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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: AI12-0218: What is the portable representation clause for
 processing IETF packets on little-endian machines?
Date: Fri, 11 May 2018 01:44:19 +0300
Organization: Tidorum Ltd
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Date: 2018-05-11T01:44:19+03:00
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On 18-05-11 01:24 , Dan'l Miller wrote:
> On Thursday, May 10, 2018 at 3:32:11 PM UTC-5, Paul Rubin wrote:
>> "Dan'l Miller" writes:
>> …  In C programs you usually just call some
>> macros to convert machine integers to and from network byte order.  In
>> the case where the machine int's bytes are already in network order, the
>> macro does nothing.
>>
>> Example 4 of this page
>>
>>   http://erlang.org/doc/programming_examples/bit_syntax.html
>>
>> shows how you describe an IP datagram in Erlang:
>>
>>     -define(IP_VERSION, 4).
>>     -define(IP_MIN_HDR_LEN, 5).
>>
>>     DgramSize = byte_size(Dgram),
>>     case Dgram of
>>         <<?IP_VERSION:4, HLen:4, SrvcType:8, TotLen:16,
>>           ID:16, Flgs:3, FragOff:13,
>>           TTL:8, Proto:8, HdrChkSum:16,
>>           SrcIP:32,
>>           DestIP:32, RestDgram/binary>> when HLen>=5, 4*HLen=<DgramSize ->
>>             OptsLen = 4*(HLen - ?IP_MIN_HDR_LEN),
>>             <<Opts:OptsLen/binary,Data/binary>> = RestDgram,
>>         ...
>>     end.
>>
>> It isn't bad, if you are used to Erlang syntax.  Those fields like
>> FragOff:13 are bit fields of the specified lengths.
>
> Ummmmmm, Paul, neither your example of C-esque bitfields in Erlang
> nor the Erlang tutorial page that you provided nor the Erland
> reference manual link in that tutorial page provided a solution
> to the byte-swapping.

Both the example, and the Erlang reference manual section linked from 
the example, describe an "endianness" attribute that can be given one of 
the values "big", "little", "native", with default "big". However, it 
seems this attribute applies only to a restricted set of types: integer, 
utf16, utf32 and float.

It is not clearly explained how endianness is interpreted for fields 
that do not align with octet boundaries.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .