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From:   Masahiro Yamada <yamada.masahiro@socionext.com>
Date:   Tue, 4 Jun 2019 21:48:12 +0900
Message-ID: <CAK7LNAS0Ph2Z6x0-UPSkJUC31NvPi09BmFrve+YJcXMrop-BGA@mail.gmail.com>
Subject: Re: [PATCH] media: do not use C++ style comments in uapi headers
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Joe Perches <joe@perches.com>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg KH <gregkh@linuxfoundation.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Tue, Jun 4, 2019 at 8:55 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Tue, Jun 4, 2019 at 1:23 PM Joe Perches <joe@perches.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.


-- 
Best Regards
Masahiro Yamada