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QCondition Class Reference


The QCondition class provides signalling of the occurrence of events between threads More...

#include <qthread.h>

Inherits Qt.

List of all member functions.

Public Members


Detailed Description

The QCondition class provides signalling of the occurrence of events between threads

QConditions allow a thread to tell other threads that some sort of event has happened; one or many threads can block waiting for a QCondition to signal an event, and a thread can call 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:

  QCondition key_pressed;

  while(1) {
     key_pressed.wait();    // This is a QCondition 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:

  QCondition 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;
  QCondition key_pressed;
  int mycount=0;

  // Worker thread code
  while(1) {
     key_pressed.wait();    // This is a QCondition 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

QCondition::QCondition ()

Constructs a new event signalling object.

QCondition::~QCondition () [virtual]

Deletes the event signalling object.

bool QCondition::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:

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 QCondition::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:

See also: wakeOne() and wakeAll().

void QCondition::wakeAll ()

This wakes all threads waiting on the QCondition. 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 QCondition::wakeOne ()

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

See also: wakeAll().


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