浅析container_of(ptr, type, member)文章来源:http://gliethttp.cublog.cn#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)//-------------------------------------------------------------下面一段程序摘自: drivers/usb/driver.cstruct usb_dynids { spinlock_t lock; struct list_head list;};struct usb_dynid { struct list_head node; struct usb_device_id id;};static const struct usb_device_id *usb_match_dynamic_id(struct usb_interface *intf, struct usb_driver *drv){ struct usb_dynid *dynid; spin_lock(&drv->dynids.lock);//gliethttp_20071018//1. drv->dynids.list为head,即:树根,父亲,正如上面的struct usb_dynids//2. dynid为child,其中drv->dynids.list.next存放了第一个child结构体中的// struct list_head类型名字为node的物理地址值.//3. 看着很复杂,其实翻译出来就简单多了(gliethttp)//模型为for(child;child != head;child = child->next)// for(dynid = container_of(drv->dynids.list.next, struct usb_dynid,nod);// dynid->node != &drv->dynids.list;// dynid = container_of(dynid->node.next, struct usb_dynid,nod)// ) list_for_each_entry(dynid, &drv->dynids.list, node) { if (usb_match_one_id(intf, &dynid->id)) { spin_unlock(&drv->dynids.lock); return &dynid->id; } } spin_unlock(&drv->dynids.lock); return NULL;}
