Re: Allocators design flaw

[Victor Porton <porton@narod.ru>]

Victor Porton wrote:

> Victor Porton wrote:
> 
>> 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.
> 
> Moreover, the same logic would lead to say that there is malloc() standard
> C flaw.
> 
> It is not constructive to criticize an ISO standard of C.

I mean, it is because we cannot change C standard for better compatibility 
with Ada. But we can change Ada 202x for better compatibility with C 
libraries.

>>> Add max alignment + log max alignment - 1 to the desired size. Add log
>>> max alignment to the returned address and round to the required
>>> alignment. Place the offset to original address in front (log alignment
>>> length). Return the rounded address. When freed use the stored offset to
>>> get the original address.
>> 
-- 
Victor Porton - http://portonvictor.org