[Spice-devel] Alignment of data within spice packets

[Christophe de Dinechin]

> On 8 Mar 2017, at 22:16, Victor Toso <lists at victortoso.com> wrote:
> 
> 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 <https://lists.freedesktop.org/archives/spice-devel/2015-August/021535.html>

Ach, I see you used SPICE_ALIGNED_CAST and SPICE_UNALIGNED_CAST. I used SPICE_REALIGN_CAST :-(

Will try to integrate your patches, thanks.

> 
> Cheers,
>        toso
>> 
>> 
>> Thanks
>> Christophe
>> 
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