Condition variables?

[Graham St Jack]

Steven Schveighoffer wrote:
> "Janice Caron" wrote
>> On 10/2/07, Graham St Jack <grahams at acres.com.au> wrote:
>>> You use it like this (incomplete pseudo-code):
>>>
>>> class ProtectedQueue(T) {
>>>      Condition ready;
>>>      int count;
>>>
>>>      this() {
>>>          ready = new Condition(this);
>>>      }
>>>
>>>      synchronized void add(T item) {
>>>          add_item_to_queue;
>>>          ++count;
>>>          ready(count > 0);
>>>      }
>>>
>>>      synchronized T remove() {
>>>          ready.wait;       // wait until count is > 0
>>>          --count;
>>>          ready(count > 0);
>>>          return remove_item_from_queue;
>>>      }
>>> }
>> I do believe that would deadlock. If thread A was waiting inside a
>> synchronized block, then thread B would never be able to enter the add
>> function, and hence would never be able to trigger the condition.
> 
> Conditions are supported by an atomic "unlock mutex and wait" operation. 
> For example,  in windows, by the SignalObjectAndWait function.  You are 
> basically saying: unlock this mutex and wait for the condition to be 
> signaled, then lock the mutex and return.  The operation has to be atomic so 
> that the thread can't be switched out of context just after unlocking and 
> miss the signal.  It is actually invalid to wait on a condition without 
> having the mutex locked.
> 
> So the code above has to do something special by unlocking the object before 
> waiting using an atomic operation.  I think this requires some language 
> support.
> 
> BTW, I'm not sure what the ready(count > 0) function does?  Can someone 
> explain it?  It appears that it is the signal, but I'm not sure why the 
> condition is required.

If you read the source for the Condition class under the example, you 
will see that all it does is sets the Condition's satisfied member, 
possibly signaling the underlying pthread_condition_t. Putting this 
boolean into the Condition class makes it FAR easier to avoid coding errors.

> 
> -Steve 
> 
>