cvs commit: src/include ar.h

Poul-Henning Kamp phk at
Mon Nov 13 09:35:25 UTC 2006

In message <20061113173927.Q75708 at>, 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

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at 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.

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