Re: [XEN PATCH v2 1/3] EFI: address a violation of MISRA C Rule 13.6

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.10.2024 07:25, Roberto Bagnara wrote:
> On 2024-09-30 15:07, Jan Beulich wrote:
>> On 30.09.2024 14:49, Federico Serafini wrote:
>>> guest_handle_ok()'s expansion contains a sizeof() involving its
>>> first argument which is guest_handle_cast().
>>> The expansion of the latter, in turn, contains a variable
>>> initialization.
>>>
>>> Since MISRA considers the initialization (even of a local variable)
>>> a side effect, the chain of expansions mentioned above violates
>>> MISRA C:2012 Rule 13.6 (The operand of the `sizeof' operator shall not
>>> contain any expression which has potential side effect).
>>
>> I'm afraid I need to ask for clarification of terminology and alike here.
>> While the Misra doc has a section on Persistent Side Effects in its
>> Glossary appendix, what constitutes a side effect from its pov isn't
>> really spelled out anywhere. Which in turn raises the question whether it
>> is indeed Misra (and not just Eclair) which deems initialization a side
>> effect. This is even more so relevant as 13.6 talks of only expressions,
>> yet initializers fall under declarations (in turn involving an expression
>> on the rhs of the equal sign).
>>
>> All the same of course affects patch 2 then, too.
> 
> MISRA C leaves the definition of "side effect" to the C Standard.
> E.g., C18 5.1.2.3p2:
> 
>    Accessing a volatile object, modifying an object, modifying a file,
>    or calling a function that does any of those operations are all
>    side effects,[omitted irrelevant footnote reference] which are
>    changes in the state of the execution environment.
> 
> The MISRA C:2012/2023 Glossary entry for "Persistent side effect"
> indirectly confirms that initialization is always a side effect.

Hmm, that's interesting: There's indeed an example with an initializer
there. Yet to me the text you quote from the C standard does not say
that initialization is a side effect - it would be "modifying an
object" aiui, yet ahead of initialization being complete the object
doesn't "exist" imo, and hence can be "modified" only afterwards.

Jan