QWaitCondition(3qt)													       QWaitCondition(3qt)

NAME

       QWaitCondition - Allows waiting/waking for conditions between threads

SYNOPSIS

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

       #include <qwaitcondition.h>

   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 this:

	   QWaitCondition key_pressed;
	   for (;;) {
	       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 this:

	   QWaitCondition key_pressed;
	   for (;;) {
	       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 condition variable) and so the task will not be performed 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
	   for (;;) {
	       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
	   for (;;) {
	       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 of two threads attempting to change the value of the same variable simultaneously are
       unpredictable.

       See also Environment Classes and Threading.

MEMBER FUNCTION DOCUMENTATION

QWaitCondition::QWaitCondition ()
       Constructs a new event signalling, i.e. wait condition, object.

QWaitCondition::~QWaitCondition () [virtual]
       Deletes the event signalling, i.e. wait condition, object.

bool QWaitCondition::wait ( unsigned long time = ULONG_MAX )
       Wait on the thread event object. The thread calling this will block until either of these 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 (the default), then the wait will never timeout (the event must be signalled). This
       function will return FALSE if the wait timed out.

       See also wakeOne() and wakeAll().

bool QWaitCondition::wait ( QMutex * mutex, unsigned long time = ULONG_MAX )
       This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

       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. If mutex is a recursive mutex, this function returns immediately. The mutex will be
       unlocked, and the calling thread will block until either of these 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 (the default), then the wait will never timeout (the event must be 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().

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 is 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

       Generated automatically from the source code.

BUGS

       If you find a bug in Qt, please report it as described in http://doc.trolltech.com/bughowto.html.  Good bug reports help us to help you.
       Thank you.

       The definitive Qt documentation is provided in HTML format; it is located at $QTDIR/doc/html and can be read using Qt Assistant or with a
       web browser. This man page is provided as a convenience for those users who prefer man pages, although this format is not officially
       supported by Trolltech.

       If you find errors in this manual page, please report them to qt-bugs@trolltech.com.  Please include the name of the manual page
       (qwaitcondition.3qt) and the Qt version (3.1.1).

Trolltech AS							  9 December 2002					       QWaitCondition(3qt)