container_of()

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#structure #struct #数据结构 #编译器

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container_of()

2008-11-19 10:52

/**
* 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) );})

#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
1.ptr为物理地址,其类型和member类型一致,最终使用typeof( ((type *)0)->member )
由编译器自动返回member的类型
2.type为包含member成员的结构体
3.offsetof(type,member)为member成员在type结构体中的偏移值,大小范围0~sizeof(type)字节
(因为以0地址为type类型数据结构的起始地址)
4.ptr- offsetof()就等于包含该ptr的type结构体父变量的物理起始地址,强制转换为(type*).

指针ptr指向结构体type中的成员member；通过指针ptr，返回结构体type的起始地址
          type
      |----------|
      |          |
      |          |
      |----------|
ptr-->| 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) );})

--------------------------------------------------
d = usb_device->dev.driver
container_of(d, struct usb_device_driver, drvwrap.driver)

     struct usb_device
|----------------------------|
|                            |
|                            |
|----------------------------|
|                            | struct device
|struct device_driver *driver|--+
|                            | -|
|----------------------------| -|
|                            | -|
|                            | -|
|----------------------------| -|
                                |
+-------------------------------+
|
|    struct usb_device_driver
| --|---------------------------|
|-- |                           |
|-- |                           |
| --|---------------------------|
+-->|struct device_driver driver| struct usbdrv_wrap drvwrap
    |int             for_devices|
    |---------------------------|
    |                           |
    |---------------------------|

--------------------------------------------

container_of宏，它的功能是得到包含某个结构成员的结构的指针:

其实现如下：

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

#define container_of(ptr, type, member) ({ /

const typeof( ((type *)0)->member ) *__mptr = (ptr); /

(type *)( (char *)__mptr - offsetof(type,member) );})

分析可知__mptr指向的是一个type结构里typeof(((type *)0)->member)类型member成员的指针，offsetof(type,member)是这个成员在结构中的偏移，单位是字节，所以为了计算type结构的起始地址，__mptr减去它自己的偏移。