鏈表結(jié)構(gòu)告抄，結(jié)合Linux內(nèi)核中 list_head 常見使用方法

list_head 定義

list_head 結(jié)構(gòu)體定義肝集，kernel/inclue/linux/types.h 如下：

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

? 然后就開始圍繞這個結(jié)構(gòu)開始構(gòu)建鏈表，然后插入、刪除節(jié)點(diǎn) ，遍歷整個鏈表等等，其實(shí)內(nèi)核已經(jīng)提供好了現(xiàn)成的接口含末，接下來就讓我們進(jìn)入kernel/include/linux/list.h中：

一. 創(chuàng)建鏈表

內(nèi)核提供了下面的這些接口來初始化鏈表：

 #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;
}

所以可以通過這個方法來初始化一個鏈表，如下是將listhead作為一個結(jié)構(gòu)體成員存放了即舌，為什么要這么做呢佣盒？后面在實(shí)際的例子應(yīng)用中會使用到

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. 添加節(jié)點(diǎn)

內(nèi)核已經(jīng)提供了添加節(jié)點(diǎn)的接口了

1. list_add

如下所示。 根據(jù)注釋可知侥涵，是在鏈表頭head后方插入一個新節(jié)點(diǎn)new沼撕。

/**
 * 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再調(diào)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

其實(shí)就是在head 鏈表頭后和鏈表頭后第一個節(jié)點(diǎn)之間插入一個新節(jié)點(diǎn)宋雏。然后這個新的節(jié)點(diǎn)就變成了鏈表頭后的第一個節(jié)點(diǎn)了。

2. list_add_tail 接口

上面所講的list_add接口是從鏈表頭header后添加的節(jié)點(diǎn)务豺。 同樣磨总，內(nèi)核也提供了從鏈表尾處向前添加節(jié)點(diǎn)的接口list_add_tail. 讓我們來看一下它的具體實(shí)現(xiàn)。

/**
 * 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 也是調(diào)用了 __list_add 笼沥，不過傳遞的參數(shù)同list_add 方法不一樣蚪燕，

所以，很清楚明了奔浅， list_add_tail就相當(dāng)于在鏈表頭前方依次插入新的節(jié)點(diǎn)（也可理解為在鏈表尾部開始插入節(jié)點(diǎn)馆纳，此時，header節(jié)點(diǎn)既是為節(jié)點(diǎn)汹桦，保持不變）

三. 刪除節(jié)點(diǎn)

內(nèi)核同樣在list.h文件中提供了刪除節(jié)點(diǎn)的接口 list_del()鲁驶， 讓我們看一下它的實(shí)現(xiàn)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) 接口就可以刪除鏈表中的任意節(jié)點(diǎn)了，但需注意舞骆，前提條件是這個節(jié)點(diǎn)是已知的钥弯，既在鏈表中真實(shí)存在，切prev督禽，next指針都不為NULL脆霎。

四. 鏈表遍歷

內(nèi)核是同過下面這些宏定義來完成對list_head鏈表進(jìn)行遍歷的，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)

而且狈惫，list.h 中也提供了list_replace( 節(jié)點(diǎn)替換) list_move(節(jié)點(diǎn)移位) 睛蛛，翻轉(zhuǎn)，查找等接口胧谈，有需要可以往對應(yīng)源碼中查看忆肾。

五. 宿主結(jié)構(gòu)

1.找出宿主結(jié)構(gòu) list_entry(ptr, type, member)

上面的所有操作都是基于list_head這個鏈表進(jìn)行的，涉及的結(jié)構(gòu)體也都是:

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

如前面一開始創(chuàng)建鏈表的時候定義的結(jié)構(gòu)體 data_list 我們這里把它叫做list_head的宿主結(jié)構(gòu)體

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

list.h中提供了list_entry宏來實(shí)現(xiàn)對應(yīng)地址的轉(zhuǎn)換菱肖，但最終還是調(diào)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定義在 kernel/include/linux/stddef.h 难菌，如下：

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

看下container_of宏的注釋：

（1）根據(jù)結(jié)構(gòu)體重的一個成員變量地址導(dǎo)出包含這個成員變量mem的struct地址。

（2）參數(shù)解釋：

? ptr ： 成員變量mem的地址

? type： 包含成員變量mem的宿主結(jié)構(gòu)體的類型

? member： 在宿主結(jié)構(gòu)中的mem成員變量的名稱

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); 這個調(diào)用返回的是 test_list 的指針

//ptr：&test_list->list_node

//type：struct data_list

//member：list_node

2. 宿主結(jié)構(gòu)的遍歷

? 因?yàn)槲覀兛梢愿鶕?jù)結(jié)構(gòu)體中成員變量的地址找到宿主結(jié)構(gòu)的地址蔑滓， 并且我們可以對成員變量所建立的鏈表進(jìn)行遍歷，那我們是不是也可以通過某種方法對宿主結(jié)構(gòu)進(jìn)行遍歷呢遇绞？

? 其實(shí)內(nèi)核中有很多這種用法键袱，ex:Asoc中的control.c， 內(nèi)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))

其中摹闽，list_first_entry 和 list_next_entry宏都定義在list.h中蹄咖，分別代表：獲取第一個真正的宿主結(jié)構(gòu)的地址； 獲取下一個宿主結(jié)構(gòu)的地址付鹿。它們的實(shí)現(xiàn)都是利用list_entry宏澜汤。

/**
 * 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)

最終使用 for 循環(huán)實(shí)現(xiàn)了宿主結(jié)構(gòu)的遍歷

首先pos定位到第一個宿主結(jié)構(gòu)地址蚜迅，然后循環(huán)獲取下一個宿主結(jié)構(gòu)地址，如果查到宿主結(jié)構(gòu)中的member成員變量（宿主結(jié)構(gòu)中struct list_head定義的字段）地址為head俊抵，則退出谁不，從而實(shí)現(xiàn)了宿主結(jié)構(gòu)的遍歷。如果要循環(huán)對宿主結(jié)構(gòu)中的其它成員變量進(jìn)行操作徽诲，這個遍歷操作就顯得特別有意義了刹帕。

List_head 實(shí)現(xiàn)隊(duì)列用法

這是個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;
}