如何将指向链表的指针转化为指向宿主结构体的指针

如何将指向list_head的指针p转化为指向宿主的指针？

#include
struct list_head {
        struct list_head *prev;
        struct list_head *next;
};
struct mm_struct {
        int a;
        int b;
        double c;
        struct list_head mmlist;
        unsigned int d;
};
struct mm_struct init_mm = { 19999 };
int main()
{
        struct mm_struct *pm = (struct mm_struct*)0;//calculator offset
        int offset = &(pm->mmlist);
        printf("offset of c: %d\n",&(pm->mmlist));
        struct list_head *pl = &init_mm.mmlist;
        struct mm_struct *pm1 = (struct mm_struct*)((int)pl - offset);//convert
        printf("pm1->a:%d\n",pm1->a);
}

[root@localhost study]# ./a.out
offset of c: 16
pm1->a:19999

代码还是很简单的，内核中也实现了相应功能的宏。如下：

433 #define list_entry(ptr, type, member) \
 434 container_of(ptr, type, member)

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

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

你还费解吗？