Re: New New Callbacks Uploaded

From: William Kempf (sirwillard_at_[hidden])
Date: 2000-11-27 10:34:31

--- In boost_at_[hidden], "Michael D. Crawford" <crawford_at_g...> wrote:
> > And
> > then there's multithreading.
> 
> I know it take extra effort, but you can do reference counting with 
> multithreading.  The ZRef reference counted smart pointer in ZooLib 
handles 
> multithreading correctly.
> 
> You could probably lift out ZRef and ZAtomic from ZooLib and use 
them on 
> their own for multithreaded reference counting without using the 
rest of 
> ZooLib.  http://zoolib.sourceforge.net
> 
> It does this by using atomic addition and subtraction on the 
reference 
> counts when it's getting operated on.  So if you have two threads 
> incrementing and decrementing the reference count it does the right 
thing.

I've not looked at ZooLib (yet!), but I can tell you that if all that 
was done were to use atomic increment and decrement on the ref-count 
then the smart pointer is *NOT* thread safe.  Copies also must copy 
the pointer, and if the whole operation isn't protected I don't see 
how it can be thread safe (though maybe I'm missing some subtle 
ordering of operations that might achieve this).

> It wasn't easy for Andy Green to get it right, considering also the 
cross-
> platform issues (and we found a small  compiler bug when doing the 
atomic 
> operations) but it does seem to work correctly.

Then we should look here for ideas for Boost Threads as well.

Bill Kempf