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

[Adrian Hoe <abyhoe@gmail.com>]

On Jun 27, 12:10 pm, "Jeffrey R. Carter"
<spam.jrcarter....@spam.acm.org> wrote:
> Adrian Hoe wrote:
>
> >    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?
>
> type C_Data is array (Positive range <>) of Interfaces.C.C_Float;
> pragma Convention (C, C_Data);


Can I use Float instead of Interfaces.C.C_Float?

One question has been raised to me:

Why use Interfaces.C.Int, Interfaces.C.C_Float since they are all new
declaration of Ada types.

I know it is for readability and maintainability but the question is
logical as the declaration in Interfaces.C is just merely creating a
new Ada type with a new name, unless there is pragma Convention
statement after every such declaration in Interfaces.C.

So, that leads to another question:

Wouldn't it be better to write as such?

type C_Data is array (Positive range <>) of Float;
pragma Convention (C, C_Data);



> function PWM_Read  (Count : in Interfaces.C.Int; Data : in C_Data)
> return Interfaces.C.Int;
> pragma Import (C, PWM_Read, "pwmRead");
>
> PWM_Count : constant := ...;
>
> PWM_Data : C_Data (1 .. PWM_Count);
> Result   : Interfaces.C.Int;
>
> Result := PWM_Read (PWM_Count, PWM_Data);

Is PWM_Data passed as pointer to pwmRead?

Thanks again.
--
Adrian Hoe
http://adrianhoe.com