1. DTLib中双向链表的设计思路
（1）数据结点之间在逻辑上构成双向循环，这有别于Linux内核链表的实现。
（2）头结点仅用于结点的定位，而Linux内核链表是将头结点作为循环的一部分。
2. 实现思路
（1）通过模板定义DualCircleList类，继承自DualLinkList类
（2）在DualCircleList内部使用Linux内核链表进行实现（另类实现）
（3）使用struct list_head定义DualCircleList的头结点
（4）特殊处理：循环遍历时忽略头结点
3. 实现要点
（1）通过list_head进行目标结点定位（position(i)）
（2）通过list_entry将list_head指针转换为目标结点指针
（3）通过list_for_each实现int find(const T& e)函数
（4）遍历函数中的next和prev需要考虑跳过头结点
【编程实验】基于Linux内核链表的双向循环链表
//LinuxList.h
#ifndef _LINUX_LIST_H #define _LINUX_LIST_H //#include <linux/types.h> //#include <linux/stddef.h> //#include <linux/poison.h> //#include <linux/prefetch.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. */ #ifndef offsetof #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) #endif #ifndef container_of #define container_of(ptr, type, member) ((type *)((char *)ptr - offsetof(type,member))) #endif #define prefetch(x) ((void)x) #define LIST_POISON1 (NULL) #define LIST_POISON2 (NULL) struct list_head{ struct list_head *next, *prev; }; struct hlist_node{ struct hlist_node *next, **pprev; }; struct hlist_head{ struct hlist_node *first; }; #define LIST_HEAD_INIT(name) { &(name), &(name) } #define LIST_HEAD(name) struct list_head name = LIST_HEAD_INIT(name) static 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! */ #ifndef CONFIG_DEBUG_LIST static void __list_add(struct list_head *node, struct list_head *prev, struct list_head *next) { next->prev = node; node->next = next; node->prev = prev; prev->next = node; } #else extern void __list_add(struct list_head *node, struct list_head *prev, struct list_head *next); #endif /** * list_add - add a new entry * @node: 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 void list_add(struct list_head *node, struct list_head *head) { __list_add(node, head, head->next); } /** * list_add_tail - add a new entry * @node: 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 void list_add_tail(struct list_head *node, struct list_head *head) { __list_add(node, 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 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. */ #ifndef CONFIG_DEBUG_LIST static void __list_del_entry(struct list_head *entry) { __list_del(entry->prev, entry->next); } static void list_del(struct list_head *entry) { __list_del(entry->prev, entry->next); entry->next = LIST_POISON1; entry->prev = LIST_POISON2; } #else extern void __list_del_entry(struct list_head *entry); extern void list_del(struct list_head *entry); #endif /** * list_replace - replace old entry by new one * @old : the element to be replaced * @node : the new element to insert * * If @old was empty, it will be overwritten. */ static void list_replace(struct list_head *old, struct list_head *node) { node->next = old->next; node->next->prev = node; node->prev = old->prev; node->prev->next = node; } static void list_replace_init(struct list_head *old, struct list_head *node) { list_replace(old, node); INIT_LIST_HEAD(old); } /** * list_del_init - deletes entry from list and reinitialize it. * @entry: the element to delete from the list. */ static void list_del_init(struct list_head *entry) { __list_del_entry(entry); 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 void list_move(struct list_head *list, struct list_head *head) { __list_del_entry(list); 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 void list_move_tail(struct list_head *list, struct list_head *head) { __list_del_entry(list); 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 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 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 int list_empty_careful(const struct list_head *head) { struct list_head *next = head->next; return (next == head) && (next == head->prev); } /** * list_rotate_left - rotate the list to the left * @head: the head of the list */ static void list_rotate_left(struct list_head *head) { struct list_head *first; if (!list_empty(head)) { first = head->next; list_move_tail(first, head); } } /** * list_is_singular - tests whether a list has just one entry. * @head: the list to test. */ static int list_is_singular(const struct list_head *head) { return !list_empty(head) && (head->next == head->prev); } static void __list_cut_position(struct list_head *list, struct list_head *head, struct