链表结构，结合Linux内核中 list_head 常见使用方法

list_head 定义

list_head 结构体定义，kernel/inclue/linux/types.h 如下：

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

​ 然后就开始围绕这个结构开始构建链表，然后插入、删除节点 ，遍历整个链表等等，其实内核已经提供好了现成的接口，接下来就让我们进入kernel/include/linux/list.h中：

一. 创建链表

内核提供了下面的这些接口来初始化链表：

 #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)

{
    WRITE_ONCE(list->next, list);
    list->prev = list;
}

所以可以通过这个方法来初始化一个链表，如下是将listhead作为一个结构体成员存放了，为什么要这么做呢？后面在实际的例子应用中会使用到

struct data_list {

    int data_len;

    char buf[64];

    struct list_head list_node;

};

struct data_list * iic_data_head;

static void init_data_head(void*) {

    iic_data_head = malloc(sizeof(struct data_list ));
    if (!iic_data_head) {

        KLOGD("no mem\n");

        return -ENOMEM;

    }
    memset(iic_data_head, 0x0, sizeof(struct data_list ));
    INIT_LIST_HEAD(&iic_data_head->**list_node**);
}

二. 添加节点

内核已经提供了添加节点的接口了

1. list_add

如下所示。 根据注释可知，是在链表头head后方插入一个新节点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再调用__list_add接口

/*
 * Insert a new entry between two known consecutive entries.
 *
 * This is only for internal list manipulation where we know
 * the prev/next entries already!
 */
#ifndef CONFIG_DEBUG_LIST
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;
}
#else
extern void __list_add(struct list_head *new,
                  struct list_head *prev,
                  struct list_head *next);
#endif

其实就是在head 链表头后和链表头后第一个节点之间插入一个新节点。然后这个新的节点就变成了链表头后的第一个节点了。

2. list_add_tail 接口

上面所讲的list_add接口是从链表头header后添加的节点。 同样，内核也提供了从链表尾处向前添加节点的接口list_add_tail. 让我们来看一下它的具体实现。

/**
 * 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);
}

list_add_tail 也是调用了 __list_add ，不过传递的参数同list_add 方法不一样，

所以，很清楚明了， list_add_tail就相当于在链表头前方依次插入新的节点（也可理解为在链表尾部开始插入节点，此时，header节点既是为节点，保持不变）

三. 删除节点

内核同样在list.h文件中提供了删除节点的接口 list_del()， 让我们看一下它的实现流程

/**
 * 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.
 */
#ifndef CONFIG_DEBUG_LIST
static inline void __list_del_entry(struct list_head *entry)
{
    __list_del(entry->prev, entry->next);
}

static inline void list_del(struct list_head *entry)
{
    __list_del(entry->prev, entry->next);
    entry->next = LIST_POISON1;
    entry->prev = LIST_POISON2;
}
#else
extern void __list_del_entry(struct list_head *entry);
extern void list_del(struct list_head *entry);
#endif

利用list_del(struct list_head *entry) 接口就可以删除链表中的任意节点了，但需注意，前提条件是这个节点是已知的，既在链表中真实存在，切prev，next指针都不为NULL。

四. 链表遍历

内核是同过下面这些宏定义来完成对list_head链表进行遍历的，如下 :

/**
 * 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)

/**
 * list_for_each_prev   -   iterate over a list backwards
 * @pos:    the &struct list_head to use as a loop cursor.
 * @head:   the head for your list.
 */
#define list_for_each_prev(pos, head) \
    for (pos = (head)->prev; pos != (head); pos = pos->prev)

/**
 * list_for_each_safe - iterate over a list safe against removal of list entry
 * @pos:    the &struct list_head to use as a loop cursor.
 * @n:      another &struct list_head to use as temporary storage
 * @head:   the head for your list.
 */
#define list_for_each_safe(pos, n, head) \
    for (pos = (head)->next, n = pos->next; pos != (head); \
        pos = n, n = pos->next)

/**
 * list_for_each_prev_safe - iterate over a list backwards safe against removal of list entry
 * @pos:    the &struct list_head to use as a loop cursor.
 * @n:      another &struct list_head to use as temporary storage
 * @head:   the head for your list.
 */
#define list_for_each_prev_safe(pos, n, head) \
    for (pos = (head)->prev, n = pos->prev; \
         pos != (head); \
         pos = n, n = pos->prev)

而且，list.h 中也提供了list_replace( 节点替换) list_move(节点移位) ，翻转，查找等接口，有需要可以往对应源码中查看。

五. 宿主结构

1.找出宿主结构 list_entry(ptr, type, member)

上面的所有操作都是基于list_head这个链表进行的，涉及的结构体也都是:

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

如前面一开始创建链表的时候定义的结构体 data_list 我们这里把它叫做list_head的宿主结构体

struct data_list{
    int data_len;
    char buf[64];
    struct list_head list_node;
};

list.h中提供了list_entry宏来实现对应地址的转换，但最终还是调用了container_of宏

 /**
 * 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_head within the struct.
 */

 #define list_entry(ptr, type, member) \
     container_of(ptr, type, 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) ({     \ 
    const typeof( ((type *)0)->member ) *__mptr = (ptr); \ 
    (type *)( (char *)__mptr - offsetof(type,member) );}) 

而offsetof定义在 kernel/include/linux/stddef.h ，如下：

#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

看下container_of宏的注释：

（1）根据结构体重的一个成员变量地址导出包含这个成员变量mem的struct地址。

（2）参数解释：

​ ptr ： 成员变量mem的地址

​ type： 包含成员变量mem的宿主结构体的类型

​ member： 在宿主结构中的mem成员变量的名称

Demo：

struct data_list {

int data_len;

char buf[64];

struct list_head list_node;

