Linux内核链表访问链表头指针,linux内核——链表结构分析
http://blog.csdn.net/tigerjibo/article/details/8299584
简单整理(使用linux3.0内核)
这里首先学习的是内核中一种抽象定义的双向链表，为了提高其扩展性。
内核中链表的描述数据结构
位置：Types.h (linux-3.0.12\include\linux) 5444 2011/11/29
222：
struct list_head {
struct list_head *next, *prev;
};
这是一个不含数据域的结构
使用内核链表结构构造自定义结构
我们可以这样定义我们的链表节点
struct my_list_node {
数据域
......
struct list_head index;
......
};
如果需要构造某类对象的特定列表，则在其结构中定义一个类型为
struct list_head的成员，通过这个成员将这类对象连接起来，形成所需列表，并通过通用链表函数对其进行操作。其优点是只需编写通用链表函数，即可构造和操作不同对象的列表，而无需为每类对象的每种列表编写专用函数，实现了代码的重用。
使用方法：以struct list_head 为基本对象，对链表进行插入、删除、合并以及遍历等各种操作。
内核链表头的初始化定义
位置：List.h (linux-3.0.12\include\linux) 21209 2011/11/29
19：
#define LIST_HEAD_INIT(name) { &(name), &(name) }
#define LIST_HEAD(name) \
struct list_head name = LIST_HEAD_INIT(name)
static inline void INIT_LIST_HEAD(struct list_head *list)
{
list->next = list;
list->prev = list;
}
其中name是一个struct list_head类型变量，list是struct list_head类型指针，上面是两种初始化的方法，使用宏定义和内联函数
自定义链表头初始化
struct list_head my_list_head;
LIST_HEAD(my_list_head);或者
INIT_LIST_HEAD(&my_list_head);
调用后头结点my_list_head的next, prev都指向自己，构成一个空链表。所以，可以借助next是否指向自己 (头结点)来判断链表是否为空。
内核链表操作的定义
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;
}
/**
* 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;
return (next == head) && (next == head->prev);
}
返回值：
为空返回1，不为空返回0
插入节点
static inline void __list_add(struct list_head *new,
struct list_head *prev,
struct list_head *next)
{
next->prev = new;
new->next = next;
new->prev = prev;
prev->next = new;
}
/**
* list_add - add a new entry
* @new: 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 inline void list_add(struct list_head *new, struct list_head *head)
{
__list_add(new, head, head->next);
}
/**
* list_add_tail - add a new entry
* @new: 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 inline void list_add_tail(struct list_head *new, struct list_head *head)
{
__list_add(new, head->prev, head);
}
遍历链表
首先看下需要用到额外的宏定义
offsetof：计算某结构体成员在结构体中的偏移地址
位置：Stddef.h (linux-3.0.12\include\linux) 435 2011/11/29
24：
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
对这个宏的讲解我们大致可以分为以下4步进行讲解：
1>( (TYPE *)0 ) 0地址强制 "转换" 为 TYPE结构类型的指针；
2>((TYPE *)0)->MEMBER 访问TYPE结构中的MEMBER数据成员；
3>&( ( (TYPE *)0 )->MEMBER)取出TYPE结构中的数据成员MEMBER的地址；
4>(size_t)(&(((TYPE*)0)->MEMBER))结果转换为size_t类型。
宏offsetof的巧妙之处在于将0地址强制转换为 TYPE结构类型的指针，TYPE结构以内存空间首地址0作为起始地址，则成员地址自然为偏移地址。可能有的读者会想是不是非要用0呢？当然不是，我们仅仅是为了计算的简便。也可以使用其他的值，只是算出来的结果还要再减去该数值才是偏移地址。
container_of：通过结构体中某个成员的地址，算出结构体的地址
位置：Kernel.h (linux-3.0.12\tools\perf\util\include\linux) 2691 2011/11/29
19：
/**
* 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)); })
说明
第一步，首先定义一个临时的数据类型(通过typeof( ((type *)0)->member )获得)与ptr相同的指针变量__mptr，然后用它来保存ptr的值。
说明：typeof是GNU C对标准C的扩展，它的作用是根据变量获取变量的类型《typeof关键字在linux 内核中很常见》
第二步，用(char *)__mptr减去member在结构体中的偏移量，得到的值就是整个结构体变量的首地址(整个宏的返回值就是这个首地址)。
遍历链表的相关宏定义
/**
* 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)
通过成员指针获得整个结构体的指针，Linux链表中仅保存了节点中struct list_head成员变量的地址，通过list_entry宏经struct list_head成员访问到作为它的所有者的节点起始地址。
/**
* 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)
