在Linux源码中，经常看到大名鼎鼎offsetof 和 container of 的宏定义，这里就此进行解析，并做了实验验证用途，仅用于自己参考记录。
1. offsetof
主要作用是获取类型的偏移量
定义如下：
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
这里我们一层一层分析下offsetof
（1）首先是TYPE *0定义了一个TYPE类型的指针，指针的偏移地址为0
（2）然后从0位置开始选择我们需要看到偏移量的成员MEMBER
（3）对其取址，得到的即为偏移量
2. container_of
主要作用是根据结构体中某成员获取该结构体的首地址指针
定义如下：
/**
* 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.
*
* It takes three arguments – a pointer, type of the container,
* and the name of the member the pointer refers to.
* The macro will then expand to a new address pointing
* to the container which accommodates the respective member.
*/
#define container_of(ptr, type, member) ({ \
const typeof( ((type *)0)->member ) *__mptr = (ptr); \
(type *)( (char *)__mptr - offsetof(type,member) );})第一行看起来和offsetof类似，只是少了取址，多了类型转换和赋值，其实是用一个__mptr代替ptr进行操作，以免误操作。
第二行这里用字符指针是因为字符类型为1个字节，所以可以方便的计算出__mptr，即ptr的地址位置，然后减去其偏移量，得到的即为TYPE类型的首地址对应的指针。
测试代码如下：
#include <stdio.h>
#include <stdlib.h>
#include "list.h"
typedef struct data_list
{
int test_int_data;
char test_char_data;
struct list_head list;
}data_list;
int main()
{
data_list data;
data.test_int_data = 123;
data.test_char_data = 'a';
/* We test offsetof here*/
printf("\n-------We test the offset marco-------\n");
printf("offset of test_int_data = %lu\n", offsetof(data_list, test_int_data));
printf("offset of test_char_data = %lu\n", offsetof(data_list, test_char_data));
printf("offset of list_head = %lu\n", offsetof(data_list, list));
/*We test container_of here*/
printf("\n-------We test the container_of marco-------\n");
data_list *ptr;
ptr = container_of(&(data.test_int_data), data_list, test_int_data);
printf("We define a new pointer ptr to struct data_list\n");
printf("After container_of, the values are as follow\n");
printf("ptr->test_int_data = %d\n", ptr->test_int_data);
printf("ptr->test_char_data = %c\n", ptr->test_char_data);
return 0;
}