On Wed, Jun 5, 2019 at 3:21 AM Arnd Bergmann <arnd@xxxxxxxx> 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