list_head *entry) { struct list_head *new_first = entry->next; list->next = head->next; list->next->prev = list; list->prev = entry; entry->next = list; head->next = new_first; new_first->prev = head; } /** * list_cut_position - cut a list into two * @list: a new list to add all removed entries * @head: a list with entries * @entry: an entry within head, could be the head itself * and if so we won't cut the list * * This helper moves the initial part of @head, up to and * including @entry, from @head to @list. You should * pass on @entry an element you know is on @head. @list * should be an empty list or a list you do not care about * losing its data. * */ static void list_cut_position(struct list_head *list, struct list_head *head, struct list_head *entry) { if (list_empty(head)) return; if (list_is_singular(head) && (head->next != entry && head != entry)) return; if (entry == head) INIT_LIST_HEAD(list); else __list_cut_position(list, head, entry); } static void __list_splice(const struct list_head *list, struct list_head *prev, struct list_head *next) { struct list_head *first = list->next; struct list_head *last = list->prev; first->prev = prev; prev->next = first; last->next = next; next->prev = last; } /** * list_splice - join two lists, this is designed for stacks * @list: the new list to add. * @head: the place to add it in the first list. */ static void list_splice(const struct list_head *list, struct list_head *head) { if (!list_empty(list)) __list_splice(list, head, head->next); } /** * list_splice_tail - join two lists, each list being a queue * @list: the new list to add. * @head: the place to add it in the first list. */ static void list_splice_tail(struct list_head *list, struct list_head *head) { if (!list_empty(list)) __list_splice(list, head->prev, head); } /** * list_splice_init - join two lists and reinitialise the emptied list. * @list: the new list to add. * @head: the place to add it in the first list. * * The list at @list is reinitialised */ static void list_splice_init(struct list_head *list, struct list_head *head) { if (!list_empty(list)) { __list_splice(list, head, head->next); INIT_LIST_HEAD(list); } } /** * list_splice_tail_init - join two lists and reinitialise the emptied list * @list: the new list to add. * @head: the place to add it in the first list. * * Each of the lists is a queue. * The list at @list is reinitialised */ static void list_splice_tail_init(struct list_head *list, struct list_head *head) { if (!list_empty(list)) { __list_splice(list, head->prev, head); INIT_LIST_HEAD(list); } } /** * 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) /** * 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 list_first_entry(ptr, type, member) list_entry((ptr)->next, type, member) /** * 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) /** * __list_for_each - iterate over a list * @pos: the &struct list_head to use as a loop cursor. * @head: the head for your list. * * This variant differs from list_for_each() in that it's the * simplest possible list iteration code, no prefetching is done. * Use this for code that knows the list to be very short (empty * or 1 entry) most of the time. */ #define __list_for_each(pos, head) for (pos = (head)->next; pos != (head); pos = pos->next) /** * list_for_each_prev - iterate over a list backwards * @pos: the &struct list_head to use as a loop cursor. * @head: the head for your list. */ #define list_for_each_prev(pos, head) for (pos = (head)->prev; prefetch(pos->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 cursor. * @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; prefetch(pos->prev), pos != (head); pos = n, n = pos->prev) /** * 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 list_for_each_entry(pos, head, member) for (pos = list_entry((head)->next, typeof(*pos), member); prefetch(pos->member.next), &pos->member != (head); pos = list_entry(pos->member.next, typeof(*pos), member)) /** * list_for_each_entry_reverse - iterate backwards 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 list_for_each_entry_reverse(pos, head, member) for (pos = list_entry((head)->prev, typeof(*pos), member); prefetch(pos->member.prev), &pos->member != (head); pos = list_entry(pos->member.prev, typeof(*pos), member)) /** * list_prepare_entry - prepare a pos entry for use in list_for_each_entry_continue() * @pos: the type * to use as a start point * @head: the head of the list * @member: the name of the list_struct within the struct. * * Prepares a pos entry for use as a start point in list_for_each_entry_continue(). */ #define list_prepare_entry(pos, head, member) ((pos) ? : list_entry(head, typeof(*pos), member)) /** * list_for_each_entry_continue - continue iteration 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. * * Continue to iterate over list of given type, continuing after * the current position. */ #define list_for_each_entry_continue(pos, head, member) for (pos = list_entry(pos->member.next, typeof(*pos), member); prefetch(pos->member.next), &pos->member != (head); pos = list_entry(pos->member.next, typeof(*pos), member)) /** * list_for_each_entry_continue_reverse - iterate backwards from the given point * @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. * * Start to iterate over list of given type backwards, continuing after * the current position. */ #define list_for_each_entry_continue_reverse(pos, head, member) for (pos = list_entry(pos->member.prev, typeof(*pos), member); prefetch(pos->member.prev), &pos->member != (head); pos = list_entry(pos->member.prev, typeof(*pos), member)) /** * list_for_each_entry_from - iterate over list of given type from the current point * @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. * * Iterate over list of given type, continuing from current position. */ #define list_for_each_entry_from(pos, head, member) for (; prefetch(pos->member.next), &pos->member != (head); pos = list_entry(pos->member.next, typeof(*pos), member)) /** * 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 list_for_each_entry_safe(pos, n, head, member) for (pos = list_entry((head)->next, typeof(*pos), member), n = list_entry(pos->member.next, typeof(*pos), member); &pos->member != (head); pos = n, n = list_entry(n->member.next, typeof(*n), member)) /** * list_for_each_entry_safe_continue - continue list iteration safe against removal * @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. * * Iterate over list of given type, continuing after current point, * safe against removal of list entry. */ #define list_for_each_entry_safe_continue(pos, n, head, member) for (pos = list_entry(pos->member.next, typeof(*pos), member), n = list_entry(pos->member.next, typeof(*pos), member); &pos->member != (head); pos = n, n = list_entry(n->member.next, typeof(*n), member)) /** * list_for_each_entry_safe_from - iterate over list from current point safe against removal * @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. * * Iterate over list of given type from current point, safe against * removal of list entry. */ #define list_for_each_entry_safe_from(pos, n, head, member) for (n = list_entry(pos->member.next, typeof(*pos), member); &pos->member != (head); pos = n, n = list_entry(n->member.next, typeof(*n), member)) /** * list_for_each_entry_safe_reverse - iterate backwards over list safe against removal * @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. * * Iterate backwards over list of given type, safe against removal * of list entry. */ #define list_for_each_entry_safe_reverse(pos, n, head, member) for (pos = list_entry((head)->prev, typeof(*pos), member), n = list_entry(pos->member.prev, typeof(*pos), member); &pos->member != (head); pos = n, n = list_entry(n->member.prev, typeof(*n), member)) /** * list_safe_reset_next - reset a stale list_for_each_entry_safe loop * @pos: the loop cursor used in the list_for_each_entry_safe loop * @n: temporary storage used in list_for_each_entry_safe * @member: the name of the list_struct within the struct. * * list_safe_reset_next is not safe to use in general if the list may be * modified concurrently (eg. the lock is dropped in the loop body). An * exception to this is if the cursor element (pos) is pinned in the list, * and list_safe_reset_next is called after re-taking the lock and before * completing the current iteration of the loop body. */ #define list_safe_reset_next(pos, n, member) n = list_entry(pos->member.next, typeof(*pos), member) /* * Double linked lists with a single pointer list head. * Mostly