QThread Class Reference
^{[QtCore module]}

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(). To create your own threads, subclass QThread and reimplement run(). For example:

    class MyThread : public QThread
    {
    public:
        void run();
    };

    void MyThread::run()
    {
        QTcpSocket socket;
        // connect QTcpSocket's signals somewhere meaningful
        ...
        socket.connectToHost(hostName, portNumber);
        exec();
    }

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 threads, 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.

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, and QWaitCondition.

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

See also start().

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.

QThread * QThread::currentThread ()   [static]

Returns a pointer to the currently executing QThread. If the current thread was not started using the QThread API, this function returns zero.

Note that QApplication creates a QThread object to represent the main thread; calling this function from main() after creating QApplication will return a valid pointer.

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. On Windows, the returned value is a pseudo-handle for the current thread that cannot be used for numerical comparison.

int QThread::exec ()   [protected]

Enters the event loop and waits until exit() is called or the main widget is destroyed, and returns the value that was set to exit() (which is 0 if exit() is called via quit()).

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.

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

See also quit() and QEventLoop.

void QThread::finished ()   [signal]

This signal is emitted when the thread has finished executing.

See also started() and terminated().

bool QThread::isFinished () const

Returns true is 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]

Causes 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 ()   [pure virtual protected]

This method is pure virtual and must be implemented in derived classes in order to do useful work. Returning from this method will end the execution of the thread.

See also 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.

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 ( QThread::Priority = InheritPriority )   [slot]

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 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 terminate at any point in its code path. Threads can be terminated while modifying data. There is no chance for the thread to cleanup 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]

Causes 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().