Subj : Re: CAS question
To   : comp.programming.threads
From : Joe Seigh
Date : Fri Mar 25 2005 04:39 pm

On Fri, 25 Mar 2005 19:51:32 -0000, Toby Douglass <toby.douglass@youknowwhattodo.summerblue.net> wrote:

> I'm currently thinking about lockless locking.
>
> One of the fundamental operations used is Compare-And-Swap.
>
> The Windows API for this is;
>
> LONG InterlockedCompareExchange( LONG volatile *Destination, LONG
> Exchange, LONG Comperand );
>
(bad example deleted)

>
> However, I've just seen this page;
>
> http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/queues.ht
> ml
>
> Which seems to imply this in-line work *can* be done.
>
> My question is: is this so?  can I perform any in-line operations on the
> arguments to CAS?
>

Yes but Microsoft's api is a little awkward.  It's better if you have an
api that returns a boolean and updates the compare value instead.  With
Microsoft's api you need to do something like

oldcount = count;
while ((oldcount = InterlockedCompareExchange(&count, oldcount + 1, cmpcount = oldcount)) != cmpcount);

With the latter api (note new and old/compare parms are  reversed) you can do

oldcount = count;
while (!cas(&count, &oldcount, oldcount + 1));


-- 
Joe Seigh

.