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

From: Masahiro Yamada
Date: Tue Jun 04 2019 - 08:52:56 EST

On Tue, Jun 4, 2019 at 8:55 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Tue, Jun 4, 2019 at 1:23 PM Joe Perches <joe@xxxxxxxxxxx> 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:

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


[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