useful for hash tables where the two pointer list head is * too wasteful. * You lose the ability to access the tail in O(1). */ #define HLIST_HEAD_INIT { .first = NULL } #define HLIST_HEAD(name) struct hlist_head name = { .first = NULL } #define INIT_HLIST_HEAD(ptr) ((ptr)->first = NULL) static void INIT_HLIST_NODE(struct hlist_node *h) { h->next = NULL; h->pprev = NULL; } static int hlist_unhashed(const struct hlist_node *h) { return !h->pprev; } static int hlist_empty(const struct hlist_head *h) { return !h->first; } static void __hlist_del(struct hlist_node *n) { struct hlist_node *next = n->next; struct hlist_node **pprev = n->pprev; *pprev = next; if (next) next->pprev = pprev; } static void hlist_del(struct hlist_node *n) { __hlist_del(n); n->next = LIST_POISON1; n->pprev = LIST_POISON2; } static void hlist_del_init(struct hlist_node *n) { if (!hlist_unhashed(n)) { __hlist_del(n); INIT_HLIST_NODE(n); } } static void hlist_add_head(struct hlist_node *n, struct hlist_head *h) { struct hlist_node *first = h->first; n->next = first; if (first) first->pprev = &n->next; h->first = n; n->pprev = &h->first; } /* next must be != NULL */ static void hlist_add_before(struct hlist_node *n, struct hlist_node *next) { n->pprev = next->pprev; n->next = next; next->pprev = &n->next; *(n->pprev) = n; } static void hlist_add_after(struct hlist_node *n, struct hlist_node *next) { next->next = n->next; n->next = next; next->pprev = &n->next; if(next->next) next->next->pprev = &next->next; } /* after that we'll appear to be on some hlist and hlist_del will work */ static void hlist_add_fake(struct hlist_node *n) { n->pprev = &n->next; } /* * Move a list from one list head to another. Fixup the pprev * reference of the first entry if it exists. */ static void hlist_move_list(struct hlist_head *old, struct hlist_head *node) { node->first = old->first; if (node->first) node->first->pprev = &node->first; old->first = NULL; } #define hlist_entry(ptr, type, member) container_of(ptr,type,member) #define hlist_for_each(pos, head) for (pos = (head)->first; pos && ({ prefetch(pos->next); 1; }); pos = pos->next) #define hlist_for_each_safe(pos, n, head) for (pos = (head)->first; pos && ({ n = pos->next; 1; }); pos = n) /** * hlist_for_each_entry - iterate over list of given type * @tpos: the type * to use as a loop cursor. * @pos: the &struct hlist_node to use as a loop cursor. * @head: the head for your list. * @member: the name of the hlist_node within the struct. */ #define hlist_for_each_entry(tpos, pos, head, member) for (pos = (head)->first; pos && ({ prefetch(pos->next); 1;}) && ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); pos = pos->next) /** * hlist_for_each_entry_continue - iterate over a hlist continuing after current point * @tpos: the type * to use as a loop cursor. * @pos: the &struct hlist_node to use as a loop cursor. * @member: the name of the hlist_node within the struct. */ #define hlist_for_each_entry_continue(tpos, pos, member) for (pos = (pos)->next; pos && ({ prefetch(pos->next); 1;}) && ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); pos = pos->next) /** * hlist_for_each_entry_from - iterate over a hlist continuing from current point * @tpos: the type * to use as a loop cursor. * @pos: the &struct hlist_node to use as a loop cursor. * @member: the name of the hlist_node within the struct. */ #define hlist_for_each_entry_from(tpos, pos, member) for (; pos && ({ prefetch(pos->next); 1;}) && ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); pos = pos->next) /** * hlist_for_each_entry_safe - iterate over list of given type safe against removal of list entry * @tpos: the type * to use as a loop cursor. * @pos: the &struct hlist_node to use as a loop cursor. * @n: another &struct hlist_node to use as temporary storage * @head: the head for your list. * @member: the name of the hlist_node within the struct. */ #define hlist_for_each_entry_safe(tpos, pos, n, head, member) for (pos = (head)->first; pos && ({ n = pos->next; 1; }) && ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); pos = n) #endif
//DualCircleList.h
