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Re: Why doesn't combine like volatiles? (volatile_ok again, sorry!)
On Nov 28, 2005, at 3:00 AM, Richard Earnshaw wrote:
Possibly, but I think the more interesting observation is listed in
parenthesis: Can a volatile access ever alias a non-volatile access?
Logic would suggest that a program is unpredictable if written in
such a
way that permits such aliases to exist, since it would mean a location
is being accessed in both non-volatile and volatile manner.
I think this is uninteresting, as I'd presume that it is valid, and
the user really wanted to do it (the below is pseudo code, no, really):
int i[4096];
void foo() {
i[0] = 0;
mmap (... &i[0], ...);
(*(volatile int *)&i[0]) = 1;
}
Anyway, why it is uninteresting is, the normal rules say they alias,
so they alias. And for people that just want to quote the standard,
don't bother, let me:
[#5] If an attempt is made to modify an object defined with
a const-qualified type through use of an lvalue with non-
const-qualified type, the behavior is undefined. If an
attempt is made to refer to an object defined with a
volatile-qualified type through use of an lvalue with non-
volatile-qualified type, the behavior is undefined.102)