/**************************************************************************** ** ** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies). ** All rights reserved. ** Contact: Nokia Corporation (qt-info@nokia.com) ** ** This file is part of the documentation of the Qt Toolkit. ** ** $QT_BEGIN_LICENSE:FDL$ ** No Commercial Usage ** This file contains pre-release code and may not be distributed. ** You may use this file in accordance with the terms and conditions ** contained in the Technology Preview License Agreement accompanying ** this package. ** ** GNU Free Documentation License ** Alternatively, this file may be used under the terms of the GNU Free ** Documentation License version 1.3 as published by the Free Software ** Foundation and appearing in the file included in the packaging of this ** file. ** ** If you have questions regarding the use of this file, please contact ** Nokia at qt-info@nokia.com. ** $QT_END_LICENSE$ ** ****************************************************************************/ /*! \class QWaitCondition \brief The QWaitCondition class provides a condition variable for synchronizing threads. \threadsafe \ingroup thread QWaitCondition allows a thread to tell other threads that some sort of condition has been met. One or many threads can block waiting for a QWaitCondition to set a condition with wakeOne() or wakeAll(). Use wakeOne() to wake one randomly selected condition or wakeAll() to wake them all. For example, let's suppose that we have three tasks that should be performed whenever the user presses a key. Each task could be split into a thread, each of which would have a \l{QThread::run()}{run()} body like this: \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 0 Here, the \c keyPressed variable is a global variable of type QWaitCondition. A fourth thread would read key presses and wake the other three threads up every time it receives one, like this: \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 1 The order in which the three threads are woken up is undefined. Also, if some of the threads are still in \c do_something() when the key is pressed, they won't be woken up (since they're not waiting on the condition variable) and so the task will not be performed for that key press. This issue can be solved using a counter and a QMutex to guard it. For example, here's the new code for the worker threads: \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 2 Here's the code for the fourth thread: \snippet doc/src/snippets/code/src_corelib_thread_qwaitcondition_unix.cpp 3 The mutex is necessary because the results of two threads attempting to change the value of the same variable simultaneously are unpredictable. Wait conditions are a powerful thread synchronization primitive. The \l{threads/waitconditions}{Wait Conditions} example shows how to use QWaitCondition as an alternative to QSemaphore for controlling access to a circular buffer shared by a producer thread and a consumer thread. \sa QMutex, QSemaphore, QThread, {Wait Conditions Example} */ /*! \fn QWaitCondition::QWaitCondition() Constructs a new wait condition object. */ /*! \fn QWaitCondition::~QWaitCondition() Destroys the wait condition object. */ /*! \fn void QWaitCondition::wakeOne() Wakes one thread waiting on the wait condition. The thread that is woken up depends on the operating system's scheduling policies, and cannot be controlled or predicted. If you want to wake up a specific thread, the solution is typically to use different wait conditions and have different threads wait on different conditions. \sa wakeAll() */ /*! \fn void QWaitCondition::wakeAll() Wakes all threads waiting on the wait condition. The order in which the threads are woken up depends on the operating system's scheduling policies and cannot be controlled or predicted. \sa wakeOne() */ /*! \fn bool QWaitCondition::wait(QMutex *mutex, unsigned long time) Releases the locked \a mutex and waits on the wait condition. The \a mutex must be initially locked by the calling thread. If \a mutex is not in a locked state, this function returns immediately. If \a mutex is a recursive mutex, this function returns immediately. The \a mutex will be unlocked, and the calling thread will block until either of these conditions is met: \list \o Another thread signals it using wakeOne() or wakeAll(). This function will return true in this case. \o \a time milliseconds has elapsed. If \a time is \c ULONG_MAX (the default), then the wait will never timeout (the event must be signalled). This function will return false if the wait timed out. \endlist The mutex will be returned to the same locked state. This function is provided to allow the atomic transition from the locked state to the wait state. \sa wakeOne(), wakeAll() */ /*! \fn bool QWaitCondition::wait(QReadWriteLock *readWriteLock, unsigned long time) \since 4.4 Releases the locked \a readWriteLock and waits on the wait condition. The \a readWriteLock must be initially locked by the calling thread. If \a readWriteLock is not in a locked state, this function returns immediately. The \a readWriteLock must not be locked recursively, otherwise this function will not release the lock properly. The \a readWriteLock will be unlocked, and the calling thread will block until either of these conditions is met: \list \o Another thread signals it using wakeOne() or wakeAll(). This function will return true in this case. \o \a time milliseconds has elapsed. If \a time is \c ULONG_MAX (the default), then the wait will never timeout (the event must be signalled). This function will return false if the wait timed out. \endlist The \a readWriteLock will be returned to the same locked state. This function is provided to allow the atomic transition from the locked state to the wait state. \sa wakeOne(), wakeAll() */