Re: Variable length raw-byte data

[Jeff Carter <jrcarter@acm.org>]

David Botton wrote:
> 
> Since Byte_List is unconstrained is there not a potential for problem here
> with additional data associated with the type for bounds?
> 
> > type Byte_List is array (Positive range <>) of Unsigned_8;

This shouldn't cause a problem, because in

Thing : Byte_List (1 .. Length);
for Thing'Address use Pointer;

Thing'Address is defined to be the address of Thing (Thing'First). If
the compiler needs to store any additional information, it has to put
that information elsewhere.

> 
> Would this be a better solution:
> 
> C_Thing (Length, Pointer)
> 
> declare
>     type Byte_List is array (1 .. Length) of Unsigned_8;
>     Thing : Byte_List;
>     for Thing'Address use Pointer;
> 
> begin
>    ...
> end;

This is certainly OK, too.

> 
> I frequently code the above as:
> 
> C_Thing (Length, Pointer)
> 
> declare
>     type Byte_List is array (1 .. Length) of Unsigned_8;
>     type Pointer_To_Byte_List is access all Byte_List;
> 
>     function To_Pointer_To_Byte_List is
>        new Ada.Unchecked_Conversion (System.Address, Pointer_To_Byte_List);
> 
>     Thing : Pointer_To_Byte_List := To_Pointer_To_Byte_List
> (Pointer'Address);
> begin
>  ...
> end;
> 
> Is there a reason that using the for Thing'Address would be a better
> solution then the second method?

Yes, the second method is not portable. While Unchecked_Conversion
between System.Address and an access type may work with some compilers,
it definitely does NOT work with all compilers.

Since you have a constrained type here, you could use
System.Address_To_Access_Conversions to obtain a valid access value, but
why add the added baggage of access-type semantics when you can make
Thing a Byte_List?

-- 
Jeff Carter
"You tiny-brained wipers of other people's bottoms!"
Monty Python & the Holy Grail