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From: fjloma@andaluciajunta.es (Francisco Javier Loma Daza)
Newsgroups: comp.lang.ada
Subject: Re: C array to ada record interface
Date: 25 Jul 2004 10:26:33 -0700
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Date: 2004-07-25T10:26:33-07:00
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Ludovic Brenta <ludovic.brenta@insalien.org> wrote in message news:<87iscdi6m5.fsf@insalien.org>...
> (Francisco Javier Loma Daza) writes:
> > Jeffrey Carter wrote in message Francisco Javier Loma Daza wrote:
> >> 
> >> > struct
> >> > {
> >> >    int a,b,c:
> >> >    int len;
> >> >    struct b* arr;
> >> > } A;
> >> > 
> >> > And I would love to interface in Ada with a record with a discriminant
> >> > 
> >> > type B_array(len: Positive) is record
> >> >     arr: array of B(1..len);
> >> > end record;
> >> > 
> >> > type A(len: Positive) is
> >> >    a,b,c: Integer;
> >> >    arr:   A(len);
> >> > end record;
> >> 
> >> Assuming you meant
> >> 
> >>     Arr : B_Array (Len => Len);
> >> 
> >> this won't work. What you have from C is a pointer; the actual array may 
> >>   not even be contiguous with the struct. You'll have to do something like
> >> 
> >> type B_Ptr is access all B;
> >> pragma Convention (C, B_Ptr);
> >> 
> >> type A_Info is record
> >>     A, B, C : Interfaces.C.Int;
> >>     Len     : Interfaces.C.Int;
> >>     Arr     : B_Ptr;
> >> end record;
> >> pragma Convention (C, A_Info);
> >> 
> >> You only use these types when talking directly to C. For the rest of 
> >> your application, you use something like the record you presented, and 
> >> have your operations that talk to C convert between the 2.
> >
> >
> > Thanks, I think you understood well
> >
> > I would want to use some more ada like array, you suggest to implement
> > that with accesors functions ... I was just wondering if it can be
> > done, the Len + Arr part, to be set up as an Ada array ...
> 
> Yes, it can be done:
> 
> with Ada.Unchecked_Conversion;
> function To_Ada_Array (Arr : in B_Ptr; Len : in Interfaces.C.int)
>                        return B_Array is
>    subtype Array_With_Known_Length is B_Array (1 .. Len);   
> 
>    function To_Ada is new Ada.Unchecked_Conversion
>       (Source => B_Ptr, Target => Array_With_Known_Length);
> 
> begin
>    -- ...
> end To_Ada_Array;
> 
> 
> In the begin/end block you have two possible strategies:
> 
>    return To_Ada (B_Ptr);
> 
> will return a B_Array where the Arr member has the same address (in
> memory) as the original C array.  If you deallocate the C array, you
> will get a Constraint_Error if you try to use the Ada array.  If you
> want to avoid this, the other strategy is to return a copy:
> 
>    declare
>       Input : Array_With_Known_Lengh := To_Ada (Arr);
>       Result : B_Array (1 .. Len) := Input;
>    begin
>       return Result;
>    end;
> 
> The advantage of having a copy is that Result.Arr has a different
> address than the C array; thus you manage the memory separately for
> the C and Ada arrays.  You can now safely deallocate the C array; the
> Ada compiler will manage the memory for the Ada array for you, since
> you declared it on the stack.
> 
> The disadvantage of this approach is that it requires more memory to
> run; if the array is very large, it could become a problem.
> 
> HTH
> 
> (PS. I have not tried to compile my example code, so there may be
> errors in it)

Thanks to all.