Linux内核--链表结构（二）

Linux内核链表定义了一系列用于链表遍历的宏，本章详细描述。

一、container_of和offsetof

首先介绍两个很好用的宏container_of和offsetof。offsetof宏用于计算结构体成员基于结构体首地址的偏移量，container_of宏用于获取结构体首地址（根据成员指针）。

#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

offsetof宏接受两个入参，分别为结构体类型和结构体成员名，该宏将0强制转换成结构体类型的指针，并取其成员的地址。结构体首地址为0，对应成员的地址即成员相对结构体首地址的偏移量。

/**
 * 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)); })

container_of宏接受三个入参，指向结构体成员的指针ptr，结构体类型type，结构体成员名member。该宏首先定义一个结构体成员类型的指针_mptr，类型的获取通过typeof，_mptr =  ptr，并将_mptr强转为char*型，减去offsetof计算的偏移量，即得到结构体首地址。

二、list_entry

/**
 * 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即根据结构体成员指针ptr取得结构体首地址，如下例子使用：

/** 结构体定义 **/
struct student{
    int id;
    char name[20];
    list_head node;
}；

struct student stu;
char* ptr = &stu.node;

/** 宏使用如下 **/
struct student* s = list_entry(ptr, struct student, node);

三、list_first_entry

/**
 * 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)

ptr为链表头节点指针，type为结构体类型，member为结构体内成员名（结构体的链表成员）。list_first_entry宏取得链表首个节点的结构体首地址（头节点不算在内）。

四、list_for_each

/**
 * 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)

从链表首节点（不包含头节点）开始往后遍历，prefetch是内核中一个预取内存函数，这样下次遍历时就能高效命中内存cache，从而提升程序性能。

#ifndef ARCH_HAS_PREFETCH
#define prefetch(x) __builtin_prefetch(x)
#endif

五、__list_for_each

/**
 * __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)

从链表首节点（不包含头节点）开始往后遍历，与list_for_each不同在于没有做prefetch预取操作。

六、list_for_each_prev

/**
 * 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)

从链表首节点（不包含头节点）开始往前遍历。

七、list_for_each_safe

/**
 * 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的加强版，支持遍历过程的节点删除操作，提高安全性。使用变量n提前保存节点pos的后继，避免遍历过程pos节点删除后，指向错误。

八、list_for_each_prev_safe

/**
 * 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_prev的加强版，支持遍历过程的节点删除操作。

九、list_for_each_entry

/**
 * 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_for_each_entry：从链表首节点（不包含头节点）开始往后遍历，pos指向的是结构体，而不是结构体内的链表节点成员。与list_for_each不同，list_for_each遍历的是链表节点，而list_for_each_entry遍历的是由链表节点串起来的结构体链表。

list_for_each_entry_reverse：与list_for_each_entry相反，是往前遍历。

十、list_for_each_entry_continue

/**
 * 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_prepare_entry：宏的目的是为list_for_each_entry_continue准备一个起始遍历的节点，如果pos非空，则取自身，如果为空，则取头节点对应的结构体（虚拟的）；

list_for_each_entry_continue：从指定的节点pos（pos指向的是结构体）的下一个开始往后遍历链表；

list_for_each_entry_continue_reverse：与list_for_each_entry_continue相反，是往前遍历。

十一、list_for_each_entry_from

/**
 * 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))

从当前节点pos开始往后遍历，pos指向的是结构体。

十二、list_for_each_entry_safe

/**
 * 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的加强版，安全地从首节点开始往后遍历，支持遍历过程删除节点。

十三、list_for_each_entry_safe_continue

/**
 * 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_continue的加强版，安全地从指定节点pos的下一处（pos指向的是结构体）开始往后遍历，支持遍历过程删除节点。

十四、list_for_each_entry_safe_from

/**
 * 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_from的加强版，支持安全地从指定节点pos处开始往后遍历，支持遍历过程删除节点。

十五、list_for_each_entry_safe_reverse

/**
 * 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_for_each_entry_reverse的加强版，支持安全地从首节点（不包含头节点）开始往前遍历，支持遍历过程删除节点。