list_head用法(一)

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 inline

struct 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

#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。

参考资料：

http://blog.csdn.net/king_208/article/details/5524175
http://www.cnblogs.com/wwang/archive/2010/11/28/1889281.html