Re: Allocators design flaw

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 2017-10-14 16:03, 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.
>>
>> 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.
> 
> As far as I understand, it will not work, because the C library I am writing
> bindings for may try to free an object allocated by me (or I my need to free
> an object allocated by the library).

The last sentence describes freeing memory, i.e. for Deallocate.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de