Re: Allocators design flaw

["Randy Brukardt" <randy@rrsoftware.com>]

"Victor Porton" <porton@narod.ru> wrote in message 
news:ort609$1i31$2@gioia.aioe.org...
> Dmitry A. Kazakov wrote:
>
>> On 2017-10-14 04:53, Victor Porton wrote:
>>> It is impossible to properly implement an allocator through a C function
>>> (such as raptor_alloc_memory() from Raptor C library) which allocates a
>>> struct and returns the pointer to the allocated struct.
>>>
>>> It is because RM13.11(21.5/3) "The Alignment parameter is a nonzero
>>> integral multiple of D'Alignment..."
>>>
>>> (If it were "The Alignment parameter is equal to D'Alignment", then we
>>> would be able just to check (in Allocate procedure implementation) that
>>>
>>> pragma Assert(Dummy_Record'Alignment mod Alignment = 0);
>>> -- where Dummy_Record is an arbitrary C-convention record
>>> -- (as all C records have the same alignment reqs)
>>>
>>> So Alignment parameter may be arbitrarily big and the C function
>>> alignment may not conform to it.
>>
>> Usually allocators return addresses already rounded and there is nothing
>> to worry about.
>>
>>> Let us think how to work around (in Ada 202x) of this design flaw.
>>
>> If any it is _alloc_memory() flaw, not Ada's.
>
> It is an Ada flaw, because Ada must be able to interact with legacy C
> libraries.

Ada can interact just fine with C libraries. But there was never any 
intention that allocators and storage pools would be part of that 
interaction. It seems to be the case here that you are trying to fit a 
square peg into a round hole.

Keep in mind that all C interfacing is by definition implementation-defined, 
since it depends heavily on the C compiler. There's only so much portability 
that can be achieved in such interfacing; it can never be 100%.

                                Randy.