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：

struct int_node {        int val;        struct int_node *next;};

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：

void insert_node(struct int_node **head, int val); void delete_node(struct int_node *head, struct int_node *current); void access_node(struct int_node *head){        struct int_node *node;        for (node = head; node != NULL; node = node->next) {                // do something here        }}

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？

在他的博客里介绍了自己的，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--，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。

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

Linux内核为链表提供了一致的访问接口:

void INIT_LIST_HEAD(struct list_head *list)；void list_add(struct list_head *new, struct list_head *head)；void list_add_tail(struct list_head *new, struct list_head *head)；void list_del(struct list_head *entry);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。

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。

代码中包含的头文件list.h是从Linux内核里抽取出来并做了一点修改的链表处理代码，使用的时候只要把这个头文件包含进来即可。

list.h:

#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

测试代码main.c

#include 
    
     #include "list.h" struct int_node {        int val;        struct list_head list;}; int main(){        struct list_head head, *plist;        struct int_node a, b;         a.val = 2;        b.val = 3;         INIT_LIST_HEAD(&head);        list_add(&a.list, &head);        list_add(&b.list, &head);         list_for_each(plist, &head) {                struct int_node *node = list_entry(plist, struct int_node, list);                printf("val = %d\n", node->val);        }         return 0;}
    

编译运行：

$ lslist.h  main.c$ gcc main.c$ ./a.out val = 3val = 2

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：

struct int_node {        int val;        struct list_head list;};

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。

#define list_for_each(pos, head) \    for (pos = (head)->next; prefetch(pos->next), pos != (head); \            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。

#define list_entry(ptr, type, member) \    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，老师曾把它形容为“覆压三百余里，隔离天日”（摘自《》），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。