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    SubjectRe: [PATCH][RESEND] do not redefine userspace's NULL #define
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    On Saturday 14 of April 2012, Linus Torvalds wrote:
    > On Fri, Apr 13, 2012 at 4:18 PM, Lubos Lunak <l.lunak@suse.cz> wrote:
    > >> 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++
    >
    > Please don't continue to spread this total bogosity.
    >
    > The reason C++ cannot use "(void *)0" has nothing to do with "stronger
    > type safety". That's a total idiotic lie by C++ apologists, and I hate
    > hearing it repeated over and over again.
    >
    > And it really *is* a lie. The C++ type system isn't even "stronger",

    $ cat b.c
    void foo()
    {
    int* a;
    void* b;
    char c;
    a = b = &c;
    }
    $ gcc -Wall b.c -c
    $ g++ -Wall b.c -c
    b.c: In function ‘void foo()’:
    b.c:6:14: error: invalid conversion from ‘void*’ to ‘int*’

    > it's just different, and it's actively *broken* wrt NULL. Always has
    > been.
    >
    > The sane thing to do for C++ would always have been to recognize that
    > "(void *)0" is not a "void pointer" - it's just NULL.

    As much as I agree that it was stupid to copy the special-casing of 0 from C
    and they should have instead special-cased (void*)0, the reality is that it
    took them until C++11 to add that to the standard, and even there they did it
    differently, so C++ now has to live with the legacy of NULL being possibly
    implemented in a stupid way.

    The reality is also that g++ people recognized this quite some time back,
    implemented NULL in a sane way, and kernel headers still replace that
    implementation with one that, as you yourself say, is technically wrong. So
    how about my patch that fixes that? The rest is a discussion about things
    that should have been but are not and is completely pointless by now.

    8<-----
    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.

    Signed-off-by: Luboš Luňák <l.lunak@suse.cz>
    ---
    include/linux/stddef.h | 12 +++++++++---
    1 files changed, 9 insertions(+), 3 deletions(-)

    diff --git a/include/linux/stddef.h b/include/linux/stddef.h
    index 6a40c76..a75f4b9 100644
    --- a/include/linux/stddef.h
    +++ b/include/linux/stddef.h
    @@ -3,12 +3,18 @@

    #include <linux/compiler.h>

    -#undef NULL
    #if defined(__cplusplus)
    -#define NULL 0
    +#ifndef NULL
    +#ifdef __GNUG__
    +#define NULL __null
    #else
    -#define NULL ((void *)0)
    +#define NULL 0
    #endif
    +#endif
    +#else /* __cplusplus */
    +#undef NULL
    +#define NULL ((void *)0)
    +#endif /* __cplusplus */

    #ifdef __KERNEL__

    --
    1.7.7

    --
    Lubos Lunak
    l.lunak@suse.cz
    --
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