linux-kernel - Re: [PATCH] media: do not use C++ style comments in uapi headers

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Message-ID: <CAK7LNARyqW3q6_46e-aYjmF8c0jUNDLdyB28zNaBEXqTV+5QSA@mail.gmail.com>
Date:   Wed, 5 Jun 2019 00:27:47 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>, Joe Perches <joe@...ches.com>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Randy Dunlap <rdunlap@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] media: do not use C++ style comments in uapi headers

On Tue, Jun 4, 2019 at 10:44 PM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Tue, Jun 04, 2019 at 09:48:12PM +0900, Masahiro Yamada wrote:
> > On Tue, Jun 4, 2019 at 8:55 PM Arnd Bergmann <arnd@...db.de> wrote:
> > >
> > > On Tue, Jun 4, 2019 at 1:23 PM Joe Perches <joe@...ches.com> wrote:
> > > >
> > > > On Tue, 2019-06-04 at 20:13 +0900, Masahiro Yamada wrote:
> > > > > On the other hand, uapi headers are written in more strict C, where
> > > > > the C++ comment style is forbidden.
> > > >
> > > > Is this a real problem for any toolchain?
> > >
> > > There is likely some code that is built with -Wpedandic -Werror --std=c89
> > > or similar. Since glibc allows this combination for its own headers, it seems
> > > best to also allow it in kernel headers that may be included by libc headers
> > > or by applications, at least where it does not hurt.
> > >
> > > Realistically though, we probably assume c99 or gnu89 in user space
> > > headers anyway, since there is no 'long long' in earlier standards.
> > >
> > >        Arnd
> >
> > In fact, I detected this issue by the following patch:
> > https://patchwork.kernel.org/patch/10974669/
> >
> > When I worked on it, I wondered which
> > c-dialect flags should be used.
> >
> > This code:
> >
> > > # Unlike the kernel space, uapi headers are written in more strict C.
> > > #  - Forbid C++ style comments
> > > #  - Use '__inline', '__asm__' instead of 'inline', 'asm'
> > > #
> > > # -std=c90 (equivalent to -ansi) catches the violation of those.
> > > # We cannot go as far as adding -Wpedantic since it emits too many warnings.
> > > #
> > > # REVISIT: re-consider the proper set of compiler flags for uapi compile-test.
> > >
> > > UAPI_CFLAGS := -std=c90 -Wpedantic -Wall -Werror=implicit-function-declaration
> >
> > Even "-std=c99 -Wpedantic" emits lots of warnings.
> >
> >
> >
> > I noticed one more thing.
> >
> > There are two ways to define fixed-width type.
> >
> > [1] #include <linux/types.h>, __u8, __u16, __u32, __u64
> >
> >       vs
> >
> > [2] #include <stdint.h>, uint8_t, uint16_t, uint32_t, uint64_t
> >
> >
> > Both are used in UAPI headers.
> > IIRC, <stdint.h> was standardized by C99.
> >
> > So, we have already relied on C99 in user-space too.
>
> Just because we have relied on it in the past, does not mean we need to
> keep relying on it.  I have had numerous complaints over the years from
> libc authors that our uapi headers are _NOT_ able to be directly
> consumed by them.  They all end up having to fix things up and include
> local "sanitized" copies.
>
> So any work we can do here to make them more sane and work properly
> everywhere is a good thing, as right now, they are broken.


Maybe, we should document UAPI header coding guideline.

Without To-Don't list,
people will do anything.


-- 
Best Regards
Masahiro Yamada