offsetof和container_of

offsetof

offsetof根据结构体的类型和成员名来计算该成员距结构体首地址的偏移量。
offsetof 的一种实现如下：

#define offsetof(type, member) ((size_t)(&((type *)0)->member))

offsetof宏首先将0转换为结构体指针类型，然后引用成员变量并取其地址。由于结构体首地址为0，所以成员变量的地址即为成员距结构体首地址的偏移量。
上面这种定义方法需要编译器的支持，现代编译器中一般采用下面的宏定义：

#define offsetof(type, member) __builtin_offsetof(type, member)

container_of

container_of根据成员地址、结构体类型和成员名来计算结构体的首地址，其定义如下：

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

示例代码

#include <stdio.h>#include <stddef.h>#define container_of(ptr, type, member) ({ \    const typeof( ((type *)0)->member ) *__mptr = (ptr); \    (type *)( (char *)__mptr - offsetof(type,member) );})struct foo{    char a;    char b[10];    char c;};intmain(int argc, char *argv[]){    struct foo st;    st.a = 1;    printf("offset of a is %d\n", (int)offsetof(struct foo, a));    printf("offset of b is %d\n", (int)offsetof(struct foo, b));    printf("offset of c is %d\n", (int)offsetof(struct foo, c));    struct foo* pst;    pst = container_of(&st.c, struct foo, c);     printf("a = %d\n", pst->a);}

参考

[1] http://www.cnblogs.com/Anker/p/3472271.html
[2] http://en.wikipedia.org/wiki/Offsetof
[3] http://www.cplusplus.com/reference/cstddef/offsetof/