Re: AI12-0218: What is the portable representation clause for processing IETF packets on little-endian machines?

["Dan'l Miller" <optikos@verizon.net>]

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.  How to actually accomplish the byte-swapping portably is the core topic of this entire thread and the core topic of AI12-0218.  Why post clearly off-topic material?  (Not only Erlang is prima facie off-topic, but bit fields in local-machine's endianness without any byte swapping is evermore off-topic, which could have been made somewhat on-topic if anything in Erlang demonstrated byte-swapping in opposing-endianness inter-machine communications, with an air of:  if Erlang can elegantly declare the byte-swapping portably on heterogenous endianness, then is there an analogous design in pre-AI12-0218 Ada?)

Everyone:
For example, in standard Ada without AI12-0218's proposal, how can one code up byte-swapping the fields of network byte order (big-endian) IETF packet headers on a little-endian machine?  Is there some clever switcheroo with processor-specific choice of child packages that contain little- versus big-endian subroutines analogous to the conditionally-compiled different definitions of C macros that both Paul and I have mentioned?

Paul:
Btw, bitfields in C and in Erlang are •the• roughly analogous language feature corresponding to representation clauses in Ada; all the undefined behavior regarding bitfields in C notwithstanding.  Indeed, AIUI, C's bitfields & their 1970s-known undefined behavior when porting among different processors were the public enemy #1 on Ichbiah et al's minds when they put bitwise representation clauses in Ada.  So you undermined your own line of reasoning there.