尾指针是相对于头指针而言的，形式与头指针相同，内容指向链表的最后一个节点。
不必等候其他线程完成，当前线程就可以“帮助”它完成操作，把尾指针向前移动(B)。
Rather than wait for the other thread to finish, the current thread can "help" it out by finishing the operation for it by moving the tail pointer forward (b).
在静止状态时，尾指针指向的链接节点的next字段总为null，而在中间状态时，这个字段为非null。
In the quiescent state, the next field of the link node pointed to by the tail is always null; in the intermediate state, it is always non-null.
返回：若成功为指针，出错或文件尾为NULL ？
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