QThread Class

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# QThread Class Reference

The QThread class provides platform-independent threads. More...

All the functions in this class are thread-safe when Qt is built with thread support.

`#include <qthread.h>`

Inherits Qt.

List of all member functions.

## Public Members

- QThread ( unsigned int stackSize = 0 )
- virtual ~QThread ()
- bool wait ( unsigned long time = ULONG_MAX )
- enum Priority { IdlePriority, LowestPriority, LowPriority, NormalPriority, HighPriority, HighestPriority, TimeCriticalPriority, InheritPriority }
- void start ( Priority priority = InheritPriority )
- void terminate ()
- bool finished () const
- bool running () const

## Static Public Members

- Qt::HANDLE currentThread ()
- void postEvent ( QObject * receiver, QEvent * event ) (obsolete)
- void exit ()

## Protected Members

- virtual void run () = 0

## Static Protected Members

- void sleep ( unsigned long secs )
- void msleep ( unsigned long msecs )
- void usleep ( unsigned long usecs )

## 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(). You inherit run() to include your code. For example:

        class MyThread : public QThread {

        public:

            virtual void run();

        };

        void MyThread::run()
        {
            for( int count = 0; count < 20; count++ ) {
                sleep( 1 );
                qDebug( "Ping!" );
            }
        }

        int main()
        {
            MyThread a;
            MyThread b;
            a.start();
            b.start();
            a.wait();
            b.wait();
        }

This will start two threads, each of which writes Ping! 20 times to the screen and exits. The wait() calls at the end of main() are necessary because exiting main() ends the program, unceremoniously killing all other threads. Each MyThread stops executing when it reaches the end of MyThread::run(), just as an application does when it leaves main().

See also Thread Support in Qt, Environment Classes, and Threading.

## Member Type Documentation

### QThread::Priority

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

- `QThread::IdlePriority` - scheduled only when no other threads are running.
- `QThread::LowestPriority` - scheduled less often than LowPriority.
- `QThread::LowPriority` - scheduled less often than NormalPriority.
- `QThread::NormalPriority` - the default priority of the operating system.
- `QThread::HighPriority` - scheduled more often than NormalPriority.
- `QThread::HighestPriority` - scheduled more often then HighPriority.
- `QThread::TimeCriticalPriority` - scheduled as often as possible.
- `QThread::InheritPriority` - use the same priority as the creating thread. This is the default.

## Member Function Documentation

### QThread::QThread ( unsigned int stackSize = 0 )

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

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.

### QThread::~QThread ()`[virtual]`

QThread destructor.

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

### Qt::HANDLE QThread::currentThread ()`[static]`

This 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, and it cannot be used for numerical comparison.

### void QThread::exit ()`[static]`

Ends the execution of the calling thread and wakes up any threads waiting for its termination.

### bool QThread::finished () const

Returns TRUE if the thread is finished; otherwise returns FALSE.

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

System independent sleep. This causes the current thread to sleep for msecs milliseconds

### void QThread::postEvent ( QObject * receiver, QEvent * event )`[static]`

This function is obsolete. It is provided to keep old source working. We strongly advise against using it in new code.

Use QApplication::postEvent() instead.

### 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().

### bool QThread::running () const

Returns TRUE if the thread is running; otherwise returns FALSE.

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

System independent sleep. This causes the current thread to sleep for secs seconds.

### void QThread::start ( Priority priority = InheritPriority )

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 argument.

If you try to start a thread that is already running, this function will wait until the the thread has finished and then restart the thread.

See also Priority.

### void QThread::terminate ()

This function terminates the execution of the thread. The thread may or may not be terminated immediately, depending on the operating system's scheduling policies. Use QThread::wait() after terminate() for synchronous termination.

When the thread is terminated, all threads waiting for the 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 cleanup after itself, unlock any held mutexes, etc. In short, use this function only if absolutely necessary.

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

System independent sleep. This causes the current thread to sleep for usecs microseconds

### bool QThread::wait ( unsigned long time = ULONG_MAX )

A thread calling this function will block 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.

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