Re: [RFC v2] Documentation about unaligned memory access

[DM <dm.n9107@gmail.com>]

On Nov 29, 2007 5:15 PM, Daniel Drake <dsd@gentoo.org> wrote:
[...]
> To avoid the unaligned memory access, you would rewrite it as follows:
>
>        void myfunc(u8 *data, u32 value)
>        {
>                [...]
>                value = cpu_to_le32(value);
>                put_unaligned(value, data);
>                [...]
>        }
>
> The get_unaligned() macro works similarly. Assuming 'data' is a pointer to
> memory and you wish to avoid unaligned access, its usage is as follows:
>
>        u32 value = get_unaligned(data);
>
> These macros work work for memory accesses of any length (not just 32 bits as
> in the examples above). Be aware that when compared to standard access of
> aligned memory, using these macros to access unaligned memory can be costy in
> terms of performance.
>

The get_unaligned call above will not do what you intended given the,
at least as I read it, implied context of myfunc. Since data is a u8*
it will only get one byte of data. To avoid misunderstandings the code
should probably read:

u32 value = get_unaligned((u32 *)data);

/DM
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/