Re: Calling Ada from C

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

On Feb 27, 4:49 am, "hannibal.h...@gmail.com"
<hannibal.h...@gmail.com> wrote:
> On Feb 23, 2:53 pm, Stephen Leake <stephen_le...@stephe-leake.org>
> wrote:
>
> > "hannibal.h...@gmail.com" <hannibal.h...@gmail.com> writes:
> > > 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.
>
> > Since the concept "unconstrained array" doesn't exist in C, this can't
> > possibly be a precise description of what you are doing.
>
> > Can you give more context, and an example of the code? Especially the
> > C declarations of the data structure you are passing to Ada.
>
> > --
> > -- Stephe
>
> The data structure in C is just a pointer to a block of data to be
> sent over a network link. This need to be converted to a byte array
> defined as:
>
> Byte_Size   : constant := 8;
>
> type Byte is range 0 .. 2**Byte_Size - 1;
> for Byte'Size use Byte_Size;
>
> type Byte_Array is array (Natural range <>) of Byte;
> pragma Pack (Byte_Array);
>
> As said, I am not extremly familiar with Ada and come from a C-
> background, so I have a few problems trying to figure out what the ada
> runtime and type system actually does. I tried something like
>
> -- This proc is exported as with C-conventions
> procedure Some_Ada_Proc(Msg : in Byte_Array;
>                         Len : in Natural) is
>   Msg_Constr : Byte_Array := Msg(0 .. Length - 1);
> begin
> ...
> end Some_Ada_Proc;
>
> This is called with:
>
> char foo[] = {0xde, 0xad, 0xbe, 0xef};
> some_ada_proc(foo, sizeof(foo)); // sizeof static array = 4
>
> But that result in a crash which I assume is due to how Ada treats
> arrays passed as arguments.

No, it's due to how Ada treats *unconstrained* arrays passed as
arguments.  When an Ada procedure has an unconstrained array as the
argument, it needs the bounds (the 'First and 'Last of the actual
array), so those have to be passed in somehow.

When the array parameter is declared with a *constrained* array type
[technically, an array subtype], the Ada routine knows the bounds
already, so they don't have to be passed in.  So there normally isn't
anything funny about how they get passed in; the Ada routine will
normally just take the address of the first element of the array, the
way you'd expect any other language to do it.

So instead of making Msg a Byte_Array, which is unconstrained, make it
some constrained subtype:

 type Byte_Array is array (Natural range <>) of Byte;
 pragma Pack (Byte_Array);

 subtype Byte_Array_Constrained is Byte_Array (Natural'First ..
Natural'Last);
   -- pick a better name than this

 procedure Some_Ada_Proc (Msg : in Byte_Array_Constrained; ...

Please see my earlier post for more details.

                          -- Adam