Re: [qemu-s390x] [RFC] error: auto propagated local_err

[Vladimir Sementsov-Ogievskiy]

18.09.2019 21:32, Eric Blake wrote:
> On 9/18/19 1:05 PM, Eric Blake wrote:
> 
>>>> #define MAKE_ERRP_SAFE() \
>>>> g_auto(ErrorPropagationStruct) (__auto_errp_prop) = {.errp = errp}; \
>>>> errp = &__auto_errp_prop.local_err
>>>>
> 
> I tried to see if this could be done with just a single declaration
> line, as in:
> 
> typedef struct ErrorPropagator {
>      Error **errp;
>      Error *local_err;
> } ErrorPropagator;
> 
> #define MAKE_ERRP_SAFE() \
>    g_auto(ErrorPropagator) __auto_errp_prop = { \
>      errp, ((errp = &__auto_errp_prop.local_err), NULL) }
> 
> But sadly, C17 paragraph 6.7.9P23 states:
> 
> "The evaluations of the initialization list expressions are
> indeterminately sequenced with respect to one another and thus the order
> in which any side effects occur is unspecified."
> 
> which does not bode well for the assignment to __auto_errp_prop.errp.
> All changes to errp would have to be within the same initializer.  Maybe:
> 
> #define MAKE_ERRP_SAFE() \
>    g_auto(ErrorPropagator) __auto_errp_prop = { \
>      .local_err = (__auto_errp_prop.err = errp, \
>          (errp = &__auto_errp_prop.local_err), NULL) }

Is it guaranteed that .errp will not be initialized to NULL after evaluating of
.local_err initializer?

> 
> but by the time you get that complicated, just using a statement is
> easier to read.
> 


-- 
Best regards,
Vladimir