Re: [Xen-devel] [PATCH] trace: fix build with gcc9

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> George Dunlap <george.dunlap@xxxxxxxxxx> 03/07/19 1:02 PM >>>
>On 3/7/19 10:34 AM, Jan Beulich wrote:
>> While I've not observed this myself, gcc 9 (imo validly) reportedly may
>> complain
>> 
>> trace.c: In function '__trace_hypercall':
>> trace.c:826:19: error: taking address of packed member of 'struct 
>> <anonymous>' may result in an unaligned pointer value
>> [-Werror=address-of-packed-member]
>>   826 |     uint32_t *a = d.args;
>
>Wait, is this saying that in this case (i.e., with a single uint32_t
>before args), you *do* get an unaligned pointer value, or just that if
>the struct changes in the future that the pointer value may become
>un-aligned?

With the __packed attribute, the compiler _could_ place the structure at
a misaligned slot on the stack. It has got nothing to do with struct layout
afaict. Note how the diagnostic says "may", not "does".


Jan


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