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    SubjectRe: [PATCH][RESEND] do not redefine userspace's NULL #define
    On Saturday 14 of April 2012, Linus Torvalds wrote:
    > On Fri, Apr 13, 2012 at 3:01 PM, Peter Seebach
    > > Maybe the thing to do would be to ensure that NULL goes to __null,
    > > then define that to be ((void *) 0) if the compiler doesn't provide
    > > it?  The magic behavior of __null seems like it'd be preferable
    > > where it is available.
    > So if gcc guarantees that __null has the correct semantics, I could
    > imagine replacing the kernel ((void *)0) with __null.

    __null apparently exists only with g++, C does not have the stronger type
    safety that prevents ((void*)0) from being usable in C++, so __null is not
    needed there. So this is really only about userspace C++ code.

    So how about this one? It does not change anything in the C mode and keeps it
    the way you want, and it fixes the C++ mode.

    headers: do not redefine userspace's NULL #define when compiling C++ code

    GCC's NULL is actually __null when compiling C++, which allows detecting some
    questionable NULL usage and warn about it, so do not redefine NULL to 0.
    Use the 0 definition only if NULL does not exists, to keep kernel headers
    self-contained (and GCC's stddef.h will possibly redefine it later to __null).
    In C mode ((void*)0) works fine.

    Signed-off-by: Luboš Luňák <>
    include/linux/stddef.h | 4 +++-
    1 files changed, 3 insertions(+), 1 deletions(-)

    diff --git a/include/linux/stddef.h b/include/linux/stddef.h
    index 6a40c76..13dfc92 100644
    --- a/include/linux/stddef.h
    +++ b/include/linux/stddef.h
    @@ -3,10 +3,12 @@

    #include <linux/compiler.h>

    -#undef NULL
    #if defined(__cplusplus)
    +#ifndef NULL
    #define NULL 0
    +#undef NULL
    #define NULL ((void *)0)


    Lubos Lunak
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