[lkml]   [2012]   [Apr]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
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
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2012-04-14 01:21    [W:0.050 / U:0.024 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site