Re: Allocators design flaw

[Victor Porton <porton@narod.ru>]

Simon Wright wrote:

> 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.

This does not help.

>> 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.

"it would have it"? What this sequence of English words mean?

-- 
Victor Porton - http://portonvictor.org