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From: "Chad R. Meiners" <crmeiners@hotmail.com>
Newsgroups: comp.lang.ada
Subject: Re: C array to Ada pointer to unconstrained array without
 copyingmemory
Date: Tue, 7 Oct 2003 18:44:09 -0400
Organization: Michigan State University
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"Duncan Sands" <duncan.sands@math.u-psud.fr> wrote in message
news:mailman.41.1065561795.25614.comp.lang.ada@ada-france.org...
> On Tuesday 07 October 2003 22:53, Chad R. Meiners wrote:
> > "Duncan Sands" <baldrick@free.fr> wrote in message
> > news:mailman.37.1065537708.25614.comp.lang.ada@ada-france.org...
> >
> > > The problem is the bounds of course.  For example, GNAT usually uses
> > > fat pointers, consisting of two normal pointers where one points to
the
> > > data, and the other to the bounds.  So the "clever stuff" will need to
> > > allocate some memory to hold the bounds and set up the fat pointer
> > > appropriately.
> > >
> > > Does anyone know a good way to do this?  The solution only needs to
> > > work with GNAT.  I appreciate that deallocating the pointer may need
> > > to be handled specially.
> >
> > See http://www.adapower.com/lang/accessmem.html method 1c using
> > Address_To_Access_Conversions.
> >
> > Page for strings, but it should work for any array type as long as you
have
> > the representation correct.
>
> Thanks Chad, but in fact that was not the question :)
> The method you refer to makes it easy to get a local
> variable with the right bounds, but that is not enough for me!
>
> Here is an example.
>
> Suppose I get the result in a local array X with the right bounds:
>
> type Array_Type is (Positive range <>) of Something;
> type Array_Pointer is access Array_Type;
>
> ...
> get data and length from C
> ...
>
> X : Array_Type (1 .. length);
> for X'Address use data; -- or another method
>
> So far so good.  Now suppose I do:
>
> Y : Array_Pointer := new Array_Type' (X);  -- (*)
>
> The object I want is Y.  What is the problem?  The problem
> is that line (*) involves allocating memory and copying the
> data from X to that memory.  I would like to end up with Y
> without performing the copy (and without reallocating
> memory for the array data).

declare
    C_Data_Ptr       :  Systems.Address := To_Address(My_C_Ptr);
    C_Data_Length :  Natural   := ...;

    type My_Ada_Type_Element is ...;
    type My_Ada_Array is array(Natural range <>) of My_Ada_Type_Element;

    subtype My_C_Data_As_An_Ada_Array_Type is
My_Ada_Array(1..C_Data_Length);
    package My_Converter is new
Address_To_Access_Conversions(My_C_Data_As_An_Ada_Array_Type);

    C_Data_In_Ada : My_Ada_Array renames
My_Converter.To_Pointer(C_Data_Ptr).all;
    -- No copying of the array done, but the array dope that you want is
built from the subtype information.
begin
...
end;