Re: [RFC][PATCH 03/12] digest_lists: Basic definitions

Greg KH
2021-06-27T15:36:04Z

On Sun, Jun 27, 2021 at 04:23:26PM +0100, Matthew Wilcox wrote:
> On Sun, Jun 27, 2021 at 12:53:47PM +0200, Greg KH wrote:
> > > +enum ops { DIGEST_LIST_ADD, DIGEST_LIST_DEL, DIGEST_LIST_OP__LAST };
> > > +
> >
> > For enums you export to userspace, you need to specify the values so
> > that all compilers get them right.
>
> I've never heard that rule before. Where does it come from?
> https://en.cppreference.com/w/c/language/enum
> says:
>
> If enumeration-constant is not followed by = constant-expression,
> its value is the value one greater than the value of the previous
> enumerator in the same enumeration. The value of the first enumerator
> (if it does not use = constant-expression) is zero.
>

I thought it was in the Documentation/driver-api/ioctl.rst file, but I
can't find it right now. Maybe it was something that Arnd said?

Arnd, is this still an issue? For some reason I thought it was always
good to have uapi .h enums be explicit as to the value in them,
otherwise some C compilers might produce other values if they were not
specified?

thanks,

greg k-h