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From: Martin Krischik <krischik@users.sourceforge.net>
Newsgroups: comp.lang.ada
Subject: Re: C bindings, Interfaces.C.Pointers etc.
Date: Thu, 13 May 2004 19:37:14 +0200
Organization: AdaCL
Message-ID: <8969563.sTNc1ZJU4A@linux1.krischik.com>
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Date: 2004-05-13T19:37:14+02:00
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Dr. Adrian Wrigley wrote:

> Thanks Martin for this suggestion.
> 
> I have given (aninvolvedimation of) my code below.
> 
> It doesn't use the I.C.Pointers package, and allows
> Ada programs to access elements of the arrays using
> standard array notation.  The main problem is that
> the bound Ada uses are over-sized, and simple use
> of 'range and 'last will fall off the ends.

There is one more way but only usable when you C and Ada compiler use the
same memory layout for variables. The example works for Strings:

   type C_Array is array (Natural range <>) of aliased Character;

   for C_Array'Component_Size use Interfaces.C.char'Size;

   pragma Convention (
             Convention => C,
             Entity     => C_Array);

   package C_Pointers
   is new
      Interfaces.C.Pointers (
         Index              => Natural,
         Element            => Character,
         Element_Array      => C_Array,
         Default_Terminator => Character'First);

   type C_String
   is record
      --  Address of some C Array
      Data : C_Pointers.Pointer;
      --  Size of the Array.
      Size : Interfaces.C.size_t;
   end record;

Ada_String  : String (1 .. Natural (Some_C_String.Size));

for Ada_String 'Address use Some_C_String.Data.all'Address;

The trick is in the last line. This is of corse an Unchecked_Convertion
without actually moving the Data. Use at your own risk. 

You can probably reduce the code involved by using an normal access to
Character for C_String.Data.

> Overall this solution meets my needs very well, and
> if I accidentally use the invalid array (upper) bound,
> the program crashes very rapidly (like it would have
> done in C with a similar bug!).

With the trick above Ada will check the array bounds - when Size containts
the right value.
 
> I'm a little surprised that I.C.Pointers doesn't seem
> to provide a convenient and efficient solution to this
> problem - using C pointers directly in array access.

I.C.Pointers is supposed to all the checked convertions needed and will work
when C and Ada use different memory layout - at least that's the theorie.
 
With Regards

Martin.

-- 
mailto://krischik@users.sourceforge.net
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