Re: Subtleties of __attribute__((packed))

["Jan Blunck" <jblunck@suse.de>]

On 12/6/06, Phil Endecott <phil_arcwk_endecott@chezphil.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.