双向链表的理解

背景

代码中经常会出现双向链表，对于双向链表的插入和删除有对应的API函数接口，但直观的图表更容易理解，所以本文会对rt-thread内核代码中提供的双向链表的一些API函数操作进行绘图，方便后续随时查看。

代码块

rt-thread中提供的代码段包括：
链表定义rtdef.h

/**
 * Double List structure
 */
struct rt_list_node
{
    struct rt_list_node *next;                          /**< point to next node. */
    struct rt_list_node *prev;                          /**< point to prev node. */
};
typedef struct rt_list_node rt_list_t;                  /**< Type for lists. */

API操作定义rtserver.h

/**
 * @addtogroup KernelService
 */

/**@{*/

/**
 * rt_container_of - return the start address of struct type, while ptr is the
 * member of struct type.
 */
#define rt_container_of(ptr, type, member) \
    ((type *)((char *)(ptr) - (unsigned long)(&((type *)0)->member)))


/**
 * @brief initialize a list object
 */
#define RT_LIST_OBJECT_INIT(object) { &(object), &(object) }

/**
 * @brief initialize a list
 *
 * @param l list to be initialized
 */
rt_inline void rt_list_init(rt_list_t *l)
{
    l->next = l->prev = l;
}

/**
 * @brief insert a node after a list
 *
 * @param l list to insert it
 * @param n new node to be inserted
 */
rt_inline void rt_list_insert_after(rt_list_t *l, rt_list_t *n)
{
    l->next->prev = n;
    n->next = l->next;

    l->next = n;
    n->prev = l;
}

/**
 * @brief insert a node before a list
 *
 * @param n new node to be inserted
 * @param l list to insert it
 */
rt_inline void rt_list_insert_before(rt_list_t *l, rt_list_t *n)
{
    l->prev->next = n;
    n->prev = l->prev;

    l->prev = n;
    n->next = l;
}

/**
 * @brief remove node from list.
 * @param n the node to remove from the list.
 */
rt_inline void rt_list_remove(rt_list_t *n)
{
    n->next->prev = n->prev;
    n->prev->next = n->next;

    n->next = n->prev = n;
}

/**
 * @brief tests whether a list is empty
 * @param l the list to test.
 */
rt_inline int rt_list_isempty(const rt_list_t *l)
{
    return l->next == l;
}

/**
 * @brief get the list length
 * @param l the list to get.
 */
rt_inline unsigned int rt_list_len(const rt_list_t *l)
{
    unsigned int len = 0;
    const rt_list_t *p = l;
    while (p->next != l)
    {
        p = p->next;
        len ++;
    }

    return len;
}

/**
 * @brief get the struct for this entry
 * @param node the entry point
 * @param type the type of structure
 * @param member the name of list in structure
 */
#define rt_list_entry(node, type, member) \
    rt_container_of(node, type, member)

/**
 * rt_list_for_each - iterate over a list
 * @pos:    the rt_list_t * to use as a loop cursor.
 * @head:   the head for your list.
 */
#define rt_list_for_each(pos, head) \
    for (pos = (head)->next; pos != (head); pos = pos->next)

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

/**
 * rt_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 rt_list_for_each_entry(pos, head, member) \
    for (pos = rt_list_entry((head)->next, typeof(*pos), member); \
         &pos->member != (head); \
         pos = rt_list_entry(pos->member.next, typeof(*pos), member))

/**
 * rt_list_for_each_entry_safe - iterate over list of given type safe against removal of list entry
 * @pos:    the type * to use as a loop cursor.
 * @n:      another type * to use as temporary storage
 * @head:   the head for your list.
 * @member: the name of the list_struct within the struct.
 */
#define rt_list_for_each_entry_safe(pos, n, head, member) \
    for (pos = rt_list_entry((head)->next, typeof(*pos), member), \
         n = rt_list_entry(pos->member.next, typeof(*pos), member); \
         &pos->member != (head); \
         pos = n, n = rt_list_entry(n->member.next, typeof(*n), member))

/**
 * rt_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 rt_list_first_entry(ptr, type, member) \
    rt_list_entry((ptr)->next, type, member)

重点函数

rt_list_init()
rt_list_insert_after()
rt_list_insert_before()
rt_list_remove()

重点函数理解

rt_list_init(rt_list_t *l)

rt_inline void rt_list_init(rt_list_t *l)
{
    l->next = l->prev = l;
}

初始化当前链表l，即当前链表l的pre和next都是指向自己。

rt_list_insert_after(rt_list_t *l, rt_list_t *n)

rt_inline void rt_list_insert_after(rt_list_t *l, rt_list_t *n)
{
    l->next->prev = n;
    n->next = l->next;

    l->next = n;
    n->prev = l;
}

将新链表n1插入到l之后

将新链表n2插入到l之后

将新链表n3插入到l之后

rt_list_insert_before(rt_list_t *l, rt_list_t *n)

rt_inline void rt_list_insert_before(rt_list_t *l, rt_list_t *n)
{
    l->prev->next = n;
    n->prev = l->prev;

    l->prev = n;
    n->next = l;
}

将新链表n1插入到l之前

将新链表n2插入到l之前

rt_list_remove(rt_list_t *n)

rt_inline void rt_list_remove(rt_list_t *n)
{
    n->next->prev = n->prev;
    n->prev->next = n->next;

    n->next = n->prev = n;
}

从已有链表中移除当前链表，如原链表

从此链表中移除n2，则