转载：http://blog.csdn.net/kasalyn/article/details/17097639
The __KERNEL__ macro is defined because there is programs (like libraries) that include kernel code and there is many things that you don't want them to include. So most modules will want the__KERNEL__ macro to be enabled.
When you compile your kernel, __KERNEL__ is defined on the command line. (in tree/kernel/Makefile---> KBUILD_CPPFLAGS := -D__KERNEL__)
User-space programs need access to the kernel headers, but some of
the info in kernel headers is intended only for the kernel. Wrapping
some statements in an#ifdef __KERNEL__/#endif block ensures that user-space programs don't see those statements.
example:
in the user space ,if you want to include the header using ‘＃ifdef __KERNEL__ XXXX', you should define the __KERNEL__.
/*为了引用kernel中的数据结构,我常用如下样式include那些服务于内核的头文件*/
#ifdef __KERNEL__
#include <linux/list.h>
#include <linux/mm.h>
#undef __KERNEL__
/*将两种类型的头文件隔离开来*/
#include <stdio.h>
#include <errno.h>
对于__KERNEL__举例：
#ifdef __KERNEL__
# define HZ CONFIG_HZ /* Internal kernel timer frequency */
# define USER_HZ 100 /* User interfaces are in "ticks" */
# define CLOCKS_PER_SEC (USER_HZ) /* like times() */
#else
# define HZ 100
#endif
在.config中：
CONFIG_HZ=100
故ISA-32中,HZ=100,而jiffies =10ms