【转】Linux内核的container_of宏

Linux内核的container_of宏

作用：

通过一个结构的成员的地址和结构名以及成员名，获得结构的首地址。

 

宏定义：

include/linux/kernel.h

/**
 * 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 - 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) );})

思路很简单：

ptr-(member在type内的偏移地址)

求member在type内的偏移地址有个小技巧，将地址0转换成一个member对象，然后这个对象的成员member的地址就是偏移量。

 

在宏的定义用用了一个C99的标准宏offsetof，它的作用是取得一个结构体中成员的偏移量。

一个比较常见的宏实现如下：

/**
 * offset_of - get the offset of a member to its struct
 * @type:   the name of the struct
 * @member: the name of the member
 */
#define offset_of(type,member) ({ /
    (size_t)&(((type*)0)->member);})

/**
 * offset_of - get the offset of a member to its struct
 * @type:	the name of the struct
 * @member:	the name of the member
 */
#define offset_of(type,member) ({ /
	(size_t)&(((type*)0)->member);})

 

在这个宏的最后一句，不能将__mptr转换成void*，因为void*指针不能参与算术运算。

 

这里是直接用typeof来定义类型。

比如这样：

const typeof(a) b;

const typeof(a) b;

 这是一个定义，将b定义成和a一样的类型，且是const的。