list_head用法(一)
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list_head用法经常在Linux的kernel里面经常看见,所以记录用法以来备忘
(1)list.h文件在include/linux/list.h
#ifndef _LIST_H#define _LIST_H#define _INLINE_ static inlinestruct list_head { struct list_head *next, *prev;};#define LIST_HEAD_INIT(name) {&(name), &(name)} #define LIST_HEAD(name) \ //定义并初始化头结点head struct list_head name = LIST_HEAD_INIT(name)#define INIT_LIST_HEAD(ptr) do {\ //初始化头结点ptr,因此需要首先定义ptr (ptr)->next = (ptr); (ptr)->prev = (ptr); \} while (0)_INLINE_ void __list_add(struct list_head *add, struct list_head *prev, struct list_head *next){ next->prev = add; add->next = next; add->prev = prev; prev->next = add;}_INLINE_ void list_add(struct list_head *add, struct list_head *head)//每次添加节点到head之后,始终都是添加到头结点之后{ __list_add(add, head, head->next);}_INLINE_ void list_add_tail(struct list_head *add, struct list_head *head)//每次添加节点都是头结点之前,由于是循环链表,就是说添加到链表尾部{ __list_add(add, head->prev, head);}_INLINE_ void __list_del(struct list_head *prev, struct list_head *next){ next->prev = prev; prev->next = next;}_INLINE_ void list_del(struct list_head *entry)//删除节点{ __list_del(entry->prev, entry->next);}_INLINE_ void list_del_init(struct list_head *entry)//删除节点,并初始化被删除的结点(也就是使被删除的结点的prev和next都指向自己){ __list_del(entry->prev, entry->next); INIT_LIST_HEAD(entry);}_INLINE_ int list_empty(struct list_head *head)//判断链表是否为空{ return head->next == head;}_INLINE_ void list_splice(struct list_head *list, struct list_head *head)//通过两个链表的head,进行连接{ struct list_head *first = list->next; if (first != list) { struct list_head *last = list->prev; struct list_head *at = head->next; first->prev = head; head->next = first; last->next = at; at->prev = last; }}#define list_entry(ptr, type, member) \ ((type *)((char *)(ptr) - (unsigned long)(&((type *)0)->member)))#define list_for_each(pos, head) \ //遍历链表,此时删除节点的操作可能会出错 for (pos = (head)->next; pos != (head); pos = pos->next) //新代码中出现prefetch() 可以不考虑,用于预取以提高遍历速度#define list_for_each_safe(pos, pnext, head) \ //遍历链表,可以同时有删除节点的操作 for (pos = (head)->next, pnext = pos->next; pos != (head); \ pos = pnext, pnext = pos->next)#undef _INLINE_#endif(2)写一个测试程序,利用各个函数实现功能点击(此处)折叠或打开#include<stdio.h>#include<stdlib.h>#include "list.h"struct int_node{ int val; struct list_head list;};int main(){ struct list_head head,*plist; struct int_node a,b,c; a.val = 1; b.val = 2; c.val = 3; INIT_LIST_HEAD(&head); //初始化链表头 list_add_tail(&a.list,&head); //添加节点 list_add_tail(&b.list,&head); list_add_tail(&c.list,&head); list_for_each(plist,&head)//遍历链表,打印结果 { struct int_node *node = list_entry(plist,struct int_node,list);//然后取得数据项,因此一般来说和list_for_each配合使用 printf("val = %d\n",node->val); }//print 1 2 3 printf("*******************************************\n"); list_del_init(&b.list); //删除节点b list_for_each(plist,&head) //重新遍历链表,打印结果 { struct int_node *node = list_entry(plist,struct int_node,list); printf("val = %d\n",node->val); }//print 1 3 printf("*******************************************\n"); struct int_node d,e; struct list_head head1; d.val = 4; e.val = 5; INIT_LIST_HEAD(&head1); //重新建立链表,表头为head1 list_add_tail(&d.list,&head1); list_add_tail(&e.list,&head1); list_splice(&head1,&head); //把两个链表进行连接 list_for_each(plist,&head) { struct int_node *node = list_entry(plist,struct int_node,list); printf("val = %d\n",node->val); }//print 4 5 1 3 printf("*******************************************\n"); if(!list_empty(&head)) //判断链表是否为空 { printf("the list is not empty!\n"); } return 0;}
(2)写一个测试程序,利用各个函数实现功能
#include<stdio.h>#include<stdlib.h>#include "list.h"struct int_node{ int val; struct list_head list;};int main(){ struct list_head head,*plist; struct int_node a,b,c; a.val = 1; b.val = 2; c.val = 3; INIT_LIST_HEAD(&head); //初始化链表头 list_add_tail(&a.list,&head); //添加节点 list_add_tail(&b.list,&head); list_add_tail(&c.list,&head); list_for_each(plist,&head)//遍历链表,打印结果 { struct int_node *node = list_entry(plist,struct int_node,list);//然后取得数据项,因此一般来说和list_for_each配合使用 printf("val = %d\n",node->val); }//print 1 2 3 printf("*******************************************\n"); list_del_init(&b.list); //删除节点b list_for_each(plist,&head) //重新遍历链表,打印结果 { struct int_node *node = list_entry(plist,struct int_node,list); printf("val = %d\n",node->val); }//print 1 3 printf("*******************************************\n"); struct int_node d,e; struct list_head head1; d.val = 4; e.val = 5; INIT_LIST_HEAD(&head1); //重新建立链表,表头为head1 list_add_tail(&d.list,&head1); list_add_tail(&e.list,&head1); list_splice(&head1,&head); //把两个链表进行连接 list_for_each(plist,&head) { struct int_node *node = list_entry(plist,struct int_node,list); printf("val = %d\n",node->val); }//print 4 5 1 3 printf("*******************************************\n"); if(!list_empty(&head)) //判断链表是否为空 { printf("the list is not empty!\n"); } return 0;}
输出结果如下:
val = 1
val = 2
val = 3
val = 1
val = 3
val = 4
val = 5
val = 1
val = 3
the list is not empty!
(3)list_for_each()与list_for_each_safe()
#define list_for_each(pos, head) \for (pos = (head)->next; prefetch(pos->next), pos != (head); \pos = pos->next)
由定义可知,list_del(pos)(将pos的前后指针指向undefined state)panic,list_del_init(pos)(将pos前后指针指向自身)导致死循环。
#define list_for_each_safe(pos, n, head) \for (pos = (head)->next, n = pos->next; pos != (head); \pos = n, n = pos->next)
由定义可知,safe函数首先将pos的后指针缓存到n,处理一个流程后再赋回pos,避免了这种情况的发生。
因此只遍历链表不删除节点时可以使用前者,若有删除节点的操作,则要使用后者。
由safe的说明可知,是专门为删除节点时准备的:iterate over a list safe against removal of list entry。其他带safe的处理也基本源于这个原因
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