Delivery-Date: Fri Oct 17 13:06:06 2008
Message-Id: <200810172014.m9HKEax4030056@bourbon.usc.edu>
To: cs551@merlot.usc.edu
Subject: Re: CSCI 551- Final project Part 1 :Doubt
Date: Fri, 17 Oct 2008 13:14:36 -0700
From: Bill Cheng <william@bourbon.usc.edu>

Someone wrote:

  > I had a doubt with Signal and Wait.
  > 
  > Say, there is a Thread X waiting on a Condition Variable CV.
  > 
  > And there is a Thread Y signaling using CV.
  > 
  > Ideally, Thread Y would signal and Thread X would wake up from Wait.
  > 
  > However, if Thread X has not begun waiting yet, and Thread Y comes
  > along and signals (even when no one is  waiting), then how is the
  > behavior ?

Then thread X won't see the signal.

  > Will the Signal be buffered and used when Thread X executes wait ?

No.  That's not how pthread signals behave.

  > In CSCI-402, we used Monitor Variables to overcome this problem. But
  > here, will Pthreads handle the buffering, or will the signal be lost ?

The signal will be lost.  You can

  > Alternatively, can I use Monitor variables here as well ?

I'm not sure exactly what you meant by "monitor variables".
If you have variables that you want to share among threads,
you need to use a mutex.  Let's assume what you meant by
a "monitor variable" is a variable protected by a mutex.

I don't really see how a signal can be missed if you code it
correctly!  Let's say thread X wants to catch a signal and
thread Y wants to send a signal and there is a mutex M and a
condition variable C and a "monitor variable" V.  So, thread
X's code should look something like:

    pthread_lock(M);
    while (V does not have the desired value) pthread_cond_wait(M,C);
    ...

Thread Y's code can look something like:

    pthread_lock(M);
    /*
     * you can switch the order of the next two lines of code
     * and it won't matter because thread Y has the mutex lock.
     */
    pthread_cond_broadcast(C);
    set V to desired value;
    pthread_unlock(M);

May be you did not signal the condition when the corresponding
mutex is locked or you checked the value of V when you did not
have the mutex locked.
--
Bill Cheng // bill.cheng@usc.edu <URL:http://merlot.usc.edu/william/usc/>