【转】玩转C链表
转自博客园，源地址：http://www.cnblogs.com/wwang/archive/2010/11/28/1889281.html
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：
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struct int_node { |
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int val; |
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struct int_node *next; |
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}; |
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：
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void insert_node(struct int_node **head, int val); |
02 |
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void delete_node(struct int_node *head, struct int_node *current); |
04 |
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void access_node(struct int_node *head) |
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{ |
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struct int_node *node; |
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for (node = head; node != NULL; node = node->next) { |
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// do something here |
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} |
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} |
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。
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struct list_head { |
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struct list_head *next, *prev; |
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}; |
Linux内核为链表提供了一致的访问接口。
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void INIT_LIST_HEAD(struct list_head *list)； |
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void list_add(struct list_head *new, struct list_head *head)； |
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void list_add_tail(struct list_head *new, struct list_head *head)； |
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void list_del(struct list_head *entry); |
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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。
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#include <stdio.h> |
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#include "list.h" |
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struct int_node { |
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int val; |
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struct list_head list; |
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}; |
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int main() |
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{ |
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struct list_head head, *plist; |
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struct int_node a, b; |
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a.val = 2; |
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b.val = 3; |
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INIT_LIST_HEAD(&head); |
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list_add(&a.list, &head); |
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list_add(&b.list, &head); |
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list_for_each(plist, &head) { |
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struct int_node *node = list_entry(plist, struct int_node, list); |
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printf("val = %d\n", node->val); |
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} |
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return 0; |
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} |
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_H
typedef 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
#define __C_LIST_H
typedef 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：
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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。
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#define list_for_each(pos, head) \ |
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for (pos = (head)->next; prefetch(pos->next), pos != (head); \ |
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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。
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#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。
