access to a volatile variable

[Sergei Organov]

Nathan Sidwell <nathan@acm.org> writes:

> Sergei Organov wrote:
> >
> > Because, for example, *whole* struct might be single hardware register:
> > 
> > struct A {
> >   int i : 1;
> >   int j : 1;
> > };
> I nearly put a comment about this, but figured that using bitfields to *access*
> volatile objects is so obviously insane that I needn't. (Notice I didn't say
> `using bitfields to *describe* the *layout* of a device register'.)
> Ho hum.

Maybe it is insane, but still it works fine for me.

> 
> > A a = *pa;  /* Access to structure as _whole_ */
> > int i = a.i;
> > int j = a.j;
> > 
> > generates _one_ access to the hardware address (at least with all C
> > compilers I ever used)
> Prove that the language guarantees that. [a particular compiler on a particular
> system may offer such a guarantee by accident or by design, I'd even like an
> example of a compiler where this is a documented feature.]

I can't prove (and I'm almost sure it couldn't be proved), that's why
I didn't claim it does. But once I check that it works this way with
particular compiler for particular target, I use it, because it is so
hardware dependent and it is very unlikely that this code will ever be
compiled for another target or by another compiler.

> > Unfortunately the latter is not valid C++ :-( Have no idea why didn't
> Why is it not valid? (What do you mean by `valid' -- that it guarantees a
> single access, or that it is compilable without diagnostic?)

It requires constructor

A(A volatile&);

to be defined for struct A, to compile 'A a = *pa', but doesn't
provide such constructor automatically (the same for assignment
operator). So the code shouldn't compile. It is explicitly mentioned
in C/C++ incompatibilities section of the Standard. I posted it here
as egcs bug some time ago, but one of gurus explained me that I was
wrong.

> 
> > they preserve compatibility with C in this area at least for
> > PODs. Don't you think it'd be fine idea to bypass this limitation in
> > egcs?
> What limitation?

To require to define 'A(A volatile&)' to compile (actually egcs
produces warning, not error).

> 
> nathan

Sergei.