链表的遍历
当我们在链表添加/修改多个节点后，我们最终是要通过查找链表中的某一个节点并对其数据进行操作，我们将逐一分析kernel/include/linux/list.h中关于链表遍历的接口。
1，list_entry用于获取struct list_head结构体指针所在结构体变量的首地址。
@ptr：指向我们要求首地址的结构体内的struct list_head成员变量，ptr的类型也为struct list_head。
@type：要求首地址的结构体类型，即struct list_head变量所在的结构体的类型。
@member：要求首地址结构体类型内struct list_head变量的变量名。
/** * 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_entry调用了contianer_of宏，container_of宏的定义与分析将在另一篇随笔中讲述。
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) /** * container_of - cast a member of a structure out to the containing structure * @ptr: the pointer to the member. * @type: the type of the container struct this is embedded in. * @member: the name of the member within the struct. * */ #define container_of(ptr, type, member) ({ \ const typeof( ((type *)0)->member ) *__mptr = (ptr); \ (type *)( (char *)__mptr - offsetof(type,member) );})
2，list_first_entry用于获取链表中第一个节点所在结构体的首地址。
@ptr：链表头节点；
@type：链表节点struct list_head变量所在结构体的类型；
@member：链表节点在所求首地址结构体内的成员变量名；
/** * 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)
3，list_for_each遍历一个链表。
@pos：struct list_head类型的指针，用于指向我们遍历的链表节点；
@head：我们要遍历链表的头节点；
/** * 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; pos != (head); pos = pos->next)
4，__list_for_each与list_for_each完全一样，遍历一个链表，同时也不做预取。
/** * __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 doesn't differ from list_for_each() any more. * We don't do prefetching in either case. */ #define __list_for_each(pos, head) \ for (pos = (head)->next; pos != (head); pos = pos->next)
5，__list_for_each_prev用于从后向前反向遍历。
/** * 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; pos != (head); pos = pos->prev)
6，list_for_each_safe安全的遍历一个链表，其机制是我们多传入一个struct list_head的指针n，用于指向pos的下一个节点，以保证我们在删除pos指向的节点时，仍能继续遍历链表的剩余节点。
/** * 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)
7，list_for_each_prev_safe反向遍历，安全查找。
/** * 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)
8，3、4、5、6、7在遍历链表时返回的是struct list_head指针的地址。当我们使用struct list_head型变量将一个节点挂到一个链表时，我们不是为了仅仅操纵这个光凸凸的节点，而是将struct list_head变量放到一个结构体内，根据对链表上struct list_head的遍历来得出strcut list_head所在结构体的首地址，list_for_each_entry正是为了完成这一功能而实现。
/** * 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); \ &pos->member != (head); \ pos = list_entry(pos->member.next, typeof(*pos), member))
我们将所求结构体类型的指针变量pos、链表的头head和所求结构体内struct list_head的变量名member传到list_for_each_entry之后， list_entry的第一个参数用head->next指向下一个节点，此节点的地址也就是在所属结构体内的struct list_head成员变量的地址，第二个参数用typeof(*pos)求得pos的结构体类型，第三个参数为所求结构体内struct list_head的变量名。
9，list_for_each_entry_reverse反向遍历链表并返回链表节点所在的结构体的首地址
/** * 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); \ &pos->member != (head); \ pos = list_entry(pos->member.prev, typeof(*pos), member))
10，list_prepare_entry用于准备一个结构体的首地址，用在list_for_each_entry_contine()中。
/** * 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))
11，list_for_each_entry_contine从当前pos的下一个节点开始继续遍历剩余的链表，不包括pos。
如果我们将pos、head、member传入list_for_each_entry，此宏将会从链表的头节点开始遍历。
/** * 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); \ &pos->member != (head); \ pos = list_entry(pos->member.next, typeof(*pos), member))
12，list_for_each_entry_contine_reverse从当前的pos的前一个节点开始继续反向遍历剩余的链表，不包括pos。
/** * 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); \ &pos->member != (head); \ pos = list_entry(pos->member.prev, typeof(*pos), member))
13，list_for_each_entry_from从pos开始遍历剩余的链表。
/** * 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 (; &pos->member != (head); \ pos = list_entry(pos->member.next, typeof(*pos), member))
14，list_for_each_entry_safe遍历链表，返回type类型的结构体的首地址，并防止因删除链表节点而导致的遍历出错。
/** * 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))
15，list_for_each_entry_safe_continue从pos节点的下一个节点开始遍历剩余的链表，并防止因删除链表节点而导致的遍历出错。
/** * 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))
16，list_for_each_entry_safe_from从pos节点开始继续遍历剩余的链表，并防止因删除链表节点而导致的遍历出错。其与list_for_each_entry_safe_contine的不同在于在第一次遍历时，pos没有指向它的下一个节点，而是从pos开始遍历。
/** * 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))
17，list_for_each_entry_safe_reverse从pos的前一个节点开始反向遍历一个链表，并防止因删除链表节点而导致的遍历出错。
/** * 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))
18，list_safe_reset_next返回当前pos节点的下一个节点的type结构体首地址。
/** * 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)
