwait().

void QThread::setPriority(Priority priority)

This function sets thepriority for a running thread. If the thread is not running, this function does nothing and returns immediately. Usestart() to start a thread with a specific priority.

Thepriority argument can be any value in theQThread::Priority enum except forInheritPriorty.

The effect of thepriority parameter is dependent on the operating system's scheduling policy. In particular, thepriority 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 alsoPriority,priority(), andstart().

void QThread::setStackSize(uint stackSize)

Sets the maximum stack size for the thread tostackSize. IfstackSize is greater than zero, the maximum stack size is set tostackSize 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 alsostackSize().

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

Enables or disables termination of the current thread based on theenabled parameter. The thread must have been started byQThread.

Whenenabled is false, termination is disabled. Future calls toQThread::terminate() will return immediately without effect. Instead, the termination is deferred until termination is enabled.

Whenenabled is true, termination is enabled. Future calls toQThread::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 threadimmediately. Note that this function will not return in this case.

See alsoterminate().

[static protected]void QThread::sleep(unsignedlong secs)

Forces the current thread to sleep forsecs seconds.

See alsomsleep() andusleep().

uint QThread::stackSize() const

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

See alsosetStackSize().

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

Begins execution of the thread by callingrun(). The operating system will schedule the thread according to thepriority parameter. If the thread is already running, this function does nothing.

The effect of thepriority parameter is dependent on the operating system's scheduling policy. In particular, thepriority 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 alsorun() andterminate().

[signal]void QThread::started()

This signal is emitted when the thread starts executing.

See alsofinished() andterminated().

[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 theterminated() signal, or useQThread::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 callingQThread::setTerminationEnabled(). Calling this function while termination is disabled results in the termination being deferred, until termination is re-enabled. See the documentation ofQThread::setTerminationEnabled() for more information.

See alsosetTerminationEnabled().

[signal]void QThread::terminated()

This signal is emitted when the thread is terminated.

See alsostarted() andfinished().

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

Forces the current thread to sleep forusecs microseconds.

See alsosleep() andmsleep().

bool QThread::wait(unsignedlong time = ULONG_MAX)

Blocks the thread until either of these conditions is met:

• The thread associated with thisQThread object has finished execution (i.e. when it returns fromrun()). 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. Iftime is ULONG_MAX (the default), then the wait will never timeout (the thread must return fromrun()). This function will return false if the wait timed out.

This provides similar functionality to the POSIXpthread_join() function.

See alsosleep() andterminate().

[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.