Subj : Re: Question : usage of conditional variables.
To   : comp.programming.threads
From : Uenal Mutlu
Date : Thu Jun 09 2005 12:16 pm

"Jamie" <TechJimin@gmail.com> wrote
>
> I am quite new to multi-threaded programming, and I am now trying to
> make two threads and communicate with each other by using conditional
> variables. However, I got stuck in a problem. If anyone give me any
> idea, I would really appreciate it!!
>
> Program Summary:
>   1) Making two threads (tA, tB)
>   2) tA send a conditional signal to tB
>   3) tB recv a conditional signal
>   4) tB send a conditional signal to tA
>   5) tA recv a conditional signal
>   6) repeat through 2 to 5.
>
> Issue:
>  I tried to run this program with a set of mutex & conditional variable
> and also two sets of mutex & conditional variables(one for tA->tB, the
> other for tB->tA). But same issue happened.
> I ran it on VMWare and it's quite slow. When I run around 300 loop, it
> looks working well, it works as the order I expected. However, when I
> increase the number of loop more than that, the order get tangled and
> it looks waiting a signal forever and don't get a signal at all..
>
> Should it be guaranted that pthread_cond_wait() is called before
> pthread_cond_signal() is called?


Try the following pseudo C++ code as a model framework:

template<typename T> class CondVar
{
public:
   CondVar();
   ~CondVar();
   void Set(T val);
   T Get();
   void WaitFor(T val);
 //...
};

CondVar<bool> cvEnding;    // gets set externally
CondVar<bool> cv;   // used for signalling

void procA()
{
  while (!cvEnding.Get())
  {
    //...
    cv.WaitFor(false);
    //...do something
    cv.Set(true);
  }
 cv.Set(true);
}

void procB()
{
  while (!cvEnding.Get())
  {
    //...
    cv.WaitFor(true);
    //...do something
    cv.Set(false);
  }
 cv.Set(false);
}

void start()
{
   cv.Set(false);
   CreateThread(procA);
   CreateThread(procB);
   cvEnding.WaitFor(true);
   //...
}

.