内核链表list.h

#ifndef _LINUX_LIST_H #define _LINUX_LIST_H /* * 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) #define INIT_LIST_HEAD(ptr) do { \ (ptr)->next = (ptr); (ptr)->prev = (ptr); \ } while (0) /* * 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 = entry->prev = 0; } /** * 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_empty - tests whether a list is empty * @head: the list to test. */ static __inline__ int list_empty(struct list_head *head) { return head->next == head; } static __inline__ int list_is_last(const struct list_head *list, const struct list_head *head) { return list->next == head; } /** * 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(struct list_head *list, struct list_head *head) { struct list_head *first = list->next; if (first != list) { 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_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) \ ((type *)((char *)(ptr)-(unsigned long)(&((type *)0)->member))) /** * list_for_each - iterate over a list * @pos: the &struct list_head to use as a loop counter. * @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 * @pos: the &struct list_head to use as a loop counter. * @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 counter. * @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) #endif 

 

 

list_for_each()与list_for_each_safe()的区别

 

参考：

http://blog.csdn.net/choice_jj/article/details/7496732

http://stackoverflow.com/questions/9207850/why-do-we-need-list-for-each-safe-in-for-deleting-nodes-in-kernel-linked-list

 

由上面两个对比来看，list_for_each_safe()函数比list_for_each()多了一个中间变量n。

当在遍历的过程中需要删除结点时，来看一下会出现什么情况：

list_for_each()：list_del(pos)将pos的前后指针指向undefined state,导致kernel panic，另如果list_del_init(pos)将pos前后指针指向自身，导致死循环。

list_for_each_safe()：首先将pos的后指针缓存到n，处理一个流程后再赋回pos，避免了这种情况发生。

因此之遍历链表不删除结点时，可以使用list_for_each()，而当由删除结点操作时，则要使用list_for_each_safe()。

其他带safe的处理也是基于这个原因。