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

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Date:   Wed, 5 Jun 2019 13:10:41 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        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 Wed, Jun 5, 2019 at 3:21 AM Arnd Bergmann <arnd@...db.de> wrote:
> > > >
> > > > 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.
>
> A related problem is that using the stdint.h types requires
> including stdint.h first, but the C library requires that including
> one standard header does not include another one recursively.
>
> So if sys/socket.h includes linux/socket.h, that must not include
> stdint.h or any other header file that does so.


This means we cannot reliably use uint{8,16,32,64}_t in UAPI headers.


[1] If we include <stdint.h> from linux/foo.h

    If sys/foo.h includes <linux/foo.h> and <stdint.h>,
    it violates the C library requirement.


[2] If we do not include <stdint.h> from linux/foo.h

    If sys/foo.h includes <linux/foo.h>, but not <stdint.h>,
    we get 'unknown type name' errors.


-- 
Best Regards
Masahiro Yamada