得到ptr指向的节点的next成员指向的结构体变量地址，此处的ptr一般是一个链表的头结点
/**
* 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)
/**
* __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)
两个宏都是用来遍历链表
pos是一个辅助指针(即链表类型),用于链表遍历
head:链表的头指针(即结构体中成员struct list_head)
/**
* 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)
逆向遍历
/**
* 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)
前面介绍了用于链表遍历的几个宏，它们都是通过移动pos指针来达到遍历的目的。但如果遍历的操作中包含删除pos指针所指向的节点，pos指针的移动就会被中断，因为list_del(pos)将把pos的next、prev置成LIST_POSITION2和LIST_POSITION1的特殊值。当然，调用者完全可以自己缓存next指针使遍历操作能够连贯起来，但为了编程的一致性，Linxu内核链表要求调用者另外提供一个与pos同类型的指针n，在for循环中暂存pos下一个节点的地址，避免因pos节点被释放而造成的断链。
/**
* 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)
功能与list_for_each_prev相同，用于逆向遍历链表。不同的是使用list_head结构体变量n作为临时存储变量。主要用于链表删除时操作。
下边遍历链表宏定义，所不同的是它是根据链表的结构体地址来进行遍历。大多数情况下，遍历链表的时候都需要获得链表节点数据项，也就是说list_for_each()和list_entry()总是同时使用。与list_for_each()不同，这里的pos是数据项结构指针类型，而不是(struct list_head 类型。
首先是从head开始遍历整个链
/**
* 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))
/**
* 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))
/**
* 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))
pos:用于遍历的指针，只是它的数据类型是结构体类型而不是strut list_head 类型
head:链表头指针
member:该结构体类型定义中struct list_head成员的变量名。
n和pos类型相同
从pos后位置开始顺序遍历到head，需要list_prepare_entry宏先对pos处理
/**
* 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); \
&pos->member != (head); \
pos = list_entry(pos->member.next, typeof(*pos), member))
从pos前一位置开始逆序遍历到head，需要list_prepare_entry宏先对pos处理
/**
* 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))
#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))
从已知的某个结点pos后一个结点开始进行遍历，与list_for_each_entry_continue不同的是,它主要用于链表进行删除时进行的遍历。
从当前pos位置开始遍历到head
#define list_for_each_entry_from(pos, head, member) \
for (; &pos->member != (head); \
pos = list_entry(pos->member.next, typeof(*pos), member))
#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))
根据pos得到其下一个节点的起始地址
/**
* 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)
链表节点的删除
/*
* 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 inline void __list_del(struct list_head * prev, struct list_head * next)
{
next->prev = prev;
prev->next = next;
}
static inline void __list_del_entry(struct list_head *entry)
{
__list_del(entry->prev, entry->next);
}
只是简单删除，对被删除的节点没做处理
static inline void list_del(struct list_head *entry)
{
__list_del(entry->prev, entry->next);
entry->next = LIST_POISON1;
entry->prev = LIST_POISON2;
}
list_del()函数将删除后的prev、next指针分别被设为LIST_POSITION2和LIST_POSITION1两个特殊值，这样设置是为了保证不在链表中的节点项不可访问。对LIST_POSITION1和LIST_POSITION2的访问都将引起页故障。
static inline void INIT_LIST_HEAD(struct list_head *list)
{
list->next = list;
list->prev = list;
}
static inline void list_del_init(struct list_head *entry)
{
__list_del_entry(entry);
INIT_LIST_HEAD(entry);
}
list_del_init这个函数首先将entry从双向链表中删除之后，并且将entry初始化为一个空链表
移动一个节点到另一个链表
/**
* 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 inline 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 inline void list_move_tail(struct list_head *list,
struct list_head *head)
{
__list_del_entry(list);
list_add_tail(list, head);
}
剩下的函数暂时不看了！！