Re: Ada array vs C pointer (call by reference)

[anon@anon.org (anon)]

The unsafe way is to use "Address" attribute.

   function pwmRead ( Count : Interfaces.C.int ;
                      Data  : System.Address ) return Interfaces.C.int ;
   pragma Import ( C, pwmRead, "pwmRead" ) ;


The standard safe method you must use two interface packages.

   Interfaces.C ;
   Interfaces.C.Pointers ;

and create a C type of pointer package for the Float type, such as

   type Float_array is array ( Interfaces.C.size_t range <> ) 
        of aliased float;

    package Float_Pointer is new 
        Interfaces.C.Pointers ( Index              => size_t,
                                Element            => Float,
                                Element_Array      => Float_array,
                                Default_Terminator => 0 ) ;


study RM B.3.2 ( 45 ) for an example that uses character array. Should 
be easy to adapt to Float.


In <d390ae47-83b1-4cfb-8e66-d6a92f38cf4e@z32g2000prh.googlegroups.com>, Adrian Hoe <abyhoe@gmail.com> writes:
>Hi,
>
>I have a C library function which takes a float * as a parameter. The
>function is as below:
>
>   int pwmRead (int pwmCount, float * data);
>
>where data is an array of float which size is determined by pwmCount.
>
>In Ada, I have:
>
>   PWM_Count : Integer := No_Of_Components_Intended;
>   PWM_Data : array (1 .. PWM_Count) of Float;
>
>My concern is how safe to pass an Ada array to a C function as a
>pointer (call by reference)? I presume Ada will allocate contiguous
>memory for array, but what if it does not? How can we establish a
>deterministic allocation and to make sure float * is accessed
>contiguously? Is there any "safe" way to do this?
>
>Thanks.
>--
>Adrian Hoe
>http://adrianhoe.com