container_of()

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。