玩转C链表
链表是C语言编程中常用的数据结构，比如我们要建一个整数链表，一般可能这么定义：
view sourceprint?1 struct int_node {
2 int val;
3 struct int_node *next;
4 };
为了实现链表的插入、删除、遍历等功能，另外要再实现一系列函数，比如：
view sourceprint?01 void insert_node(struct int_node *head, struct int_node *current);
02
03 void delete_node(struct int_node *head, struct int_node *current);
04
05 void access_node(struct int_node *head)
06 {
07 struct int_node *node;
08 for (node = head; node != NULL; node = node->next) {
09 // do something here
10 }
11 }
如果我们的代码里只有这么一个数据结构的话，这样做当然没有问题，但是当代码的规模足够大，需要管理很多种链表，难道需要为每一种链表都要实现一套插入、删除、遍历等功能函数吗？
熟悉C++的同学可能会说，我们可以用标准模板库啊，但是，我们这里谈的是C，在C语言里有没有比较好的方法呢？
Mr.Dave在他的博客里介绍了自己的实现，这个实现是个很好的方案，各位不妨可以参考一下。在本文中，我们把目光投向当今开源界最大的C项目--Linux Kernel，看看Linux内核如何解决这个问题。
Linux内核中一般使用双向链表，声明为struct list_head，这个结构体是在include/linux/types.h中定义的，链表的访问是以宏或者内联函数的形式在include/linux/list.h中定义。
view sourceprint?1 struct list_head {
2 struct list_head *next, *prev;
3 };
Linux内核为链表提供了一致的访问接口。
view sourceprint?1 void INIT_LIST_HEAD(struct list_head *list)；
2 void list_add(struct list_head *new, struct list_head *head)；
3 void list_add_tail(struct list_head *new, struct list_head *head)；
4 void list_del(struct list_head *entry);
5 int list_empty(const struct list_head *head)；
以上只是从Linux内核里摘选的几个常用接口，更多的定义请参考Linux内核源代码。
我们先通过一个简单的实作来对Linux内核如何处理链表建立一个感性的认识。
view sourceprint?01 #include <stdio.h>
02 #include "list.h"
03
04 struct int_node {
05 int val;
06 struct list_head list;
07 };
08
09 int main()
10 {
11 struct list_head head, *plist;
12 struct int_node a, b;
13
14 a.val = 2;
15 b.val = 3;
16
17 INIT_LIST_HEAD(&head);
18 list_add(&a.list, &head);
19 list_add(&b.list, &head);
20
21 list_for_each(plist, &head) {
22 struct int_node *node = list_entry(plist, struct int_node, list);
23 printf("val = %d\n", node->val);
24 }
25
26 return 0;
27 }
看完这个实作，是不是觉得在C代码里管理一个链表也很简单呢？
代码中包含的头文件list.h是我从Linux内核里抽取出来并做了一点修改的链表处理代码，现附在这里给大家参考，使用的时候只要把这个头文件包含到自己的工程里即可。
代码
#ifndef __C_LIST_H#define __C_LIST_Htypedef unsigned char u8;typedef unsigned short u16;typedef unsigned int u32;typedef unsigned long size_t;#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->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) (type *)((char *)ptr -offsetof(type,member))/* * These are non-NULL pointers that will result in page faults * under normal circumstances, used to verify that nobody uses * non-initialized list entries. */#define LIST_POISON1 ((void *) 0x00100100)#define LIST_POISON2 ((void *) 0x00200200)struct list_head { struct list_head *next, *prev;};/** * 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) #define LIST_HEAD_INIT(name) { &(name), &(name) }#define LIST_HEAD(name) \ struct list_head name = LIST_HEAD_INIT(name)static inline void INIT_LIST_HEAD(struct list_head *list){ list->next = list; list->prev = list;}/** * list_for_each - iterate over a list * @pos: the &struct list_head to use as a loop counter. * @head: the head for your list. */#define list_for_each(pos, head) \ for (pos = (head)->next; pos != (head); pos = pos->next)/** * list_for_each_r - iterate over a list reversely * @pos: the &struct list_head to use as a loop counter. * @head: the head for your list. */#define list_for_each_r(pos, head) \ for (pos = (head)->prev; pos != (head); pos = pos->prev) /* * Insert a new entry between two known consecutive entries. * * This is only for internal list manipulation where we know * the prev/next entries already! */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;}/** * list_add - add a new entry * @new: new entry to be added * @head: list head to add it after * * Insert a new entry after the specified head. * This is good for implementing stacks. */static inline void list_add(struct list_head *new, struct list_head *head){ __list_add(new, head, head->next);}/** * list_add_tail - add a new entry * @new: new entry to be added * @head: list head to add it before * * Insert a new entry before the specified head. * This is useful for implementing queues. */static inline void list_add_tail(struct list_head *new, struct list_head *head){ __list_add(new, head->prev, head);}/* * Delete a list entry by making the prev/next entries * point to each other. * * This is only for internal list manipulation where we know * the prev/next entries already! */static inline void __list_del(struct list_head * prev, struct list_head * next){ next->prev = prev; prev->next = next;}/** * list_del - deletes entry from list. * @entry: the element to delete from the list. * Note: list_empty on entry does not return true after this, the entry is * in an undefined state. */static inline void list_del(struct list_head *entry){ __list_del(entry->prev, entry->next); entry->next = LIST_POISON1; entry->prev = LIST_POISON2;}/** * list_empty - tests whether a list is empty * @head: the list to test. */static inline int list_empty(const struct list_head *head){ return head->next == head;}static inline void __list_splice(struct list_head *list, struct list_head *head){ struct list_head *first = list->next; struct list_head *last = list->prev; struct list_head *at = head->next; first->prev = head; head->next = first; last->next = at; at->prev = last;}/** * list_splice - join two lists * @list: the new list to add. * @head: the place to add it in the first list. */static inline void list_splice(struct list_head *list, struct list_head *head){ if (!list_empty(list)) __list_splice(list, head);}#endif // __C_LIST_H
list_head通常是嵌在数据结构内使用，在上文的实作中我们还是以整数链表为例，int_node的定义如下：
view sourceprint?1 struct int_node {
2 int val;
3 struct list_head list;
4 };
使用list_head组织的链表的结构如下图所示：
遍历链表是用宏list_for_each来完成。
view sourceprint?1 #define list_for_each(pos, head) \
2 for (pos = (head)->next; prefetch(pos->next), pos != (head); \
3 pos = pos->next)
在这里，pos和head均是struct list_head。在遍历的过程中如果需要访问节点，可以用list_entry来取得这个节点的基址。
view sourceprint?1 #define list_entry(ptr, type, member) \
2 container_of(ptr, type, member)
我们来看看container_of是如何实现的。如下图所示，我们已经知道TYPE结构中MEMBER的地址，如果要得到这个结构体的地址，只需要知道MEMBER在结构体中的偏移量就可以了。如何得到这个偏移量地址呢？这里用到C语言的一个小技巧，我们不妨把结构体投影到地址为0的地方，那么成员的绝对地址就是偏移量。得到偏移量之后，再根据ptr指针指向的地址，就可以很容易的计算出结构体的地址。
list_entry就是通过上面的方法从ptr指针得到我们需要的type结构体。
Linux内核代码博大精深，陈莉君老师曾把它形容为“覆压三百余里，隔离天日”（摘自《阿房宫赋》），可见其内容之丰富、结构之庞杂。内核里有着众多重要的数据结构，具有相关性的数据结构之间很多都是用本文介绍的链表组织在一起，看来list_head结构虽小，作用可真不小。
Linux内核是个伟大的工程，其源代码里还有很多精妙之处，值得C/C++程序员认真去阅读，即使我们不去做内核相关的工作，阅读精彩的代码对程序员自我修养的提高也是大有裨益的。 |
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