Linux内核链表-通用链表的实现

　　最近编程总想着参考一些有名的开源代码是如何实现的，因为要写链表就看了下linux内核中对链表的实现。

　　链表是一种非常常见的数据结构，特别是在动态创建相应数据结构的情况下更是如此，然而在操作系统内核中，动态创建相应的数据结构尤为频繁。由于不带数据域所以Linux中的这种链表是通用的，在如何情况下，只要需要链表的数据结构包含它就行了。

　　链表只包含两个指针　

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

　　数据结构如果需要链表只需要包含它就行

typedef struct s_DASTUC
{
  int ds_stus;
  int ds_type;
  struct list_head ds_list;
  int ds_count;
}

　　通过链表的地址获取链表所在的数据结构地址

#define list_entry(ptr, type, number)    \
                    ((type *)((char *)(ptr) - (unsigned long)(&((type *))->number)))

网上看到过有人说为什么不把链表放到数据结构首部，这样链表所在的地址就是数据结构的地址，何必实现list_entry(ptr, type, number)，不要去限制数据结构调用链表的写法。

　　详细的网上都有博客了，这里博主只是为了记下知识点而已，下面是从Linux源码List.h拿出来的部分实现：

 #ifndef _MYLIST_H
 #define _MYLIST_H

 #define LIST_POISON1    NULL
 #define LIST_POISON2    NULL

 /*
  * Simple doubly linked list implementation.
  *
  * Some of the internal functions ("__xxx") are useful when
  * manipulating whole lists rather than single entries, as
  * sometimes we already know the next/prev entries and we can
  * generate better code by using them directly rather than
  * using the generic single-entry routines.
  */

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

 #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)
 {
     list->next = list;
     list->prev = list;
 }

 /*
  * Insert a new entry between two known consecutive entries.
  *
  * This is only for internal list manipulation where we know
  * the prev/next entries already!
  */
 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 - 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_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);
 }

 /*
  * Delete a list entry by making the prev/next entries
  * point to each other.
  *
  * This is only for internal list manipulation where we know
  * the prev/next entries already!
  */
 static inline void __list_del(struct list_head * prev, struct list_head * next)
 {
     next->prev = prev;
     prev->next = next;
 }

 /**
  * 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 = LIST_POISON1;
     entry->prev = LIST_POISON2;
 }

 /**
  * list_replace - replace old entry by new one
  * @old : the element to be replaced
  * @new : the new element to insert
  *
  * If @old was empty, it will be overwritten.
  */
 static inline void list_replace(struct list_head *old,
                 struct list_head *new)
 {
     new->next = old->next;
     new->next->prev = new;
     new->prev = old->prev;
     new->prev->next = new;
 }

 static inline void list_replace_init(struct list_head *old,
                     struct list_head *new)
 {
     list_replace(old, new);
     INIT_LIST_HEAD(old);
 }

 /**
  * 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);
 }

 /**
  * list_move - delete from one list and add as another's head
  * @list: the entry to move
  * @head: the head that will precede our entry
  */
 static inline void list_move(struct list_head *list, struct list_head *head)
 {
     __list_del(list->prev, list->next);
     list_add(list, head);
 }

 /**
  * list_move_tail - delete from one list and add as another's tail
  * @list: the entry to move
  * @head: the head that will follow our entry
  */
 static inline void list_move_tail(struct list_head *list,
                   struct list_head *head)
 {
     __list_del(list->prev, list->next);
     list_add_tail(list, head);
 }

 /**
  * list_is_last - tests whether @list is the last entry in list @head
  * @list: the entry to test
  * @head: the head of the list
  */
 static inline int list_is_last(const struct list_head *list,
                 const struct list_head *head)
 {
     return list->next == head;
 }

 /**
  * list_empty - tests whether a list is empty
  * @head: the list to test.
  */
 static inline int list_empty(const struct list_head *head)
 {
     return head->next == head;
 }

 /**
  * list_empty_careful - tests whether a list is empty and not being modified
  * @head: the list to test
  *
  * Description:
  * tests whether a list is empty _and_ checks that no other CPU might be
  * in the process of modifying either member (next or prev)
  *
  * NOTE: using list_empty_careful() without synchronization
  * can only be safe if the only activity that can happen
  * to the list entry is list_del_init(). Eg. it cannot be used
  * if another CPU could re-list_add() it.
  */
 static inline int list_empty_careful(const struct list_head *head)
 {
     struct list_head *next = head->next;
     return (next == head) && (next == head->prev);
 }

 #define __list_for_each(pos, head) \
                     for (pos = (head)->next; pos != (head); pos = pos->next)

 #define list_first_entry(ptr, type, member) \
                     list_entry((ptr)->next, type, member)
 #define list_entry(ptr, type, number)    \
                     ((type *)((char *)(ptr) - (unsigned long)(&((type *))->number)))

 #endif