Linux内核链表基础

1、内核链表的定义在include/linux/list.h
struct list_head {
struct list_head *next, *prev;
};
容易看出，Linux内核链表为双向链表。

2、Linux链表与普通链表区别
我们通常定义的链表是在链表节点中嵌入元素,比如
struct MyList
{
int StudentID; /* 被嵌入的元素 */
struct MyList *prev;
struct MyList *next;
}
而LInux为了移植方便性和通用性，在元素结构体中嵌入链表节点
strcut MyList
{
int StudentID;
struct list_head head; /* 链表节点作为结构体元素 */
}

3、Linux内核链表中提供的操作链表函数
(1)初始化

static inline void INIT_LIST_HEAD(struct list_head *list)
{
list->next = list; /* 下一个节点指向自己 */
list->prev = list; /* 前一个节点指向自己 */
}
(2)添加链表节点

/**
* 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); /* 节点插入到head和head->next之间 */
}
而__list_add函数如下
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;
}
(3)删除节点
方法一：
/**
* 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 = (void *)0xDEADBEEF; /* 将指针指向2个不可访问的位置 */
entry->prev = (void *)0xBEEFDEAD;
}
其中调用的__list_del函数如下，
static inline void __list_del(struct list_head *prev, struct list_head *next)
{
next->prev = prev; /* */
prev->next = next;
}
注意list_del函数中的最后两条语句，类似于free()的作用。
当用户打算访问地址0xDEADBEEF或0xBEEFDEAD时，将产生页中断。

方法二：
为了更安全的删除节点，可使用list_del_init
/**
* list_del_init - deletes entry from list and reinitialize it.
* @entry: the element to delete from the list.
*/
static inline void list_del_init(struct list_head *entry)
{
__list_del(entry->prev, entry->next);
INIT_LIST_HEAD(entry);
}

(4)提取结构的数据信息
按通常的方式使用链表很容易获取数据信息，
但使用Linux内核链表要访问数据则比较困难，
关键是如何求取链表节点地址和数据地址的偏移量。
注意list_entry传递的参数！type指传递的是类型，不是变量。
/**
* 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include/linux/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)); })
而其中，
[a]#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
[b]typeof()是gcc的扩展，和sizeof()类似

(5)、遍历链表
/**
* 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; prefetch(pos->next), pos != (head); \
pos = pos->next)
遍历链表也是一个宏定义，且是一个没加{}的for循环，
因此调用此函数时一定要加上{}。
使用时注意参数pos是指针类型，我们要定义一个list_head类型的
结构体变量指针传送给这个函数。

4、链表使用实例

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/slab.h>
#include <linux/list.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("monkeyzx");
MODULE_DESCRIPTION("Mylist Module");
MODULE_VERSION("V1.0");

struct student
{
	char name[100];
	int id;
	struct list_head list;
};

struct student *pstudent;
struct student *tmp_student;
struct list_head student_list;
struct list_head *pos;

int __init mylist_init()
{
	int i = 0;

	INIT_LIST_HEAD(&student_list);

	pstudent = kmalloc(sizeof(struct student)*5, GFP_KERNEL);
	memset(pstudent, 0, sizeof(struct student)*5);

	for(i=0; i<5; i++)
	{
		sprintf(pstudent[i].name, "Student %d", i+1);
		pstudent[i].id = i + 1;
		list_add(&(pstudent[i].list), &student_list);
	}

	list_for_each(pos, &student_list)
	{
		tmp_student = list_entry(pos, struct student, list);
		printk("<0>student %d name: %s\n", tmp_student->id, tmp_student->name);

	}
}

void __exit mylist_exit()
{
	int i = 0;

	for(i=0; i<5; i++)
	{
		list_del(&(pstudent[i].list));
	}

	kfree(pstudent);
}

module_init(mylist_init);
module_exit(mylist_exit);

makefile文件如下

#######################################################################################

ifneq ($(KERNELRELEASE),)

obj-m := mylist.o

else
	
KDIR := /lib/modules/2.6.18-53.el5/build
all:
	make -C $(KDIR) M=$(PWD) modules
clean:
	rm -f *.ko *.o *.mod.o *.mod.c *.symvers

endif

########################################################################################