linux-kernel - Re: Subtleties of __attribute_

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Date:	Wed, 6 Dec 2006 16:04:22 +0100
From:	"Jan Blunck" <jblunck@...e.de>
To:	"Phil Endecott" <phil_arcwk_endecott@...zphil.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Subtleties of __attribute__((packed))

On 12/6/06, Phil Endecott <phil_arcwk_endecott@...zphil.org> wrote:
> Dear All,
>
> I used to think that this:
>
> struct foo {
>    int a  __attribute__((packed));
>    char b __attribute__((packed));
>    ... more fields, all packed ...
> };
>
> was exactly the same as this:
>
> struct foo {
>    int a;
>    char b;
>    ... more fields ...
> } __attribute__((packed));
>
> but it is not, in a subtle way.
>

The same code is generated. The difference is that usually packing the
whole struct isn't as error-prone as packing every element. Besides
that the gcc warns about packing objects that have an alignment of 1.
This is the reason why we should use the second approach.
-
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