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To: Bruce Evans <bde@zeta.org.au>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
In-Reply-To: Your message of "Mon, 13 Nov 2006 19:30:11 +1100."
	<20061113173927.Q75708@delplex.bde.org> 
Date: Mon, 13 Nov 2006 09:35:19 +0000
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Cc: Joseph Koshy <jkoshy@FreeBSD.org>, src-committers@FreeBSD.org,
	cvs-all@FreeBSD.org, cvs-src@FreeBSD.org
Subject: Re: cvs commit: src/include ar.h 
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In message <20061113173927.Q75708@delplex.bde.org>, Bruce Evans writes:
>On Mon, 13 Nov 2006, Joseph Koshy wrote:
>
>> jkoshy      2006-11-13 04:28:29 UTC
>>
>>  FreeBSD src repository
>>
>>  Modified files:
>>    include              ar.h
>>  Log:
>>  Attempt to improve application portability by marking `struct ar_hdr'
>>  as `packed'.
>>
>>  The C standard leaves the alignment of individual members of a C
>>  struct upto the implementation, so pedantically speaking portable
>>  code cannot assume that the layout of a `struct ar_hdr' in memory
>>  will match its layout in a file.  Using a __packed attribute
>>  declaration forces file and memory layouts for this structure to
>>  match.
>>
>>  Submitted by:   ru
>
>I don't see how this can be more portable.

I agree with bruce that __packed is not the way to go.

For things that represent communication protocols, even if
they happen through files, I advocate using the functions
in sys/endian.h to explicitly decode and encode fields into
bytestrings.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.