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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Dispatching callback handed over to C
Date: Fri, 06 Apr 2012 12:41:29 +0100
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Date: 2012-04-06T12:41:29+01:00
List-Id: <comp.lang.ada>

Maciej Sobczak <see.my.homepage@gmail.com> writes:

> On 5 Kwi, 11:13, Natasha Kerensikova <lithium...@gmail.com> wrote:
>
>> So for my binding, what about something like :
>>
>>    type Opaque_Data is null record;
>>    pragma Convention (C, Opaque_Data);
>>
>>    type Opaque_Pointer is access all Opaque_Data;
>>    pragma Convention (C, Opaque_Pointer);
> [...]
>
> Looks good!
>
>> As far as I cen tell, Opaque_Pointer refers only to types compatibles
>> with C,
>
>> Or is there some trap in the above code that I'm missing?
>
> That according to AARM, the C counterpart needs to correspond to
> Opaque_Data, that is, a pointer to an empty struct with the same
> alignment requirements.

My approach in TASH[1] was to write a script in Tcl to write (at
installation) a C program which #include's tcl.h etc to generate an Ada
spec[2] containing eg

   --  Size macros defined in tcl.h.

   NUM_STATIC_TOKENS : constant := 20;

   TCL_DSTRING_STATIC_SIZE : constant := 200;

   --  Sizes of structs defined in tcl.h.

   Tcl_Interp_Size : constant := 24;
   Tcl_Interp_Alignment : constant := 8;

but this is only needed where the user has to declare an instance of an
object. Otherwise, and this must be especially true where the C type is
void *, the library gives you back a handle and your only responsibility
is to give it back when called for.

Typically I wrote

   type Tcl_Interp_Rec (<>) is private;
   type Tcl_Interp is access all Tcl_Interp_Rec;
   pragma Convention (C, Tcl_Interp);

private

   type Tcl_Interp_Rec is new Interfaces.C.char_array
     (0 .. Tcl_Record_Sizes.Tcl_Interp_Size - 1);
   for Tcl_Interp_Rec'Alignment
     use Tcl_Record_Sizes.Tcl_Interp_Alignment;

I think I could easily have got away with 

   type Tcl_Interp_Rec is null record;

and left out all the size/alignment stuff!

[1] https://sourceforge.net/projects/tcladashell/
[2] this may have been a step further than necessary!