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From: sjw <simon.j.wright@mac.com>
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Subject: Re: Gem 39 - compiler specific?
Date: Sat, 5 Sep 2009 23:54:50 -0700 (PDT)
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Date: 2009-09-05T23:54:50-07:00
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On Sep 5, 9:45=A0pm, Maciej Sobczak <see.my.homep...@gmail.com> wrote:

> You meant pragma Import (C, P);

Yes

> B.3/68:
>
> "An Ada access T parameter, or an Ada out or in out parameter of an
> elementary type T, is passed as a t* argument to a C function, where t
> is the C type corresponding to the Ada type T."
>
> This is the only guarantee concerning access types when interfacing
> with C.
> The paragraph is clearly constrained to those types that have some
> corresponding type on the C side. For example access to
> Interfaces.C.int will be passed as int*.
> But I see nothing about access to types that have no C counterpart. In
> particular, access values to class-wide types are not covered at all.
>
> This is the reason why I think that passing address (seen as void* on
> the C side) is the proper way.
> All corrections to this reasoning are highly welcome.

The last time I had to do something like this, I said something like

with Ada.Streams;
package Accessing is

   type Stream_Access is access all
Ada.Streams.Root_Stream_Type'Class;
   for Stream_Access'Size use 32;

   procedure Pass (SA : Stream_Access);
   pragma Import (C, Pass);

end Accessing;

but that was a long time ago, and GNAT now says

accessing.ads:7:20: warning: "Pass.SA" involves a tagged type which
does not correspond to any C type

which is a pretty strong hint!