Linux双向链表(二)——逻辑判断

双链表判断函数

1、判断结点是不是链表head的最后一个结点

/** * 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 inline int list_is_last(const struct list_head *list,const struct list_head *head){return list->next == head;}

方法很简单：检测 list指向的entry的next结点是否是head，如果是，则返回true，否则，返回false！

用途：对于有序链表，从头遍历链表，需要这样做遍历结束条件

2、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;}

有上篇博文可知在链表初始化时，宏定义

#define LIST_HEAD_INIT(name) { &(name),&(name) }

把name的next和prev初始化为指向自身，以表示链表为空。由此，返回head->next == head的逻辑值即可。

返回值：如果链表为空，返回true，否则返回false

用途：操纵一个链表时，首先要保证链表非空，否则，可能导致潜在运行bug

3、不安全方式（并行时）判断链表是否为空

/** * 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 inline int list_empty_careful(const struct list_head *head){struct list_head *next = head->next;//局部变量保存，如果未执行return之前，另CPU操作了链表，结果是不正确的，因此在并行架构上要“careful”——小心使用，具体细节看上面代码作者的注释return (next == head) && (next == head->prev);}

4、判断链表是否只有一个结点

/** * list_is_singular - tests whether a list has just one entry. * @head: the list to test. */static inline int list_is_singular(const struct list_head *head) {return !list_empty(head) && (head->next == head->prev);//非空且只有一个，使用图中①②}

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