Re: Interfacing with C

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

On May 18, 1:58 pm, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:
> On Mon, 18 May 2009 04:29:44 -0700 (PDT), cedric wrote:
> > I would like to connect a C interface to another software package with
> > my Ada programs.
>
> > The h-file of the api contains the following pre-processor commands:
>
> > #ifdef SPSSXD_EXPORTS
> > # define SPSSXD_API __declspec(dllexport)
> > #else
> > # if MS_WINDOWS
> > #   define SPSSXD_API __declspec(dllimport)
> > # else
> > #   define SPSSXD_API
> > # endif
> > #endif
>
> From this you should figure out the call convention. It can be either C or
> stdcall. The rest can be usually ignored. Usually this stuff tells whether
> the entry point is a reference or else a vector for dynamic run-time
> linking or else an entry point, when the library itself is compiled,
> nothing interesting for an Ada client.
>
> > Some functions headers are
>
> > SPSSXD_API bool IsBackendReady();
> > SPSSXD_API int SetUpFromSPSS(SpssAdapter* anAdapter, SpssXDSmb*
> > anSmb);
>
> > What do I have to do to get a connection between the C functions and
> > my Ada program with regard to the commands of the pre-processor?
>
> Print the preprocessor output if you are in doubts. Otherwise it could be
> like this:
>
>    with Interfaces.C;  use Interfaces.C;
>    ...
>    function IsBackendReady return Int; -- C does not have Boolean
>    pragma Import (stdcall, IsBackendReady, "IsBackendReady");
>
>    type SpssAdapter is record
>        ... -- Components of struct SpssAdapter
>    end record;
>    pragma Convention (C, SpssAdapter);
>
>    type anSmb is record
>        ... -- Components of struct anSmb
>    end record;
>    pragma Convention (C, anSmb);
>
>    function SetUpFromSPSS
>       (  anAdapter : access SpssAdapter;
>           anSmb : access SpssXDSmb
>       );
>    pragma Import (stdcall, SetUpFromSPSS, "SetUpFromSPSS");
>
> --
> Regards,
> Dmitry A. Kazakovhttp://www.dmitry-kazakov.de

Funny, I wrote the bindings for these two particular functions
yesterday afternoon at the Ada-Belgium general assembly for one of our
attendees. The types SpssAdapter and SpssXDSmb are opaque in the
specification (they are declared as:

class SpssAdapter;
class SpssXDSmb;

and the only function using them takes pointers to them). My solution
was something like:

package SPSS is
   type Adapter is null record;
   type Adapter_Access is access all Adapter;
   pragma Convention (C, Adapter_Access);

   type XD_SMB is null record;
   type XD_SMB_Access is access all XD_SMB;
   pragma Convention (C, XD_SMB_Access);

   function Set_Up_From_SPSS
     (An_Adapter : in Adapter; An_SMB : in XD_SMB_Access)
     return Interfaces.C.int;
   pragma Import (Convention => C, Entity => Set_Up_From_SPSS,
     Linker_Name => "SetUpFromSPSS");
end SPSS;

This solution avoids the need to define an Ada type equivalent to, and
with the same representation as, the C type, and it respects the
opaqueness of the types in the C API.

--
Ludovic Brenta.