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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: C bindings, Interfaces.C.Pointers etc.
Date: 12 May 2004 05:50:03 +0100
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Date: 2004-05-12T05:50:03+01:00
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Jeffrey Carter <spam@spam.com> writes:

> Dr. Adrian Wrigley wrote:
> 
> > Hi folks!
> > I have a problem creating a binding to a C library.
> > The library makes extensive use of arrays of arrays of structs.
> > Both levels of arrays are of variable length.
> >  struct node { int index; double value; };
> >  struct problem { node **x; };
> > When I attempt to define Ada types for this type of thing,
> > I find I can't use access types to the arrays with the
> > necessary C convention.
> > "warning: this access type does not correspond to C pointer"
> > is the compiler error.
> 
>     -- struct node { int index; double value; };
>     type Node_Info is record
>        Index : Interfaces.C.Int;
>        Value : Interfaces.C.Double;
>     end record;
>     pragma Convention (C, Node_Info);
> 
>     -- struct problem { node **x; };
>     type Node_Ptr     is access all Node_Info;
>     pragma Convention (C, Node_Ptr);
>     type Node_Ptr_Ptr is access all Node_Ptr;
>     pragma Convention (C, Node_Ptr_Ptr);
>     type Problem_Info is record
>        X : Node_Ptr_Ptr;
>     end record;
>     pragma Convention (C, Problem_Info);
> 
> This compiles fine. This is an exact duplicate of the C
> declarations.

But not, I think, of the implications. Where is the equivalent of 

  problem p;
  node *np = *p.x;
  np++;               // <<<<

I've used constrained subtypes in the past:

  type Node_Array is array (Integer range <>) of Node_Info;
  
then in a subprogram taking a Node_Array parameter Nodes,

  subtype Specific_Node_Array is Node_Array (Nodes'First .. Nodes'Last);
  type Node_Array_Access is access Specific_Node_Array;
  pragma Convention (C, Node_Array_Access);

then declare a local binding to the C operation.

Same again for Node_Ptr_Array, though the Ada side of this is going to
be rather difficult since the Node_Ptrs in this are really
Node_Array_Accesses and the Node_Arrays are not all of the same
length.

Perhaps the simplest way would be to use System.Address:

   (Arr (Arr'First))'Address

>               To deal with this using a higher level Ada
> abstraction, you'd wrap something around this to provide that
> abstraction.



-- 
Simon Wright                               100% Ada, no bugs.