#ifndef _DUALCIRCLELIST_H_ #define _DUALCIRCLELIST_H_ #include "LinuxList.h" #include "DualLinkList.h" namespace DTLib { template <typename T> class DualCircleList : protected DualLinkList<T> { protected: //自定义链表结点类型 struct Node : public Object { list_head head; T value; }; list_head* m_current; int m_step; list_head m_header; list_head* position(int index) const { //const对象不能对成员变量取地址，需去掉const限定 list_head* ret = const_cast<list_head*>(&m_header); for(int i=0; i<index; i++){ ret = ret->next; } return ret; } int mod(int value) { return (this->m_length==0) ? 0: (value % this->m_length); } public: DualCircleList() { this->m_length = 0; m_current = NULL; m_step = 1; //初始化Linux内核链表，形成循环链表 INIT_LIST_HEAD(&this->m_header); } bool insert(const T& elem) { return insert(this->m_length, elem); } bool insert(int index, const T& elem) { bool ret = (0<=index); if (ret){ index = index % (this->m_length + 1); Node* node = new Node(); if(node != NULL){ node->value = elem; list_add_tail(&node->head, position(index)->next); this->m_length++; }else{ THROW_EXCEPTION(NotEnoughMemoryException, "No memory to insert new element..."); } } return ret; } bool remove(int index) { bool ret = true; index = mod(index); ret = ((0<=index) && (index<this->m_length)); if(ret){ list_head* toDel = position(index)->next; if(m_current == toDel){ m_current = toDel->next; } list_del(toDel); this->m_length--; delete list_entry(toDel, Node, head); } return ret; } bool set(int index, const T& elem) //O(n) { bool ret = true; index = mod(index); ret = (0 <= index) && (index < this->m_length); if(ret){ list_entry(position(index)->next, Node, head)->value = elem; } return ret; } T get(int index) //O(n) { T ret; if(get(index, ret)){ return ret; }else{ THROW_EXCEPTION(IndexOutOfBoundsException, "Invalid paramter index to get element ..."); } } bool get(int index, T& elem) //O(n) { bool ret = true; index = mod(index); ret = (0 <= index) && (index < this->m_length); if(ret){ list_head* current = position(index); elem = list_entry(current->next, Node, head)->value; } return ret; } int find(const T& elem) const //O(n) { int ret = -1; int i = 0; list_head* slider = NULL; list_for_each(slider, &m_header){ if(list_entry(slider, Node, head)->value == elem){ ret = i; break; } i++; } return ret; } int length() const { return this->m_length; } void clear() { while(this->m_length >0) remove(0); } ~DualCircleList() { clear(); } public: bool move(int index, int step = 1) { bool ret = (step > 0); index = mod(index); ret = (0 <= index) && (index < this->m_length); if(ret){ m_current = position(index)->next; this->m_step = step; } return ret; } bool end() { return (m_current == NULL) || (this->m_length == 0); } T current() { if(!end()){ return list_entry(m_current, Node, head)->value; }else{ THROW_EXCEPTION(InvalidOperationException, "No value at current position ..."); } } bool next() { int i = 0; while((i<m_step) && !end()){ //只有非头结点i才计数，否则跳过头结点 if(m_current != &m_header){ i++; } m_current = m_current->next; } //是不是又指向了头结点？ if(m_current == &m_header){ m_current = m_current->next; } return (i == m_step); } bool prev() { int i = 0; while((i<m_step) && !end()){ //只有非头结点i才计数，否则跳过头结点 if(m_current != &m_header){ i++; } m_current = m_current->prev; } //是不是又指向了头结点？ if(m_current == &m_header){ m_current = m_current->prev; } return (i == m_step); } }; } #endif // _DUALCIRCLELIST_H_
//main.cpp
#include <iostream> #include "DualCircleList.h" using namespace std; using namespace DTLib; int main() { DualCircleList<int> dl; for(int i=0; i<5; i++){ dl.insert(i); dl.insert(5); } cout << "begin" << endl; dl.move(dl.length()-1); while(dl.find(5) != -1){ if(dl.current() == 5){ cout << dl.current() << " "; dl.remove(dl.find(dl.current())); }else{ dl.prev(); } } cout << endl; cout << "end" << endl; //注意：10>dl.length()，会循环打印10个元素 for(int i=0; i<10; i++){ cout << dl.get(i) << " "; } cout << endl; //以下代码会死循环，因为这是双向循环链表 // for(dl.move(0); !dl.end(); dl.next()){ // cout << dl.current() << endl; // } return 0; } /*输出结果 begin 5 5 5 5 5 end 0 1 2 3 4 0 1 2 3 4 */
4.思考题：pn1==pn2吗？
struct Node : public Object //注意Object是个虚基类 { list_head head; T value; }; Node node; list_head* ld = &node.head; Node* pn1 = reinterpret_cast<Node*>(ld); //error, 遗漏了虚函数表指针字段 Node* pn2 = list_entry(ld, Node, head); //ok
5. 小结
（1）Linux内核链表是带头结点的双向循环链表
（2）DualCircleList使用Linux内核链表进行内部实现
（3）DualCircleList在循环遍历时需要跳过头结点
（4）将list_head指针转换为目标结点指针时，使用list_entry宏。