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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Interfacing to C: big structures
Date: Tue, 26 Feb 2008 20:28:48 -0500
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Date: 2008-02-26T20:28:48-05:00
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"Alex R. Mosteo" <amosteo@unizar.es> writes:

> I see. So, for example, let's say there's this C function:
>
> void init (struct S *s) {...};
>
> and you pass the Ada array address to it.
>
> It may crash/ill-behave, if the C code takes for granted that the s
> pointer is, say, 8-aligned, and some C generated code within "init"
> depends on this assumption.
>
> It may be slow, otherwise, if no such assumption exists.

Right.  The crash vs. slow issue is defined by the hardware,
in practise.  x86 machines tolerate unaligned refs (slowly)
whereas RISC machines tend to trap on unaligned refs.

> I'm missing some other possibilities?

No, I don't think you're missing anything.

> And, finally, is adding an alignment clause enough to solve this issue?
> If so, how do you query the alignment of a C datatype?

I'm not a C expert, but I don't think there's any easy way.

> (Could a conservative approach be to force the Ada side type to be
> max-aligned for the architecture?)

Yes.  Probably "for S'Alignment use Long_Float'Alignment" will work OK
on most machines.  Is there a cleaner way to do that?

- Bob