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Newsgroups: comp.lang.ada
Date: Mon, 19 May 2014 17:22:31 -0700 (PDT)
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Subject: Re: How to import C pointers to structures?
From: Adam Beneschan <adambeneschan@gmail.com>
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Xref: news.eternal-september.org comp.lang.ada:19914
Date: 2014-05-19T17:22:31-07:00
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On Monday, May 19, 2014 5:10:14 PM UTC-7, Shark8 wrote:
> On 19-May-14 16:44, Adam Beneschan wrote:
>=20
> >> You generally *can't*  do a real/meaningful import w/o knowing the
> >> layout.
>=20
> > I'm not sure that's quite true ... there are lots of situations
> > where you simply want to "pass through" a pointer, e.g. in
> > callback situations, where a function sets up a callback and says
> > "when you do the callback, please pass this pointer to it"; and
> > whoever registers the callback will take the pointer, store it
> > somewhere, and use it when it's needed, without ever trying to
> > look at what it points to.
>=20
> While those are /common/ cases, I would be hesitant to call them=20
> /general/ cases.
>=20
> > Unfortunately, I don't see the equivalent of "void *" anywhere in
> > Interfaces.C, which is what the sort of thing you'd want.  You
> > could probably use Interfaces.C.Strings.chars_ptr (the equivalent
> > of "char *").
>=20
> Instantiate Interfaces.C.Pointers on a null-record or, perhaps,=20
> System.Address?

I wouldn't trust System.Address; there's no guarantee that the Ada implemen=
tation of this Ada type is represented the same way as a C pointer, an Ada =
access object, or anything else.  The first idea, to instantiate Interfaces=
.C.Pointers, should be trustworthy--that's an excellent idea.

                               -- Adam