Linux内核链表的浅析和模拟

linux内核的链表设计非常独特。和通常的把数据结构装入链表不同，linux反其道而行之，把链表节点装入数据结构。这样的做法很好地实现了对数据的封装。并且为所有的链表操作提供了统一的接口。简单而高效。关于链表所有操作的函数都在/linux/list.h文件里

PS：由于list.h文件没有署名的注释，民间猜测内核的链表机制很有可能就是Linux Torvalds本人的作品

内核中的链表通常是一个双向循环链表，节点定义如下(linux/types.h)：

struct list_head {
	struct list_head *next, *prev;
};

这种设计方式让链表灵活了很多，比如可以在一个数据结构中插入两个list_head，使该结构同时存在于两个双链队列中，比如：内核中用于内存页面管理的page结构(mm.h)

typedef struct page{
     struct list_head list;
     struct list_head lru;
     ......
}

这种设计方式还有一个最大的好处，内核中对链表中的所有操作（插入，删除，合并，遍历）都以list_head结构为参数，实现了统一的操作接口。

另一个问题随之而来，如何访问到每个节点的数据呢。且看linux精巧的设计：

由于在C中，一个结构中的变量偏移在编译时就被ABI固定下来，利用这一点，内核中的链表就可以通过list_head找到节点的入口，从而访问到节点中各个元素。

通过list.h中的宏list_entry实现：

/**
 * 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)

container_of 定义在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) );})

offsetof定义在stddef.h中

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

返回成员MEMBER在结构TYPE中的偏移

简要解释下：

offsetof中，把0强制转换为结构的初始地址，然后对其成员取地址，所得的值即为该成员相对于结构入口的偏移。size_t是unsigned int类型，转换是为了便于后续计算。

container_of中，typeof是c语言关键字的一个新扩展，返回参数类型。该宏首先将ptr赋值给member类型的指针_mptr，然后用_mptr减去偏移，所得即为该节点的入口地址。

各种链表的操作接口，在list.h中定义，下面简要地罗列：

1、初始化链表：

linux没有链表头，使第一个节点的指针指向自己即完成初始化。

两种方式：1）

#define LIST_HEAD_INIT(name) { &(name), &(name) }
#define LIST_HEAD(name) struct list_head name = LIST_HEAD_INIT(name)

#define INIT_LIST_HEAD(ptr) do { 	(ptr)->next = (ptr); (ptr)->prev = (ptr); } while (0)

static inline int list_empty(const struct list_head *head)
{
		return head->next == head;
}

2、添加节点：

static inline void __list_add(struct list_head *new,
			      struct list_head *prev,
			      struct list_head *next)
{
	next->prev = new;
	new->next = next;
	new->prev = prev;
	prev->next = new;
}

头尾添加，分别如下调用即可：

__list_add(new, head, head->next);
__list_add(new, head->prev, head);

3、删除节点

static inline void __list_del(struct list_head * prev, struct list_head * next)
{
	next->prev = prev;
	prev->next = next;
}

4、搬移节点，把属于一个链表的节点移动到另一个链表

static inline void list_move(struct list_head *list, struct list_head *head);

5 还有一些其他操作，比如合并两个链表：

static inline void __list_splice(const struct list_head *list,
				 struct list_head *prev,
				 struct list_head *next)
{
	struct list_head *first = list->next;
	struct list_head *last = list->prev;

	first->prev = prev;
	prev->next = first;

	last->next = next;
	next->prev = last;
}

/**
 * list_splice - join two lists, this is designed for stacks
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 */
static inline void list_splice(const struct list_head *list,
				struct list_head *head)
{
	if (!list_empty(list))
		__list_splice(list, head, head->next);
}

/**
 * list_for_each	-	iterate over a list
 * @pos:	the &struct list_head to use as a loop cursor.
 * @head:	the head for your list.
 */
#define list_for_each(pos, head) 	for (pos = (head)->next; pos != (head); pos = pos->next)

 * 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);		     &pos->member != (head); 		     pos = list_entry(pos->member.next, typeof(*pos), member))

自己写了一个小程序作为本文结束，运用linux链表的设计思想，在同一链表中链接两种不同数据结构并遍历之。

#include<stdio.h>
#include<stdlib.h>

#define get_entry(ptr, structure, member) 	((structure *)((char *)ptr - (unsigned int)&(((structure *)0) -> member)))

#define init_list_head(p) do {	(p) -> prev = (p); (p) -> next = (p); }while(0)

typedef struct link_pointer_tag
{
	struct link_pointer_tag *prev;
	struct link_pointer_tag *next;
}link_pointer;

typedef struct 
{
	int element1;
	link_pointer p;
}link_node1;

typedef struct 
{
	char element2;
	link_pointer p;
}link_node2;

link_pointer* head;

link_pointer* create_node1()
{
	int ele;
	link_node1* tmp_node1;
	if ((tmp_node1 = (link_node1*)malloc(sizeof(link_node1))) == NULL)
	{
		printf("Not enough memery!\n");
		exit(0);
	}	
	printf("Input the element of node: element1(int)\n");
	scanf("%d", &ele);
	tmp_node1 -> element1 = ele;
	return &(tmp_node1 -> p);
}

link_pointer* create_node2()
{
	char ele;
	link_node2* tmp_node2;
	if ((tmp_node2 = (link_node2*)malloc(sizeof(link_node2))) == NULL)
	{
		printf("Not enough memery!\n");
		exit(0);
	}	
	printf("Input the element of node: element2(char)\n");
	scanf("%c", &ele);
	tmp_node2 -> element2 = ele;
	return &(tmp_node2 -> p);
}

void add_node(link_pointer* ptr, link_pointer* pre, link_pointer* next)
{
	pre -> next = ptr;
	ptr -> prev = pre;
	ptr -> next = next;
	next -> prev = ptr;
}

void create_list()
{
	int lenth, i;
	head = NULL;
	printf("Input lenth:");
	scanf("%d", &lenth);
	for (i = 1; i <= lenth; i++)	
	{
		setbuf(stdin,NULL);
		printf("The number of node %d:\n", i);
		if(i % 2 != 0)			
		{
			if(i == 1)
			{
				head = create_node1();
				init_list_head(head);
			}
			else 
			{	
				add_node(create_node1(), head -> prev, head);
			}
		}
		else
		{	
			add_node(create_node2(), head -> prev, head);
		}
	}
}

void show_all_node()
{
	link_pointer* tmp = head;
	int count = 1;
	do
	{
		printf("The %dth node: ", count);
		if (count % 2 != 0)
			printf("%d\n", get_entry(tmp, link_node1, p) -> element1);
		if (count % 2 == 0)
			printf("%c\n", get_entry(tmp, link_node2, p) -> element2);
		count++;
	} while ((tmp = tmp -> next) != head);
}

int main()
{
	create_list();
	show_all_node();
	return 0;
}

Linux内核链表的浅析和模拟,古老的榕树,5-wow.com