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Newsgroups: comp.lang.ada
Subject: Re: trying to call HDF5 from ada -- problems with constants
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From: Ludovic Brenta <ludovic.brenta@insalien.org>
Date: Thu, 26 Aug 2004 20:29:19 +0200
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Date: 2004-08-26T20:30:25+02:00
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Christopher Broeg writes:
> tx. I just realized that with gnat I can import the constants
> directly. I realized that the ada specifications in the RM do never
> explicitly mention functions or procedures with respect to pragma
> import. So I did:
>
>     H5T_STD_I32BE_g : Hid_t;
>     pragma Import(C,H5T_STD_I32BE_g , "H5T_STD_I32BE_g");
>
> and it works! All you have to do is call H5open manually, which is
> done automatically with the c preprocessor. Is this behaviour
> compiler-dependant, or ADA standard?

It is Ada standard, but the implementation depends on how well the Ada
compiler, C compiler, and linker get along with each other.  GNAT, GCC
and GNU ld get _very_ well along together :)

BTW, you can call H5open from within elaboration code if that helps
you, like this:

package P is
   pragma Elaborate_Body (P); -- force elaboration of the body (optional)

   H5T_STD_I32BE_g : Hid_t;
   pragma Import(C,H5T_STD_I32BE_g , "H5T_STD_I32BE_g");
end P;

package body P is
begin
   H5T_STD_I32BE_g := H5open;  -- elaboration code
end P;

The elaboration runs before your main subprogram.

> I'm surprised, because there was this long thread on this very topic
> explicitly stating that ada cannot import c constants... But I guess
> it only referred to preprocessor constants. All "variing" constants
> are of course real c constants visible in the library.

That's right, Ada cannot see preprocessor constants, she can only see
C's "static" variables.  This is really done by the linker.

-- 
Ludovic Brenta.