Re: [9fans] plan or side effect

["Thomas Bushnell, BSG" <tb+usenet@becket.net>]

"AMSRL-CI-CN" <gwyn@arl.army.mil> writes:

> "Thomas Bushnell, BSG" <tb+usenet@becket.net> wrote in message
> news:87u1s2y1uu.fsf@becket.becket.net...
> > In the presence of concurrence, even this is not sufficient, because a
> > different thread could clobber the value.  However, C does not
> > guarantee synchronization in this case unless the variable is marked
> > "volatile".
> 
> Actually the C standard does not address threads at all.
> It is nice that "volatile" helps, but I'm sure it doesn't totally
> solve the concurrent data access problem for threads.

No, certainly not!  I misspoke.  I shouldn't have said
"synchronization", which does imply more, and certainly you still need
mutexes or semaphores or something.

"volatile" is a declaration to the compiler that the value could be
changing at any time, unbeknownst to the compiler, and so values can't
be cached in registers.  

If "acquire mutex" is a function call, then you don't even need to
declare the variable volatile, since the compiler knows that the
function call could clobber all of memory.  So it isn't even necessary
for threads, if you *are* using mutexes, and for other reasons you
certainly do need to.  So the reference to threads was a needless
confusion; sorry.

Thomas