A condition variable, usually used to communicate information about changing state to a thread that is waiting for something to happen. A condition variable can be used to “wake up” a thread when some arbitrary condition has changed.
A condition variable is associated with a single mutex, and several condition variables may share the same mutex.
Informs one of the other threads who are currently blocked on
wait()that the relevant condition has changed. If multiple threads are currently waiting, at least one of them will be woken up, although there is no way to predict which one. It is possible that more than one thread will be woken up.
The caller must be holding the mutex associated with the condition variable before making this call, which will not release the mutex.
If no threads are waiting, this is a no-op: the notify event is lost.
void output(std::ostream &out) const¶
This method is declared virtual in ConditionVarDebug, but non-virtual in
void wait(double timeout)¶
Waits on the condition. The caller must already be holding the lock associated with the condition variable before calling this function.
wait() will release the lock, then go to sleep until some other thread calls
notify()on this condition variable. At that time at least one thread waiting on the same
ConditionVarDirectwill grab the lock again, and then return from wait().
It is possible that wait() will return even if no one has called
notify(). It is the responsibility of the calling process to verify the condition on return from wait, and possibly loop back to wait again if necessary.
Note the semantics of a condition variable: the mutex must be held before wait() is called, and it will still be held when wait() returns. However, it will be temporarily released during the wait() call itself.
Waits on the condition, with a timeout. The function will return when the condition variable is notified, or the timeout occurs. There is no way to directly tell which happened, and it is possible that neither in fact happened (spurious wakeups are possible).
See wait() with no parameters for more.
- void notify(void)¶