linux源码分析之list的实现_linux list_empty

目录

list_add

list_add_tail

list_del

list_empty

list_splice

list_entry

list_for_each

---

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

list_add

头插法,将新元素new使用头插法插入到head后面

/*
 * 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

尾插法,将新元素插入尾部,因为是循环链表,直接插在head前面也就相当于插在链表

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 - 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_del

删除链表中的指定元素

/*
 * These are non-NULL pointers that will result in page faults
 * under normal circumstances, used to verify that nobody uses
 * non-initialized list entries.
 */
#define LIST_POISON1  ((void *) 0x00100100)
#define LIST_POISON2  ((void *) 0x00200200)
 
/*
 * 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_empty

判断链表是否为空

/**
 * 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_splice

合并链表,将list链表头插到head链表

/**
 * 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;
}
 
static inline void __list_splice(const struct list_head *list,
				 struct list_head *head)
{
	struct list_head *first = list->next;
	struct list_head *last = list->prev;
	struct list_head *at = head->next;
 
	first->prev = head;
	head->next = first;
 
	last->next = at;
	at->prev = last;
}
 
/**
 * list_splice - join two lists
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 */
static inline void list_splice(const struct list_head *list,
				struct list_head *head)
{
	if (!list_empty(list))
		__list_splice(list, head);
}

list_entry

获取结构体实例的首地址

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

//最终的宏定义如下:
//ptr:结构体实例中特定成员的地址
//type:结构体类型
//member:结构体实例中特定成员的名称
#define list_entry(ptr, type, member)  ({			\
	const typeof( ((type *)0)->member ) *__mptr = (ptr);	\
	(type *)( (char *)__mptr - &((type *)0)->member) );})
 
/*
&(type *)0)->member: 在0x0地址上面建立sizeof(type)个字节的type结构体， ->member 取出这个0地址上type结构体里的member成员,最后获取到member距离0x0的偏移.
typeof( ((type *)0)->member ): 获取member的类型
const typeof( ((type *)0)->member ) *__mptr = (ptr): 定义一个指针__mptr指向实际地址
(type *)( (char *)__mptr - &((type *)0)->member) ):m_ptr代表的地址减去member在结构体中的偏移,即为结构体实例的首地址
*/

写到这,突然有个感慨:有些自己觉得不合理的东西,可能是因为自己的无知造成的.

list_for_each

遍历链表

//__builtin_prefetch() 是 gcc 的一个内置函数。它通过对数据手工预取的方法，减少了读取延迟，从而提高了性能，但该函数也需要 CPU 的支持。
void __builtin_prefetch (const void *addr, ...);
 
#define prefetch(x) __builtin_prefetch(x)
 
/**
 * 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; prefetch(pos->next), pos != (head); \
        	pos = pos->next)