浅析container_of(ptr, type, member)

    指针ptr指向结构体type中的成员member；通过指针ptr，返回结构体type的起始地址,下面来自定义在include/linux/kernel.h中

/**
 * 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) );})

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

1.ptr为物理地址,其类型和member类型一致,最终使用typeof( ((type *)0)->member )
  由编译器自动返回member的类型
2.type为包含member成员的结构体
3.offsetof(type,member)为member成员在type结构体中的偏移值,大小范围0~sizeof(type)字节 (因为以0地址为type类型数据结构的起始地址)
4.ptr- offsetof()就等于包含该ptr的type结构体父变量的物理起始地址,强制转换为(type*).

应用举例：

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

#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))
//-------------------------------------------------------------
list_entry((head)->next, typeof(*pos), member)返回(head)->next物理指针所处位置向前减去offsetof()个字节数据之后, 其父变量pos的物理地址,父变量的类型在编译时由typeof(*pos)自动返回(gliethttp).
所以list_for_each_entry遍历head下面挂接的类型为typeof(*pos)的childs结构体们,当然每个child结构体包含struct list_head node之类相似的双向链表list_head类型项,就这样通过循环pos将依次指向双向链表上的各个child.(member就是child类型中被定义的变量名)
//-------------------------------------------------------------