Re: [boost] Re: Threads docs updated in CVS

From: Peter Dimov (pdimov_at_[hidden])
Date: 2001-08-13 12:47:07

From: "Scott McCaskill" <scott_at_[hidden]>
> > OK, but why is this better than the event-based solution? IOW what is
the
> > rationale for the added complexity?
>
> That's a different question.

Well this is the question being discussed, isn't it? :-)

>  I don't have a lot of experience with events
> and WFMO, but I can see one possible advantage of CVs over WFMO.  WFMO can
> only tell you one of two things (depending on how it's used) about which
of
> the objects in question became signaled:
>
> 1) one particular object became signaled, the others may or may not have
> become signalled;
>
> 2) all of the objects became signaled
>
> In the case where the predicate consists of multiple parts (analogous to
> waiting for more than one object in WFMO), using CVs I can find out as
much
> info as I want about which part(s) of the predicate became true, whereas
> with WFMO I have only the two options above.  Granted, you could achieve
the
> same results by using more than just WFMO, but then the solution would be
> more complex than CVs, not less.

Not really; boost::waitFor() need not be constrained in the way the win32
API is, if this functionality is important.

[...]

> As I said, I don't have a lot of experience with events, but maybe someone
> who does can show me the most elegant way to do the equivalent of the
> following using events:
>
> mutex m;
> condition cv;
> volatile bool a = false, b = false, c = false;
>
> void threadfunc()
> {
>     mutex::scoped_lock lock( m );
>     while ( !( a || b || c ) )
>         cv.wait( lock );
>     if ( a )
>         do_something_a();
>     if ( b )
>         do_something_b();
>     if ( c )
>         do_something_c();
> }

Did you forget a loop? There's no point threading otherwise.

>
> void notifyThread( bool _a, bool _b, bool _c )
> {
>     { mutex::scoped_lock lock( m ); a = _a; b = _b; c = _c; }
>     cv.notify_one();
> }

synchronized_queue<int> q;
event e;

void threadfunc()
{
  for(;;)
  {
    waitFor(e);

    while(!q.empty())
    {
       int m = q.pop();
       if(m & 1) do_something_a();
       if(m & 2) do_something_b();
       if(m & 4) do_something_c();
    }
  }
}

void notifyThread(bool a, bool b, bool c)
{
   q.push(a + 2 * b + 4 * c);
   e.raise();
}

Did I utterly miss the point? :-)

--
Peter Dimov
Multi Media Ltd.