QWaitCondition - Allows waiting/waking for conditions
       between threads

       #include <qthread.h>

       Inherits Qt.

   Public Members
       QWaitCondition ()
       virtual ~QWaitCondition ()
       bool wait ( unsigned long time = ULONG_MAX )
       bool wait ( QMutex * mutex, unsigned long time = ULONG_MAX
           )
       void wakeOne ()
       void wakeAll ()


DESCRIPTION

       The QWaitCondition class allows waiting/waking for
       conditions between threads

       QWaitConditions allow 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 event or wakeAll() to wake them all. For
       example, say we have three tasks that should be performed
       every time the user presses a key; each task could be
       split into a thread, each of which would have a run() body
       like so:

         QWaitCondition key_pressed;
         while(1) {
            key_pressed.wait();    // This is a QWaitCondition global variable
            // Key was pressed, do something interesting
            do_something();
         }

       A fourth thread would read key presses and wake the other
       three threads up every time it receives one, like so:

         QWaitCondition key_pressed;
         while(1) {
            getchar();
            // Causes any thread in key_pressed.wait() to return from
            // that method and continue processing
            key_pressed.wakeAll();
         }

       Note that the order the three threads are woken up in is
       undefined, and that if some or all of the threads are
       still in do_something() when the key is pressed, they
       won't be woken up (since they're not waiting on the

       for that key press. This can be avoided by, for example,
       doing something like this:

         QMutex mymutex;
         QWaitCondition key_pressed;
         int mycount=0;
         // Worker thread code
         while(1) {
            key_pressed.wait();    // This is a QWaitCondition global variable
            mymutex.lock();
            mycount++;
            mymutex.unlock();
            do_something();
            mymutex.lock();
            mycount--;
            mymutex.unlock();
         }
         // Key reading thread code
         while(1) {
            getchar();
            mymutex.lock();
            // Sleep until there are no busy worker threads
            while(count>0) {
              mymutex.unlock();
              sleep(1);
              mymutex.lock();
            }
            mymutex.unlock();
            key_pressed.wakeAll();
         }

       The mutexes are necessary because the results if two
       threads attempt to change the value of the same variable
       simultaneously are unpredictable.


MEMBER FUNCTION DOCUMENTATION


QWaitCondition::QWaitCondition ()

       Constructs a new event signalling object.


QWaitCondition::~QWaitCondition () [virtual]

       Deletes the event signalling object.


bool QWaitCondition::wait ( QMutex * mutex, unsigned long time =

       ULONG_MAX )
       Release the locked mutex and wait on the thread event
       object. The mutex must be initially locked by the calling
       thread. If mutex is not in a locked state, this function
       returns immediately. The mutex will be unlocked, and the
       thread calling will block until one of 2 conditions is
       met:

       Another thread signals it using wakeOne() or wakeAll().
              This function will return TRUE in this case.

              (default argument), then the wait will never
              timeout (the event must signalled). This function
              will return FALSE if the wait timed out.

       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.

       See also wakeOne() and wakeAll().


bool QWaitCondition::wait ( unsigned long time = ULONG_MAX )

       Wait on the thread event object. The thread calling this
       will block until one of 2 conditions is met:

       Another thread signals it using wakeOne() or wakeAll().
              This function will return TRUE in this case.

       time milliseconds has elapsed. If time is ULONG_MAX
              (default argument), then the wait will never
              timeout (the event must signalled). This function
              will return FALSE if the wait timed out.

       See also wakeOne() and wakeAll().


void QWaitCondition::wakeAll ()

       This wakes all threads waiting on the QWaitCondition. The
       order in which the threads are woken up depends on the
       operating system's scheduling policies, and cannot be
       controlled or predicted.

       See also wakeOne().


void QWaitCondition::wakeOne ()

       This wakes one thread waiting on the QWaitCondition. The
       thread that woken up depends on the operating system's
       scheduling policies, and cannot be controlled or
       predicted.

       See also  wakeAll().


SEE ALSO

       http://doc.trolltech.com/qwaitcondition.html
       http://www.trolltech.com/faq/tech.html


COPYRIGHT

       Copyright 1992-2001 Trolltech AS,
       http://www.trolltech.com.  See the license file included
       in the distribution for a complete license statement.


AUTHOR

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       In case of content or formattting problems with this
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       Please include the name of the manual page
       (qwaitcondition.3qt) and the Qt version (2.3.1).


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