};

struct data_list* test_list;

container_of(&test_list->list_node, struct data_list, list_node); 这个调用返回的是 test_list 的指针

//ptr：&test_list->list_node

//type：struct data_list

//member：list_node

2. 宿主结构的遍历

​ 因为我们可以根据结构体中成员变量的地址找到宿主结构的地址， 并且我们可以对成员变量所建立的链表进行遍历，那我们是不是也可以通过某种方法对宿主结构进行遍历呢？

​ 其实内核中有很多这种用法，ex:Asoc中的control.c， 内核在list.h中提供了下面的宏：

/**
 * list_for_each_entry  -   iterate over list of given type
 * @pos:    the type * to use as a loop cursor.
 * @head:   the head for your list.
 * @member: the name of the list_struct within the struct.
 */
#define list_for_each_entry(pos, head, member)              \
    for (pos = list_first_entry(head, typeof(*pos), member);    \
         &pos->member != (head);                    \
         pos = list_next_entry(pos, member))

/**
 * list_for_each_entry_reverse - iterate backwards over list of given type.
 * @pos:    the type * to use as a loop cursor.
 * @head:   the head for your list.
 * @member: the name of the list_struct within the struct.
 */
#define list_for_each_entry_reverse(pos, head, member)          \
    for (pos = list_last_entry(head, typeof(*pos), member);     \
         &pos->member != (head);                    \
         pos = list_prev_entry(pos, member))

其中，list_first_entry 和 list_next_entry宏都定义在list.h中，分别代表：获取第一个真正的宿主结构的地址； 获取下一个宿主结构的地址。它们的实现都是利用list_entry宏。

/**
 * list_first_entry - get the first element from a list
 * @ptr:    the list head to take the element from.
 * @type:   the type of the struct this is embedded in.
 * @member: the name of the list_struct within the struct.
 *
 * Note, that list is expected to be not empty.
 */
#define list_first_entry(ptr, type, member) \
    list_entry((ptr)->next, type, member)
    
/**
 * list_next_entry - get the next element in list
 * @pos:    the type * to cursor
 * @member: the name of the list_struct within the struct.
 */
#define list_next_entry(pos, member) \
    list_entry((pos)->member.next, typeof(*(pos)), member)

最终使用 for 循环实现了宿主结构的遍历

首先pos定位到第一个宿主结构地址，然后循环获取下一个宿主结构地址，如果查到宿主结构中的member成员变量（宿主结构中struct list_head定义的字段）地址为head，则退出，从而实现了宿主结构的遍历。如果要循环对宿主结构中的其它成员变量进行操作，这个遍历操作就显得特别有意义了。

List_head 实现队列用法

这是个demo

static struct my_data_list * iic_data_head;
static int my_queue_data_lists(u8* data, int data_len)
{
    struct my_data_list * list_node_data = NULL;
    int ret = 0;
    if (data==NULL || data_len<=0) {
        KLOGD("error data to add to queue");
        ret = -EINVAL;
    }

    list_node_data = malloc(sizeof(struct my_data_list));
    list_node_data->data_buff.data = malloc(data_BUFFER_SIZE);

    if (list_node_data) {
        list_node_data->list_node.next = NULL;
        list_node_data->list_node.prev = NULL;
        
        pthread_mutex_lock(&queue_mutex);
        list_node_data->data_buff.len = data_len;
        memcpy(list_node_data->data_buff.data, data, data_len);
        list_add_tail(&list_node_data->list_node, &iic_data_head->list_node);

        KLOGD("Add list_node:%p value:%s len:%d cmd:0x%02x%02x\n",

           iic_data_head->list_node.prev,

           list_node_data->data_buff.data, list_node_data->data_buff.len,

           data[data_CMD_H_INDEX], data[data_CMD_L_INDEX]);

        pthread_mutex_unlock(&queue_mutex);
    } else {
        ret = -ENOMEM;
    }
    return ret;
}

/**

\* 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) );})

static int my_dequeue_data_lists(u8* value)

{

    struct list_head* list_node;
    struct my_data_list* list_node_data;
    int value_len = 0;

    // mutex_lock(&iic_data_head->read_mutex);

    pthread_mutex_lock(&queue_mutex);
    if (!list_empty(&iic_data_head->list_node))
    {
        list_node = iic_data_head->list_node.next;
        list_node_data = container_of(list_node,struct my_data_list, list_node);
        value_len = list_node_data->data_buff.len;
        memcpy(value, list_node_data->data_buff.data, value_len);
        KLOGD("Out list_node:%p value:%s len:%d cmd:0x%02x%02x\n",
           list_node, list_node_data->data_buff.data,
           list_node_data->data_buff.len,
           value[data_CMD_H_INDEX], value[data_CMD_L_INDEX]);
        list_del(list_node);
        free(list_node_data->data_buff.data);
        free(list_node_data);
    }
    // mutex_unlock(&iic_data_head->read_mutex);
    pthread_mutex_unlock(&queue_mutex);
    return value_len;

}

static int my_init_list_head(void)

{
    iic_data_head = malloc(sizeof(struct my_data_list));
    memset(iic_data_head, 0x0, sizeof(struct my_data_list));

    if (!iic_data_head) {
        KLOGD("no mem\n");
        return -ENOMEM;
    }

    INIT_LIST_HEAD(&iic_data_head->list_node);
    return 0;
}