Re: CSingleLock - known behaviour?
On Wed, 25 Jun 2008 16:16:09 -0400, Joseph M. Newcomer
<newcomer@flounder.com> wrote:
How about this one:
CSemaphore queuesem(...);
and the code is
void Receiver()
{
CSingleLock lock(&queuesem);
while(TRUE)
{
lock.Lock();
...remove element from queue
}
}
which cannot be written as
void Receiver()
{
while(TRUE)
{
CSingleLock lock(&queuesem);
lock.Lock();
...remove element from queue
}
}
Note that in the first example, the lock cannot be acquired a second time and in the
second example, each time you leave the body of the loop the semaphore is released, even
though that is the responsibility of the other thread.
There are symmetric examples for CEvent, where you want to wait for the event in a loop.
joe
I've always believed that the concept of "locking" doesn't really apply to
events, so it's just more bad design that you can use a CEvent with
CSingleLock, and that CEvent contains a function named "Unlock". Jeez, I
was saying that 10 years ago:
http://groups.google.com/group/microsoft.public.vc.language/msg/534cab3386d12348
After reviewing the implementation of CEvent::Unlock in VC 2008, I rest my
case:
BOOL CEvent::Unlock()
{
return TRUE;
}
Furthermore, although I've used events, I've never needed an RAII wrapper
for setting/resetting them, and indeed, my class library doesn't define
one.
Now that you mention it, the lock template class I last modified on
1/3/1999 does maintain a count. I guess I did this for the semaphore class
wrapper I wrote, and I forgot about it, as I haven't used a semaphore in
all that time. (The class library I wrote about three years ago doesn't
currently contain a semaphore class, just mutexes, events, condition
variables, and so forth.) So, I've been limiting my "lock thinking" to
mutexes, and CSingleLock clearly won't work for your semaphore example.
That said, it's trivial to write your own lock class that will work.
So, when I asked for a code example, I was really thinking about one that
would demonstrate the usefulness of recursive lock objects as applied to
mutexes.
--
Doug Harrison
Visual C++ MVP