Re: [Qemu-devel] [RFC] error: auto propagated local_err

[Eric Blake <eblake@redhat.com>]

On 9/19/19 1:47 AM, Vladimir Sementsov-Ogievskiy wrote:

>> "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?

Probably not.

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

Either two declarations (the second being an unused dummy variable
declared solely for its initializer's side-effects) or a declaration and
a statement are the only sane ways I can see to provide guaranteed
ordering.  It's hidden behind a macro, so I don't care which.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org