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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Binary and XML serialization of types
Date: Fri, 31 Jan 2014 11:49:10 +0100
Organization: cbb software GmbH
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Date: 2014-01-31T11:49:10+01:00
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On Fri, 31 Jan 2014 01:51:37 -0800 (PST), hanslad@gmail.com wrote:

> One question: You have implemented the litle endian Unsigned_16 Get like this:
> 
>  procedure Get
>              (  Data    : in out Stream_Element_Array;
>                 Pointer : in out Stream_Element_Offset;
>                 Value   : out Unsigned_16
>              )  is
>    begin
>       if (  Pointer < Data'First
>          or else
>             (  Pointer > Data'Last
>             and then
>                Pointer > Data'Last + 1
>          )  )
>       then
>          Raise_Exception
>          (  Layout_Error'Identity,
>             "Pointer is out of bounds"
>          );
>       elsif Pointer + 1 > Data'Last then
>          Raise_Exception (End_Error'Identity, "End of data");
>       end if;
>       Value :=
>          (  Shift_Left (Unsigned_16 (Data (Pointer + 1)), 8)
>          or             Unsigned_16 (Data (Pointer))
>          );
>       Pointer := Pointer + 2;
>    end Get;
> 
> Is it assumed that the system is little endian. How will this work on a big endian system?

The number is built up from stream elements using arithmetic operations.
These operation should work regardless the endianness of the host machine.

P.S. I cannot find a reference in RM which states the semantics of
Shift_Left. I presume that it is such that

    Shift_Left (2#0001#, 1) = 2#0010#

independently on the machine endianness.

P.P.S. Conversion from Stream_Element to Unsigned_16 must also be
independent on the machine endianness. The bit-endianness of Stream_Element
must be handled by the hardware, e.g. by the Ethernet controller, when it
stores received octets into the host machine memory.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de