Re: Calling Ada from C

["Adam Beneschan" <adam@irvine.com>]

On Feb 22, 7:46 am, "hannibal.h...@gmail.com"
<hannibal.h...@gmail.com> wrote:
> I have a slight problem trying to call an Ada function from a C
> function. I need to pass in an unconstrained array to the Ada
> function. The problem is how I specify the size.
>
> This is probably very simple, but I am more or less just getting
> started with Ada, comming from a C-background.
>
> My code yealds warnings like this (when compiled with GNAT):
>
> foo.ads:50:23: warning: type of argument "Insert_C.Packet" is
> unconstrained array
> foo.ads:50:23: warning: foreign caller must pass bounds explicitly
>
> I have been searching a lot in order to figure out what to do about
> this, but all the FFI documentation is for calling C-functions from
> Ada, and some very simple examples of how to call ada functions that
> take primitive arguments (ints, and similar items).
>
> Anyone who know how to pass in the bounds explicitly?

Don't use an unconstrained array.  If I were to write an procedure P
to be called from C that would take, say, an unknown-size array of
integers, I'd do it like this:

   type Int_Array is array (Integer range <>) of Integer;
      -- this is the unconstrained array

   subtype Constrained_Int_Array is Int_Array (0 .. Integer'Last);
      -- or 1 .. Integer'Last, if you prefer

   procedure P (Arr : in out Constrained_Int_Array;  Len : in
Integer);

Then, inside P, just make sure you use Len; don't use Arr'Range or
Arr'Last which will be useless.  Not the safest thing, since now your
Ada program won't do the automatic bounds checking, but it's portable.

Or, you could write a second Ada routine to do all the work:

   procedure P2 (Arr : in Int_Array);  -- use the unconstrained array

and make P simply a wrapper:

   procedure P (Arr : in out Constrained_Int_Array;  Len : in Integer)
is
   begin
      P2 (Arr (Arr'First .. Arr'First + Len - 1));
   end P;


When you pass unconstrained arrays around as parameters from one Ada
subprogram to another, the subprograms will know the Ada conventions
for passing those arrays around.  A C program won't know how to do
this, unless you find out how your particular Ada implementation does
this and make your C program set up the data in the same way, but that
will make your program non-portable.  That's why I would stick to
constrained arrays when calling Ada from non-Ada (or vice versa).

                          -- Adam