Re: Robert Dewar's great article about the Strengths of Ada over other langauges in multiprocessing!

[gpriv@axonx.com]

On Mar 13, 9:03 am, "Alex R. Mosteo" <devn...@mailinator.com> wrote:
> Peter C. Chapin wrote:
> > This paper:
>
> >http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf
>
> > describes some of these issues in a C++ context and is written by two
> > authors who probably know what they are talking about. Their conclusion
> > is that it is pretty much impossible to write correct multi-threaded
> > code in C++ without (non-standard) compiler assistance. I'm paraphrasing
> > here. It is my understanding, however, that this matter will be
> > addressed in C++ 0x.
>
> > Peter
>
> After reading the pointers given in this thread, I'm under the impression that
> indeed you need to use volatile for unprotected variables accessed across
> threads? For, example, in Ada:
>
> declare
>    Global_Exit : Boolean := False;
>    --  pragma Volatile (Global_Exit);
>    --  pragma Atomic   (Global_Exit);
> begin
>    --  Lots o'tasks here that monitor Global_Exit for termination.
> end;
>
> Here you don't need a protected, since once someone makes Global_Exit true, the
> end arrives and all is well. I would normally mark that as Atomic which, as it
> implies volatile, is OK.
>
> But let's say one knows that boolean is atomic for the architecture, and makes
> the (risky) decision of not explicitly saying so to the compiler. I understand
> that, without volatile, tasks never writing to Global_Exit could optimize away
> any check for changes on it, and see it as a constant? (Indeed I guess Gnat
> would issue a warning about Global_Exit being constant).
>
> Although in Ada I don't see much (or any?) use for volatile, non-atomic shared
> variables outside of protected objects.
>
> Some flaw in my understanding?

You are correct you should use volatiles. I don't think there is any
need to have volatile inside the protected objects.  Access will be
properly serialized anyway.

G.