internal / external include files

internal / external include files

[Martin Kaiser]

Dear all,

is it correct that the files under include/uapi/ can be included by a
user space application?

I just ran into an error where my application ended up including
include/uapi/linux/stddef.h (with a number of intermediate steps).

This fails because the kernel's stddef.h includes
include/linux/compiler_types.h and this file is internal to
the kernel.

What is the correct way to solve this? Should I fix my include path to
make sure that my application picks the stddef.h in the compiler's
sysroot rather than the kernel's stddef.h?

Or should include/uapi/linux/stddef.h guard the kernel-internal parts
using #ifdef __KERNEL__?

Thanks,
Martin

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Re: internal / external include files

[Greg KH]

On Wed, Jul 24, 2019 at 11:11:00AM +0200, Martin Kaiser wrote:
> Dear all,
> 
> is it correct that the files under include/uapi/ can be included by a
> user space application?

Yes.

> I just ran into an error where my application ended up including
> include/uapi/linux/stddef.h (with a number of intermediate steps).

That's odd, what did you ask to be included that caused that?

> This fails because the kernel's stddef.h includes
> include/linux/compiler_types.h and this file is internal to
> the kernel.

What kernel version did this happen for?

> What is the correct way to solve this? Should I fix my include path to
> make sure that my application picks the stddef.h in the compiler's
> sysroot rather than the kernel's stddef.h?

The system stddef.h should always be used "first".

> Or should include/uapi/linux/stddef.h guard the kernel-internal parts
> using #ifdef __KERNEL__?

There should no longer be any need for that.

thanks,

greg k-h

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Re: internal / external include files

[Martin Kaiser]

Thus wrote Greg KH (greg@kroah.com):

> That's odd, what did you ask to be included that caused that?

I called gcc with -I $KERNEL_ROOT/include/uapi. This is the chain of
files that were included.

my_app.c
   $GCC_SYSROOT/usr/include/netinet/in.h
      $GCC_SYSROOT/usr/include/sys/socket.h
         $KERNEL_ROOT/include/uapi/asm-generic/socket.h
            $KERNEL_ROOT/include/uapi/linux/posix_types.h
               $KERNEL_ROOT/include/uapi/linux/stddef.h


and the error is

$KERNEL_ROOT/include/uapi/linux/stddef.h:2:34: fatal error:
linux/compiler_types.h: No such file or directory
#include <linux/compiler_types.h>

> > This fails because the kernel's stddef.h includes
> > include/linux/compiler_types.h and this file is internal to
> > the kernel.

> What kernel version did this happen for?

I get the error with today's linux-next tree. 4.14 is fine.

> > What is the correct way to solve this? Should I fix my include path to
> > make sure that my application picks the stddef.h in the compiler's
> > sysroot rather than the kernel's stddef.h?

> The system stddef.h should always be used "first".

My understanding of the gcc manual is that directories specified with -I
are searched before the system directories.

https://gcc.gnu.org/onlinedocs/gcc-9.1.0/gcc/Directory-Options.html#Directory-Options

So it seems that -I /my/kernel/include/uapi is not a good idea since the
kernel's include files take precedence then.

Best regards,
Martin

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Re[3]: internal / external include files

[Konstantin Andreev]

Hi, Martin.

The [$KERNEL_ROOT/include/uapi/linux/stddef.h] you are talking about is just a few lines:

| #include <linux/compiler_types.h>
|
| #ifndef __always_inline
| #define __always_inline inline
| #endif

it turns into

| #ifndef __always_inline
| #define __always_inline inline
| #endif

after [make headers_install] installs it into [/usr/include/linux/stddef.h].

Userspace applications shall not use kernel tree directly.

Regards, Konstantin.

Martin Kaiser, 25 jul 2010 00:20 msk:
>
> I called gcc with -I $KERNEL_ROOT/include/uapi. This is the chain of files that were included.
>
> my_app.c
>     $GCC_SYSROOT/usr/include/netinet/in.h
>        ...
>
> and the error is
>
> $KERNEL_ROOT/include/uapi/linux/stddef.h:2:34: fatal error: linux/compiler_types.h: No such file or directory #include <linux/compiler_types.h>
>
> Best regards, Martin

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