Re: Thread Checking the Queue data in an infinite loop

From:
"Doug Harrison [MVP]" <dsh@mvps.org>
Newsgroups:
microsoft.public.vc.mfc
Date:
Tue, 15 Jan 2008 11:05:27 -0600
Message-ID:
<85ppo39ec1l6uhppr12gvf03bauo5tba61@4ax.com>
On Tue, 15 Jan 2008 06:17:35 -0800 (PST), prams <pramodjoisb@yahoo.com>
wrote:

Hello All,
Mr. Doug Harrison:
Thank you for your response.
As mentioned in your psuedo code I am already doing this with my
wrapper class which has a Mutex to provide synchronized read and write
access to the Queue.


The only reason that may be sufficient here is because there is only one
reader. The issue is that in general, both queue operations below must be
locked as a unit:

   // NB: He said he is fine with draining the queue.
   if (!q.empty())
      q.drain();

If your Queue class isn't locking the empty() test, it needs to start. If
you had more than one reader, and you didn't lock the composite operation,
you would have a race condition between the separately locked empty() test
and drain() operation.

But with this Code also I think you cannot avoid
unneccessarry wakeups since events are signalled once we add to the
queue and Reader thread will drain the queue and then calls wait.


I said in my first post in this thread that unnecessary wake-ups will occur
with the algorithm you described and explained how to deal with them. I've
never said they wouldn't occur. Beyond the fact that they can occur and you
have to code around them, I don't think they're significant.

Also I got a link to a PDF File which suggests use of semaphores for a
similar problem.
Good Link: http://sleepygeek.org/stuff/tysvc.pdf
But I am scared to use semaphores, as I have never used. I am trying
to solve using semaphores also.
Please let me know whether pipes(which signal event) are the perfect
solution. Then I would once for all go with that.


I don't have any more to say about pipes beyond what I wrote a couple of
messages ago, in the same message where I linked to the Wiki article that
explained the "bounded buffer" problem and solved it in terms of
semaphores.

--
Doug Harrison
Visual C++ MVP

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