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The QWaitCondition class allows waiting/waking for conditions between threads. More...
#include <qthread.h>
Inherits Qt.
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 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 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.
See also wakeOne() and wakeAll().
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().
See also wakeOne().
See also wakeAll().
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Copyright © 2001 Trolltech | Trademarks | Qt version 3.0.0-beta1-beta1
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