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/*!

    \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()

*/