linux内核学习list_for_each_entry(转)
2010-07-27 21:51
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在Linux内核源码中,经常要对链表进行操作,其中一个很重要的宏是list_for_each_entry:
意思大体如下:
假设只有两个结点,则第一个member代表head,
list_for_each_entry的作用就是循环遍历每一个pos中的member子项。
图1:
pos: pos:
___________ ____________
| | | |
| | | |
| ........... | | ................ |
| | | |
| | | |
| member: | _________|__> member |
| { | | | { |
| *prev; | | | *prev; |
| *next;--|---------- | *next;--------------------
| } | | } | |
|—^———— | |____________| |
| |
| |
|_____________________________________________|
宏list_for_each_entry:
/**
401 * list_for_each_entry - iterate over list of given type
402 * @pos: the type * to use as a loop cursor.
403 * @head: the head for your list.
404 * @member: the name of the list_struct within the struct.
405 */
406#define list_for_each_entry(pos, head, member) /
407 for (pos = list_entry((head)->next, typeof(*pos), member); /
408 prefetch(pos->member.next), &pos->member != (head); /
409 pos = list_entry(pos->member.next, typeof(*pos), member))
复制代码
list_entry((head)->next, typeof(*pos), member)返回(head)->next物理指针所处位置向前减去offsetof()个字节数据之后, 其父变量pos的物理地址,父变量的类型在编译时由typeof(*pos)自动返回.
所以list_for_each_entry遍历head下面挂接的类型为typeof(*pos)的childs结构体们,当然每个child结构体包含struct list_head node之类相似的双向链表list_head类型项,就这样通过循环pos将依次指向双向链表上的各个child.(member就是child类型中被定义的变量名)
其中用到了函数list_entry():
这个函数的作用在图1中表示就是可以通过已知的指向member子项的指针,获得整个结构体的指针(地址)
/**
329 * list_entry - get the struct for this entry
330 * @ptr: the &struct list_head pointer.
331 * @type: the type of the struct this is embedded in.
332 * @member: the name of the list_struct within the struct.
333 */
334#define list_entry(ptr, type, member) /
335 container_of(ptr, type, member)
复制代码
和函数prefetch:
#define prefetch(x) __builtin_prefetch(x)
复制代码
其中用到了builtin_prefetch:
prefetch的含义是告诉cpu那些元素有可能马上就要用到,告诉cpu预取一下,这样可以提高速度
其中用到了函数container_of():
/**
487 * container_of - cast a member of a structure out to the containing structure
488 * @ptr: the pointer to the member.
489 * @type: the type of the container struct this is embedded in.
490 * @member: the name of the member within the struct.
491 *
492 */
493#define container_of(ptr, type, member) ({ /
494 const typeof( ((type *)0)->member ) *__mptr = (ptr); /
495 (type *)( (char *)__mptr - offsetof(type,member) );})
其中又用到了offsetof()函数:
lxr上找到的源码:
#define offset_of(type, memb) /
47 ((unsigned long)(&((type *)0)->memb))
offsetof(TYPE, MEMBER)
该宏在Linux内核代码(版本2.6.22)中定义如下:
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER);
分析:
(TYPE *)0,将 0 强制转换为 TYPE 型指针,记 p = (TYPE *)0,p是
4000
指向TYPE的指针,它的值是0。那么 p->MEMBER 就是 MEMBER 这个元素了,而&(p->MEMBER)就是MENBER的地址,而基地址为0,这样就巧妙的转化为了TYPE中的偏移量。再把结果强制转 换为size_t型的就OK了,size_t其实也就是int。
typedef __kernel_size_t size_t;
typedef unsigned int __kernel_size_t;
可见,该宏的作用就是求出MEMBER在TYPE中的偏移量。
意思大体如下:
假设只有两个结点,则第一个member代表head,
list_for_each_entry的作用就是循环遍历每一个pos中的member子项。
图1:
pos: pos:
___________ ____________
| | | |
| | | |
| ........... | | ................ |
| | | |
| | | |
| member: | _________|__> member |
| { | | | { |
| *prev; | | | *prev; |
| *next;--|---------- | *next;--------------------
| } | | } | |
|—^———— | |____________| |
| |
| |
|_____________________________________________|
宏list_for_each_entry:
/**
401 * list_for_each_entry - iterate over list of given type
402 * @pos: the type * to use as a loop cursor.
403 * @head: the head for your list.
404 * @member: the name of the list_struct within the struct.
405 */
406#define list_for_each_entry(pos, head, member) /
407 for (pos = list_entry((head)->next, typeof(*pos), member); /
408 prefetch(pos->member.next), &pos->member != (head); /
409 pos = list_entry(pos->member.next, typeof(*pos), member))
复制代码
list_entry((head)->next, typeof(*pos), member)返回(head)->next物理指针所处位置向前减去offsetof()个字节数据之后, 其父变量pos的物理地址,父变量的类型在编译时由typeof(*pos)自动返回.
所以list_for_each_entry遍历head下面挂接的类型为typeof(*pos)的childs结构体们,当然每个child结构体包含struct list_head node之类相似的双向链表list_head类型项,就这样通过循环pos将依次指向双向链表上的各个child.(member就是child类型中被定义的变量名)
其中用到了函数list_entry():
这个函数的作用在图1中表示就是可以通过已知的指向member子项的指针,获得整个结构体的指针(地址)
/**
329 * list_entry - get the struct for this entry
330 * @ptr: the &struct list_head pointer.
331 * @type: the type of the struct this is embedded in.
332 * @member: the name of the list_struct within the struct.
333 */
334#define list_entry(ptr, type, member) /
335 container_of(ptr, type, member)
复制代码
和函数prefetch:
#define prefetch(x) __builtin_prefetch(x)
复制代码
其中用到了builtin_prefetch:
prefetch的含义是告诉cpu那些元素有可能马上就要用到,告诉cpu预取一下,这样可以提高速度
其中用到了函数container_of():
/**
487 * container_of - cast a member of a structure out to the containing structure
488 * @ptr: the pointer to the member.
489 * @type: the type of the container struct this is embedded in.
490 * @member: the name of the member within the struct.
491 *
492 */
493#define container_of(ptr, type, member) ({ /
494 const typeof( ((type *)0)->member ) *__mptr = (ptr); /
495 (type *)( (char *)__mptr - offsetof(type,member) );})
其中又用到了offsetof()函数:
lxr上找到的源码:
#define offset_of(type, memb) /
47 ((unsigned long)(&((type *)0)->memb))
offsetof(TYPE, MEMBER)
该宏在Linux内核代码(版本2.6.22)中定义如下:
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER);
分析:
(TYPE *)0,将 0 强制转换为 TYPE 型指针,记 p = (TYPE *)0,p是
4000
指向TYPE的指针,它的值是0。那么 p->MEMBER 就是 MEMBER 这个元素了,而&(p->MEMBER)就是MENBER的地址,而基地址为0,这样就巧妙的转化为了TYPE中的偏移量。再把结果强制转 换为size_t型的就OK了,size_t其实也就是int。
typedef __kernel_size_t size_t;
typedef unsigned int __kernel_size_t;
可见,该宏的作用就是求出MEMBER在TYPE中的偏移量。
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