Re: Adding offset to 'Address

[Adam Beneschan <adam@irvine.com>]

On Sep 12, 2:40 pm, Robert A Duff <bobd...@shell01.TheWorld.com>
wrote:
> Adam Beneschan <a...@irvine.com> writes:
> >   (Ethernet_Header'Size + System.Storage_Unit - 1) /
> > System.Storage_Unit
>
> > which rounds the size in bits up to the next multiple of
> > System.Storage_Unit.  [System.Storage_Unit will equal 8 on most
> > systems.]
>
> It seems to me that this sort of thing won't work unless
> System.Storage_Unit is 8 bits.  So you don't want rounding up.
> It seems better to say:
>
>     pragma Assert (System.Storage_Unit = 8);
>     ... Raw_Bytes.all'Address + Ethernet_Header'Size/8 ...
>
> If you really need to do this sort of thing, that is.

That's very likely to work.  I'd still worry that, since the
definition of 'Size is slightly fuzzy, an implementation could still
choose a value that is not a multiple of 8 in a surprising way:

   type Rec1 is record
      Value_1 : Integer;  -- 32 bits, say
      Value_2 : Integer;
      Control_Flag : Boolean;
   end record;

On an 8-bit machine, is Rec1'Size 72 or 65?  I don't think the RM
provides an answer to this.  The minimum size required to represent
Rec1 is 65, and an implementation *could* decide that if Rec1 were a
component of a larger record, and that larger record has another
Boolean component, it could stick that component in the same byte as
Control_Flag.  Or it could just always allocate 9 bytes for Rec1
regardless.  So I still think rounding up is a good idea:

    pragma Assert (System.Storage_Unit = 8);
     ... Raw_Bytes.all'Address + (Ethernet_Header'Size+7)/8 ...

But the real answer, I think, is that if you're going to be using this
type for communication with other machines, you need to use
representation clauses to specify every detail of the type anyway,
including a 'Size clause:

   for Ethernet_Header'Size use ...;

and now you can make sure that it's a multiple of 8:

     pragma Assert (System.Storage_Unit = 8);
     pragma Assert (Ethernet_Header'Size mod 8 = 0);

     ... Raw_Bytes.all'Address + Ethernet_Header'Size/8 ...

                                    -- Adam