#include "list.h"
struct kool_list{
int to;
struct list_head list;
int from;
};
int main(int argc, char **argv){
struct kool_list *tmp;
struct list_head *pos, *q;
unsigned int i;
struct kool_list mylist;
INIT_LIST_HEAD(&mylist.list); /*初始化链表头*/
/* 给mylist增加元素 */
for(i=5; i!=0; --i){
tmp= (struct kool_list *)malloc(sizeof(struct kool_list));
/* 或者INIT_LIST_HEAD(&tmp->list); */
printf("enter to and from:");
scanf("%d %d", &tmp->to, &tmp->from);
list_add(&(tmp->list), &(mylist.list));
/* 也可以用list_add_tail() 在表尾增加元素*/
}
printf("\n");
printf("traversing the list using list_for_each()\n");
list_for_each(pos, &mylist.list){
/* 在这里 pos->next 指向next 节点, pos->prev指向前一个节点.这里的节点是
struct kool_list类型. 但是，我们需要访问节点本身，而不是节点中的list字段，宏list_entry()正是为此目的。*/
tmp= list_entry(pos, struct kool_list, list);
printf("to= %d from= %d\n", tmp->to, tmp->from);
}
printf("\n");
/* 因为这是循环链表，也可以以相反的顺序遍历它，
*为此，只需要用'list_for_each_prev'代替'list_for_each'，
* 也可以调用list_for_each_entry() 对给定类型的节点进行遍历。
* 例如:
*/
printf("traversing the list using list_for_each_entry()\n");
list_for_each_entry(tmp, &mylist.list, list)
printf("to= %d from= %d\n", tmp->to, tmp->from);
printf("\n");
/*现在，我们可以释放 kool_list节点了.我们本可以调用 list_del()删除节点
* 但为了避免遍历链表的过程中删除元素出错，因此调用另一个更加安全的宏 list_for_each_safe()，
* 具体原因见后面的分析＊/
printf("deleting the list using list_for_each_safe()\n");
list_for_each_safe(pos, q, &mylist.list){
tmp= list_entry(pos, struct kool_list, list);
printf("freeing item to= %d from= %d\n", tmp->to, tmp->from);
list_del(pos);
free(tmp);
}
return 0;
}
2. 关于删除元素的不安全性
为什么说调用list_del()删除元素有安全隐患？具体看源代码：
/*
* 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;
}
可以看出，当执行删除操作的时候， 被删除的节点的两个指针被指向一个固定的位置（entry->next = LIST_POISON1;
entry->prev = LIST_POISON2;）。而list_for_each(pos, head)中的pos指针在遍历过程中向后移动，即pos = pos->next，如果执行了list_del()操作，pos将指向这个固定位置的next, prev,而此时的next, prev没有任何意义，别无选择，出错。
而list_for_each_safe(p, n, head) 宏解决了上面的问题：
/**
* 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)
它采用了一个同pos同样类型的指针n 来暂存将要被删除的节点指针pos，从而使得删除操作不影响pos指针！
实际上，list.h的设计可谓精益求精，煞费苦心，用简洁的代码突破计算机科学中传统的链表实际机制，不仅考虑了单处理机，还利用了Paul E. McKenney提出的RCU（读拷贝更新）的技术，从而提高了多处理机环境下的性能。关于RCU，请看
