[Spice-devel] Alignment of data within spice packets

[Victor Toso]

Hi,

On Wed, Mar 08, 2017 at 07:21:15PM +0100, Christophe de Dinechin wrote:
> I have several errors like this:
>
> spice-channel.c:1923:12: error: cast from 'uint8_t *' (aka 'unsigned char *') to
>       'uint32_t *' (aka 'unsigned int *') increases required alignment from 1 to
>       4 [-Werror,-Wcast-align]
>   ...(uint32_t *)((uint8_t *)c->peer_msg + GUINT32_FROM_LE(c->peer_msg->caps_offset));
>      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Is there any guarantee that the result pointer will be 32-bit aligned?
> In other words, is it guaranteed that the c->peer_msg->caps_offset
> will be a multiple of 4? I don’t see this written in the spec.
>
> Based on the answer, the correct fix is:
>
> - If aligned, to cast to a uint32_t * in a way that disables the warning
>
> - If misaligned, to replaced *p with a read operation that is safe when mis-aligned (OS dependent I guess)

Yes, we should check case-by-case. I started doing some work around this
(nearly one and half year ago already!) but I got stuck on explaining
the rationale for each cast which would avoid the warnings.

You can see the comments and suggestions at the time are in the
following thread:

https://lists.freedesktop.org/archives/spice-devel/2015-August/021535.html

Cheers,
        toso
>
>
> Thanks
> Christophe
> 
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