Re: [PATCH] [v2] Kbuild: move to -std=gnu11

Masahiro Yamada
2022-02-28T17:14:26Z

On Mon, Feb 28, 2022 at 8:25 PM Mark Rutland <mark.rutland@arm.com> wrote:
>
> Hi Arnd,
>
> This is great!
>
> On Mon, Feb 28, 2022 at 11:27:43AM +0100, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@arndb.de>
> >
> > During a patch discussion, Linus brought up the option of changing
> > the C standard version from gnu89 to gnu99, which allows using variable
> > declaration inside of a for() loop. While the C99, C11 and later standards
> > introduce many other features, most of these are already available in
> > gnu89 as GNU extensions as well.
> >
> > An earlier attempt to do this when gcc-5 started defaulting to
> > -std=gnu11 failed because at the time that caused warnings about
> > designated initializers with older compilers. Now that gcc-5.1 is the
> > minimum compiler version used for building kernels, that is no longer a
> > concern. Similarly, the behavior of 'inline' functions changes between
> > gnu89 and gnu11, but this was taken care of by defining 'inline' to
> > include __attribute__((gnu_inline)) in order to allow building with
> > clang a while ago.
> >
> > One minor issue that remains is an added gcc warning for shifts of
> > negative integers when building with -Werror, which happens with the
> > 'make W=1' option, as well as for three drivers in the kernel that always
> > enable -Werror, but it was only observed with the i915 driver so far.
> > To be on the safe side, add -Wno-shift-negative-value to any -Wextra
> > in a Makefile.
> >
> > Nathan Chancellor reported an additional -Wdeclaration-after-statement
> > warning that appears in a system header on arm, this still needs a
> > workaround.
>
> FWIW, I had a go at moving to c99 a few weeks ago (to be able to use
> for-loop-declarations in some concurrency primitives), and when I tried, I also
> saw declaration-after-statement warnings when building modpost.c, which is easy
> enough to fix:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=treewide/gnu99&id=505775bd6fd0bc1883f3271f826963066bbdc194
>

I do not understand this statement:

"Usually such warnings are implciitly enabled as part of `-std=gnu89`,
and in preparation for changing the standard used, this patch explciitly
enales the warnings with `-Wdeclaration-after-statement`, which takes
effect regardless of which version of the C standard is in use."

modpost is already built with -std=gnu89.

If Wdeclaration-after-statement is implied by gnu89,
why did nobody notice this before?

--
Best Regards
Masahiro Yamada