Linux--内核链表

Posted 2020-10-13

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(struct list_head *new, struct list_head *head)

　　// new 的节点被插入到 head（head为任意链表头） 的后面

/**
 * 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;
}

list_add_tail(struct list_head *new, struct list_head *head)
  new 的节点插入到 head 的前面

/**
 * 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);
}

 

(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(ptr, type, member)  // list_head的指针 ， 外部结构的数据类型（struct statu），成员名
　　　　　　　　　　　　　　　　　　　//返回指向 data 的指针

/**
 * 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)); })

（5）链表的遍历

/**
 * 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)

举一个栗子：

struct list_head *entry;

struct list_head dev;  //链表头

list_for_each(entry, dev)

{

card = list_entry(entry, struct list_head, card);
if(card->dev_midi == minor)
break;
}