linux内核代码container_of

typedef unsigned int    __kernel_size_t;
typedef __kernel_size_t        size_t;
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

/**
* container_of - cast a member of a structure out to the containing structure
* @ptr:        the pointer to the member.
* @type:       the type of the container struct this is embedded in.
* @member:     the name of the member within the struct.
*
*/
#define container_of(ptr, type, member) ({                      \
const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
(type *)( (char *)__mptr - offsetof(type,member) );})

struct person
{
char name[10];
int age;
int num;
} ocjPerson;

int main(void)
{
struct person* p =  container_of(&ocjPerson.age, struct person, age);

printf("the value of p = %x\n", p);
printf("the value of &ocjPerson = %x\n", &ocjPerson);

return 0;
}

/*
打印结果：
　　the value of p == the value of &ocjPerson

分析：
container_of(&ocjPerson.age, struct person, age);
({                      \
const typeof( ((struct person *)0)->age ) *__mptr = (&ocjPerson.age);    \
(struct person *)( (char *)__mptr - ((size_t) &((struct person *)0)->age) );})

A. typeof是GNU C对标准C的扩展，它的作用是根据变量获取变量的类型

B.#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
分析一下这个offsetof宏的运行机理：
一共4步
1. ( (TYPE *)0 ) 将零转型为TYPE类型指针;
2. ((TYPE *)0)->MEMBER 访问结构中的数据成员;
3. &( ( (TYPE *)0 )->MEMBER )取出数据成员的地址;
4.(size_t)(&(((TYPE*)0)->MEMBER))结果转换类型。巧妙之处在于将0转换成(TYPE*)，结构以内存空间首地址0作为起始地址，则成员地址自然为偏移地址；

*/