Re: Calling Ada from C

["Jeffrey R. Carter" <jrcarter@acm.org>]

hannibal.holm@gmail.com wrote:
> 
> 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;

Anything you use to talk to C should be convention C. Since, from below, 
in C this seems to be char, you should use Interfaces.C.Char and 
Interfaces.C.Char_Array. Since Char_Array is convention C, Ada won't 
expect it to have bounds associated with it.

> 
> 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);

Again, you should use Interfaces.C.Int instead of Natural.

Where does Length come from?

> This is called with:
> 
> char foo[] = {0xde, 0xad, 0xbe, 0xef};
> some_ada_proc(foo, sizeof(foo)); // sizeof static array = 4

This is where I get char from above.

If using Char_Array still crashes, you can use a very large constrained 
subtype of Char_Array:

subtype Many_Chars is Interfaces.C.Char_Array (Interfaces.C.Size_T);

and slice the result with 0 .. Interfaces.C.Size_T (Len). This can then 
be converted into your array of Bytes.

-- 
Jeff Carter
"I would never want to belong to any club that
would have someone like me for a member."
Annie Hall
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