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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: AI12-0218: What is the portable representation clause for
 processing IETF packets on little-endian machines?
Date: Sat, 12 May 2018 08:08:40 +0100
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Xref: reader02.eternal-september.org comp.lang.ada:52308
Date: 2018-05-12T08:08:40+01:00
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"Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org> writes:

> On 05/11/2018 11:39 PM, Simon Wright wrote:
>>
>> whereas Ada2012 says
>>
>>      "To convert a record from one representation to another, two
>>      record types with a common ancestor type need to be declared,
>>      with no inherited subprograms."
>>
>> which seems to require _three_ types.
>
> The example in 13.6 shows this with only 2 types. This is because a
> type is defined to be its own ancestor (ARM 3.4.1).

I saw the example. I stand by my statement that it's confusing.

Perhaps there needs to be a way to mark a language usage as
domain-specific rather than natural; in English, I am not my own
ancestor (or descendant).

>> But in any case your code doesn't answer my question: what
>> representation trick could convert between
>>
>>     type T is record
>>        J : Integer;
>>     end T;
>>
>> and
>>
>>     type BE_T is new T;
>>     for BE_T use record
>>         ?????
>>     end record;
>
> I was showing how to use records with representations to get the
> compiler to do byte swapping for you. There's no way to use them to
> change the byte order of an integer type directly.

Not quite the same, but I think I showed upthread a conversion that does
this:

   function To_Fixed_32_16 (S : Four_Byte_Slice) return Fixed_32_16 is
      function Convert is new Ada.Unchecked_Conversion (Four_Byte_Slice,
                                                        Fixed_32_16);
   begin
      if Big_Endian then
         return Convert (S);
      else
         return Convert ((1 => S (4),
                          2 => S (3),
                          3 => S (2),
                          4 => S (1)));
      end if;
   end To_Fixed_32_16;

Of course, in C you'd use ntohl(), but we'd have to wrap something like
the above in a generic.