list_for_each_entry

最新推荐文章于 2025-09-18 15:14:51 发布

转载最新推荐文章于 2025-09-18 15:14:51 发布 · 420 阅读

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本文详细解读了C语言中用于遍历列表的宏函数list_for_each_entry的工作原理，包括如何初始化循环变量，使用prefetch进行预取优化，以及循环条件的实现。同时介绍了内部使用的list_entry和container_of函数的作用。

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/**
* list_for_each_entry - iterate over list of given type
* @pos: the type * to use as a loop cursor.
* @head: the head for your list.
* @member: the name of the list_struct within the struct.
*/
#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_for_each_entry	-	iterate over list of given type
 * @pos:	the type * to use as a loop cursor.
 * @head:	the head for your list.
 * @member:	the name of the list_struct within the struct.
 */
#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))

对程序中for循环的三步分析：

（1），pos = list_entry((head)->next, typeof(*pos), member)

pos相当于循环中返回的循环变量，这里就是返回一个结构体指针。实现过程如下：

函数list_entry():

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/**
* list_entry - get the struct for this entry
* @ptr: the &struct list_head pointer.
* @type: the type of the struct this is embedded in.
* @member: the name of the list_struct within the struct.
*/
#define list_entry(ptr, type, member) \
container_of(ptr, type, member)

/**
 * list_entry - get the struct for this entry
 * @ptr:	the &struct list_head pointer.
 * @type:	the type of the struct this is embedded in.
 * @member:	the name of the list_struct within the struct.
 */
#define list_entry(ptr, type, member) \
	container_of(ptr, type, member)

跟进：container_of这个函数：

这个不做重点分析，这个函数的做用是：它的作用显而易见，那就是根据一个结构体变量中的一个域成员变量的指针来获取指向整个结构体变量的指针。

[cpp] view plain copy

print ?

/**
* 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)); })

/**
 * 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)); })

所以list_entry()的作用为：如上图所示，可以以通过已知的指向member子项的指针，获得整个结构体的指针（地址）

（2）， prefetch(pos->member.next),&pos->member!= (head);

prefetch的含义是告诉cpu那些元素有可能马上就要用到，告诉cpu预取一下，这样可以提高速度，用于预取以提高遍历速度；

&pos->member !=(head) ，这个判断循环条件。

（3）， pos= list_entry(pos->member.next, typeof(*pos), member))

和第（1）实现相似，用于逐项向后（next 方向）移动 pos。