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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Allocators design flaw
Date: Sat, 14 Oct 2017 09:27:13 +0200
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Date: 2017-10-14T09:27:13+02:00
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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.

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