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From: "Rupert Pigott" <darkboo-remove-this-ng.@hotmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Import a type from C
Date: Wed, 8 Jan 2003 20:46:23 -0000
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"Stephen Leake" <stephen.a.leake.1@gsfc.nasa.gov> wrote in message
news:u7kfqjnvv.fsf@gsfc.nasa.gov...
> "Frank J. Lhota" <NOSPAM.lhota.adarose@verizon.net> writes:
>
> >     pragma Convention( C, My_Data );
> >
> > The Convention pragma can be used to specify that this record type
should be
> > laid out in the same way that a C compiler would.
>
> Note that "a C compiler" really means "the C compiler that the Ada
> compiler knows about, with its default command line args". For
> example, GNAT will assume Gnu C, _without_ -fpacked. ObjectAda
> probably assumes MS C, again without the "packed" flag. That has
> caused me some major problems, when linking against C code compiled
> _with_ the -fpacked flag.
>
> So I always use a rep spec, instead of relying on Convention (C). Then
> I write some C code that checks the struct size, to be sure there is
> no padding.

Perhaps I'm overly pessimistic here... IIRC there is no guarantee
of the order of members within a structure in C either. One of those
little gotchas (along with no alignment control) that **** portable
code and force you to go the long way around...

Cheers,
Rupert