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From: Jerry <lanceboyle@qwest.net>
Newsgroups: comp.lang.ada
Subject: Re: Trouble writing Ada callback from C
Date: Fri, 11 Sep 2009 22:28:05 -0700 (PDT)
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Date: 2009-09-11T22:28:05-07:00
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On Sep 11, 1:24=A0am, Ludovic Brenta <ludo...@ludovic-brenta.org> wrote:
>
> I take it you mean "not known at compile time" because it is indeed
> "known at call time".
>
> C doesn't have proper strings; it has pointers to null-terminated
> arrays of characters, which do not carry bounds information. So, the
> specification of your callback is not "return a string in an array"
> but "write N characters starting where label points, where N+1 <
> a_length, followed by a null character (ASCII.NUL)". =A0Here is a
> possible implementation (not compiled or tested either):
>
> with Interfaces.C.Strings; use Interfaces.C.Strings;
> procedure geolocation_labeler (label : in chars_ptr; length : in
> size_t) is
> =A0 =A0Result : constant String :=3D " N"; -- or whatever
> begin
> =A0 =A0Update (Item =3D> label, Offset =3D> 0, Str =3D> To_C (Result), Ch=
eck =3D>
> False);
> =A0 =A0-- See AARM B.3.1(43, 50.a/2)
> exception
> =A0 =A0when others =3D>
> =A0 =A0 =A0 -- Important: callbacks must handle all exceptions because th=
e C
> layer
> =A0 =A0 =A0 -- calling us cannot propagate or handle them.
> =A0 =A0 =A0 null; -- or report it.
> end geolocation_labeler;
>
> HTH
>
> --
> Ludovic Brenta.

Thanks, Ludovic.

Unfortunately, the C overlords are giving me a string of length 0--the
first character is the null character. So Update quits with

raised INTERFACES.C.STRINGS.UPDATE_ERROR

as is expected since Update can't write before the null character.

I'm now (desperately) studying the GNAT code in i-cstrin.adb and
attempting to make something like its Poke command, and then using it
in a modified Update which I'm calling Unsafe_Update which is just
like Update but with these lines deleted:

      if Check and then Offset + Chars'Length  > Strlen (Item) then
         raise Update_Error;
      end if;

I'm getting several errors which I'm trying to work through.

Jerry