list_entry , container_of

/*refer to
http://blog.csdn.net/yinkaizhong/article/details/4093795
*/
[root@localhost chapter15]# cat test.c 
#include<stdio.h>
struct student{
    char name[20]; 
    char sex;
}stu={"zhangsan",'m'};

main()
{
    struct student *stu_ptr;    //存储container_of宏的返回值
    int offset;            //存储offsetof宏的返回值
//下面三行代码等同于 container_of（＆stu.sex,struct student, sex ）参数带入的情形

    const typeof(((struct student*)0)->sex) *_mptr = &stu.sex;
//首先定义一个 _mptr指针， 类型为struct student结构体中sex成员的类型
//typeof 为获取（（（struct student＊）0）->sex)的类型，此处此类型为char
//（（struct student＊）0）在offsetof处讲解

    offset = (int)(&((struct student *)0)->sex);
/*（（struct student＊）0）为 把 0地址 强制转化为指向student结构体类型的指针
该指针从地址 0 开始的 21个字节用来存放name 与 sex（char name〔20〕与 char sex共21字节）
sex存放在第20个字节出（从0字节开始）
&((struct student *)0)->sex 取出sex地址(此处即为20) 并强制转化为整形
所以offset为20，后面的printf结果将证明这一点*/

    stu_ptr = (struct student *)((char*)_mptr - offset);
/*（（char＊）_mptr - offset）此处先把_mptr指针转化为字符形指针
（为什么这么做呢？ 如果_mptr为整形指针 _mptr - offset 相当于减去 sizeof（int）＊offset个字节）
减去 offset值 相当于 得到_mptr所在结构体的首地址（即stu的地址）
然后我们把 该地址 强制转化为 struct student类型即可正常使用了*/

    printf("offset of stu.sex = %d\n",offset); 
    printf("stu_ptr->name:%s\nstu_ptr->sex:%c\n", stu_ptr->name, stu_ptr->sex);
    return 0;
}
[root@localhost chapter15]# 

[root@localhost chapter15]# ./test 
offset of stu.sex = 20
stu_ptr->name:zhangsan
stu_ptr->sex:m 

/**********include/linux/kernel.h******************/
#define container_of(ptr, type, member) ({			\
	const typeof( ((type *)0)->member ) *__mptr = (ptr);	\
	(type *)( (char *)__mptr - offsetof(type,member) );})

/**********include/linux/stddef.h*****************/
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

主要就是取offset这一步，利用了借助将0强制转化为student指针类型，
取其sex元素地址，便是相对student首址的偏移量
&((struct student *)0)->sex

给定
结构体的一个成员的地址，
此成员在此结构体中名字，
此结构体类型
可以找到此结构体的地址


include/linux/kernel.h

/**
 * 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) );})

include/linux/list.h

/*
 * Simple doubly linked list implementation.
 *
 * Some of the internal functions ("__xxx") are useful when
 * manipulating whole lists rather than single entries, as
 * sometimes we already know the next/prev entries and we can
 * generate better code by using them directly rather than
 * using the generic single-entry routines.
 */

struct list_head {
	struct list_head *next, *prev;
};

/**
 * list_entry - get the struct for this entry
 * @ptr:	the &struct list_head pointer.
 * @type:	the type of the struct this is embedded in.
 * @member:	the name of the list_struct within the struct.
 */
#define list_entry(ptr, type, member) \
	container_of(ptr, type, member)

/**
 * list_for_each	-	iterate over a list
 * @pos:	the &struct list_head to use as a loop cursor.
 * @head:	the head for your list.
 */
#define list_for_each(pos, head) \
	for (pos = (head)->next; prefetch(pos->next), pos != (head); \
        	pos = pos->next)

/**
 * list_for_each_entry    -    iterate over list of given type
 * @pos:    the type * to use as a loop cursor.
 * @head:    the head for your list.
 * @member:    the name of the list_struct within the struct.
 */
#define list_for_each_entry(pos, head, member)                \
    for (pos = list_entry((head)->next, typeof(*pos), member);    \
         prefetch(pos->member.next), &pos->member != (head);     \
         pos = list_entry(pos->member.next, typeof(*pos), member))