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From: Victor Porton <porton@narod.ru>
Newsgroups: comp.lang.ada
Subject: Re: Allocators design flaw
Date: Sat, 14 Oct 2017 17:12:26 +0300
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Date: 2017-10-14T17:12:26+03:00
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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.

> 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