Re: Boeing and Dreamliner

["Robert I. Eachus" <rieachus@attbi.com>]

Wesley Groleau wrote:
> John R. Strohm wrote:
> 
>> types in Ada resemble those of pointers in other languages, the Ada
>> compiler-writer is under NO obligation to implement access types using
>> physical pointers.
>>
>> Is this correct?
> 
> What is a physical pointer?  An address?
> If so, you are correct.  I have found bugs
> where a programmer thought (incorrectly)
> that they could pass an access value to C
> for a pointer instead of passing The_Access.all'Address

I would go further and say that there are many places in Ada where an 
access value CANNOT be implemented as a simple physical pointer.  The 
simplest example is an access to subprogram, where you need to pass both 
the code AND the environment/closure.  This is not new to Ada, and in 
fact I could probably recall offhand how this was handled in several 
PL/I compilers.  (Multics PL/I used three "physical" pointers, while 
Stratus used two.)  Then there are things like bit pointers and bit 
slices that need to point to something smaller than an address.

Wesley Groleau's idiom will always work if anything works.  But there 
are things you just can't do.  (For example passing a pointer to a 
nested Ada subprogram to C probably won't work if there are any 
"uplevel" references.  One of the nice things about OpenVMS is the 
common calling sequence for all languages, so if you do pass a "fat" 
pointer, the C environment knows what to do with it.)




-- 

                                                        Robert I. Eachus

�In an ally, considerations of house, clan, planet, race are 
insignificant beside two prime questions, which are: 1. Can he shoot? 2. 
Will he aim at your enemy?� -- from the Laiden novels by Sharon Lee and 
Steve Miller.