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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Allocators design flaw
Date: Sat, 14 Oct 2017 09:02:47 +0100
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Date: 2017-10-14T09:02:47+01:00
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Victor Porton <porton@narod.ru> writes:

> 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)

See the arguments in the AI:
http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ai05s/ai05-0193-1.txt?rev=1.8&raw=N

> So Alignment parameter may be arbitrarily big and the C function alignment 
> may not conform to it.

No bigger than 'Max_Alignment_For_Allocation.

> Let us think how to work around (in Ada 202x) of this design flaw.

If the C struct needs alignment greater than that of the elements of
which it is composed (see the AI for example) it would have it.