__KERNEL__ macro

   The __KERNEL__ macro is defined because there is programs (like libraries) than 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.

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>