container_of()宏解析

        在内核中很多地方都会使用到宏container_of()。

        都知道宏container_of()的作用是：根据结构体中一个域成员变量的指针获取指向整个结构体的指针、现在来分析这个宏是如何实现的；

        内核中container_of()代码如下：

/**
 * 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)); })

        内核中offsetof()代码如下：

#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

一、理论解析

       先来说说offsetof()宏；

       1. (type *)0：将0转化为TYPE型指针；

       2. (TYPE *)0)->MEMBER：表示访问TYPE型指针0指向结构体的成员变量MEMBER；

       3. &((TYPE *)0)->MEMBER)：表示取成员变量MEMBER的地址；

       4. ((size_t) ：类型转化

       巧妙之处在于：将0转换成(TYPE*)，然后结构体以内存空间首地址0作为起始地址，则成员地址就成为偏移地址。

       再来说说container_of()宏；

       1.  typeof(((type *)0)->member)的作用是获取成员member的结构类型；

        2. typeof(((type *)0)->member)*__mptr == (ptr);表示定义临时变量__mptr，将指针ptr赋值给__mptr；

        3. (type *)( (char *)__mptr - offsetof(type,member) )在(type *)作用下进行将字节型的结构体起始指针转换为type *型的结构体起始指针；

二、案例解析：

        以类型struct example_st为例：

struct example_st{
	type_fst mem1;
	type_sec mem2;
	type_thd mem3;
	type_fth mem4;
};

         进行地址标记：

        container_of(sec, struct example_st, mem2))代入宏

container_of(sec, struct example_st, mem2) ({          \
	const typeof(((struct example_st *)0)->mem2)*__mptr = (sec);    \
        (struct example_st *)((char *)__mptr - offsetof(struct example_st, mem2)); })

        第三行offsetof是计算mem2在结构体struct example_st,中偏移量，等于0x0008;
        第二行可知__mptr = 0xa018;

        第三行计算结果是：0xa018 - 0x0008 = 0xa0010;

三、程序实例

list.h

/**********************************************
 * Author: lewiyon@hotmail.com
 * File name: list.h
 * Description: define some macroes
 * Date: 2011-12-29
 *********************************************/
#ifndef __LIST_H
#define __LIST_H

//typedef unsigned long size_t;

#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

#define container_of(ptr, type, member) ({          \
        const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
        (type *)( (char *)__mptr - offsetof(type,member) );})

#endif

test.c

/**********************************************
 * Author: lewiyon@hotmail.com
 * File name: test.h
 * Description: test the macroes in the core.h 
 * Date: 2011-12-29
 *********************************************/

#include <malloc.h>
#include "list.h"

struct student{
    char *name;
    char sex;
    unsigned long id;
    int age;
    int money;
};

static struct student *id_stu(unsigned long *id_no)
{
    return (container_of(id_no, struct student, id));
}

int main(int argc, char *argv[])
{
    unsigned long offset;
    struct student *stu, *stu_tst;

    stu_tst = NULL;
    stu = (struct student *)malloc(sizeof(struct student));
    stu->name = "lewiyon"; 
    stu->sex = 'M'; 
    stu->id = 2004035000; 
    stu->age = 26; 
    stu->money =0;
    printf("-------------------------------\n");
    printf("stu         = %p\n", stu);
    printf("&stu->name  = %p\n", &stu->name);
    printf("&stu->sex   = %p\n", &stu->sex);
    printf("&stu->id    = %p\n", &stu->id);
    printf("&stu->age   = %p\n", &stu->age);
    printf("&stu->money = %p\n", &stu->money);
    printf("stu->name   = %s\n", stu->name);
    printf("-------------------------------\n");
    
    stu_tst = id_stu(&stu->id);
    offset = offsetof(struct student, id);
    printf("offset          = %p\n", offset);
    printf("stu_tst         = %p\n", stu_tst);
    printf("&stu_tst->name  = %p\n", &stu_tst->name);
    printf("&stu_tst->sex   = %p\n", &stu_tst->sex);
    printf("&stu_tst->id    = %p\n", &stu_tst->id);
    printf("&stu_tst->age   = %p\n", &stu_tst->age);
    printf("&stu_tst->money = %p\n", &stu_tst->money);
    printf("stu_tst->name   = %s\n", stu_tst->name);

    return 0;
}