Re: [PATCH] xen: Use __auto_type

[Nicola Vetrini <nicola.vetrini@bugseng.com>]

On 2025-05-12 20:25, Elliott Mitchell wrote:
> On Mon, May 12, 2025 at 03:00:18PM +0200, Jan Beulich wrote:
>> On 12.05.2025 14:09, Andrew Cooper wrote:
>> >
>> > Now for the (new) controversial part.  Since sending this, Linux has
>> > decided to just #define auto __auto_type for C < 23, in order to start
>> > writing C23 compatible code from now.  It's more succinct, and has
>> > better longevity.
>> >
>> > We might want to consider the same, although it will introduce a new
>> > example of defining a keyword, which we'd have to call out in the
>> > MISRA/Eclair config.
>> 
>> I'm not outright opposed, as I don't think we use "auto" with its
>> original semantics, but it feels somewhat odd.
> 
> Problem is "auto" already has a defined meaning in C.  Having this will
> subtly break contributions from authors who weren't familiar with
> everything in Xen's headers.  For anyone who does anything with 
> projects
> besides Xen this will encourage bad habits.
> 
> I believe many projects have a rule of *never* #define C keywords.  I'm
> surprised such made it into the Linux kernel.  I expect it will be 
> ripped
> out in the near future.
> 
> MISRA *doesn't* absolutely forbid this?

It does, and in fact I don't think that is a wise decision (it's not 
quite UB I think because Xen does not use standard library headers, but 
still). However Xen does already #define "inline" with a specific 
rationale. I could find only [1] as a reference to the discussion in 
Linux, but perhaps I missed something. Do you have more recent thread 
@Andrew?

[1] 
https://lore.kernel.org/lkml/d4f87590-6cbb-4ee9-bead-7d958fc1fa83@p183/#R

-- 
Nicola Vetrini, B.Sc.
Software Engineer
BUGSENG (https://bugseng.com)
LinkedIn: https://www.linkedin.com/in/nicola-vetrini-a42471253