Re: Importing C structs?

["Warren W. Gay VE3WWG" <ve3wwg@cogeco.ca>]

chris wrote:

> Freejack wrote:
> 
>> I'm finally starting to get an idea(at least I think I am.) on handling
>> interfacing to C.
> 
> Really?  I don't think I'll ever get the hang of it.

Depending upon the mandates that you're working with
for your project(s), I'll offer this general advice
when interfacing to C facilities:

For complex interactions, like those that involve C
structures, it is sometimes much quicker and reliable
(in the portability sense) to build a layer of C
between the two environments (C wrapper routines).

This helps in several ways:

  - C macros are easily tested and handled
  - sizeof(type) differences are more easily
    addressed (or identified when out of bounds)
  - difficult C structures can be mapped to Ada
    arguments that are more easily handled.
  - C preprocessor work can make the wrapper
    more platform agnostic without affecting
    Ada code changes.
  - Permits more flexibility on the Ada side,
    WRT to records (tagged or controlled etc.)

If OTOH, you must have "as much Ada as possible", or the
ultimate in efficiency, then you'll have more work cut
out for you, at least in the more difficult C/C++ cases.

WRT to C structures and portability, what you may find
is that all your effort will be trashed on another
platform where they have defined some members slightly
differently (different sizes, pad members, different
alignment rules and filling/packing, different order
etc.)  So, if this is a concern, a wrapper is much
safer in the portability sense.

...
> This is from my ongoing work on a libjpeg binding...
> 
>    type Jpeg_Error_Mgr is record
>       Error_Exit          : Error_Exit_Handler;
>       Emit_Message        : Emit_Message_Handler;
>       Output_Message      : Output_Message_Handler;
>       Format_Message      : Format_Message_Handler;
>       Reset_Error_Mgr     : Reset_Error_Handler;
>       Msg_Code            : C.Int;
>       Msg_Parm            : System.Address;
>       Trace_Level         : C.Int;
>       Num_Warnings        : C.Long;
>       Jpeg_Message_Table  : Jpeg_Message_TableT;
>       Last_Jpeg_Message   : C.Int;
>       Addon_Message_Table : Jpeg_Message_TableT;
>       First_Addon_Message : C.Int;
>       Last_Addon_Message  : C.Int;
>    end record;
> 
>    pragma Convention (C, Jpeg_Error_Mgr);

-- 
Warren W. Gay VE3WWG
http://home.cogeco.ca/~ve3wwg