struct cdev {

     struct kobject kobj;                    // 内嵌的kobject对象 

     struct module *owner;                   // 所属模块

     const struct file_operations *ops;      // 文件操作结构体

     struct list_head list;                  //linux内核所维护的链表指针

     dev_t dev;                              //设备号

     unsigned int count;                     //设备数目

 };

 #define MINORBITS    20

 #define MINORMASK    ((1U << MINORBITS) - 1)

 #define MAJOR(dev)    ((unsigned int) ((dev) >> MINORBITS))   //获得主设备号

 #define MINOR(dev)    ((unsigned int) ((dev) & MINORMASK))    //获得此设备号

 #define MKDEV(ma,mi)    (((ma) << MINORBITS) | (mi))          //由主次设备号得到设备号

 /**

  * cdev_init() - initialize a cdev structure

  * @cdev: the structure to initialize

  * @fops: the file_operations for this device

  *

  * Initializes @cdev, remembering @fops, making it ready to add to the

  * system with cdev_add().

  */

 void cdev_init(struct cdev *cdev, const struct file_operations *fops)

 {

     memset(cdev, , sizeof *cdev);

     INIT_LIST_HEAD(&cdev->list);

     kobject_init(&cdev->kobj, &ktype_cdev_default);

     cdev->ops = fops;

 }

 /**

  * cdev_alloc() - allocate a cdev structure

  *

  * Allocates and returns a cdev structure, or NULL on failure.

  */

 struct cdev *cdev_alloc(void)

 {

     struct cdev *p = kzalloc(sizeof(struct cdev), GFP_KERNEL);

     if (p) {

         INIT_LIST_HEAD(&p->list);

         kobject_init(&p->kobj, &ktype_cdev_dynamic);

     }

     return p;

 }

 /**

  * register_chrdev_region() - register a range of device numbers

  * @from: the first in the desired range of device numbers; must include

  *        the major number.

  * @count: the number of consecutive device numbers required

  * @name: the name of the device or driver.

  *

  * Return value is zero on success, a negative error code on failure.

  */

 int register_chrdev_region(dev_t from, unsigned count, const char *name)

 {

     struct char_device_struct *cd;

     dev_t to = from + count;

     dev_t n, next;

     for (n = from; n < to; n = next) {

         next = MKDEV(MAJOR(n)+, );

         if (next > to)

             next = to;

         cd = __register_chrdev_region(MAJOR(n), MINOR(n),

                    next - n, name);

         if (IS_ERR(cd))

             goto fail;

     }

     return ;

 fail:

     to = n;

     for (n = from; n < to; n = next) {

         next = MKDEV(MAJOR(n)+, );

         kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n));

     }

     return PTR_ERR(cd);

 }

 /**

  * alloc_chrdev_region() - register a range of char device numbers

  * @dev: output parameter for first assigned number

  * @baseminor: first of the requested range of minor numbers

  * @count: the number of minor numbers required

  * @name: the name of the associated device or driver

  *

  * Allocates a range of char device numbers.  The major number will be

  * chosen dynamically, and returned (along with the first minor number)

  * in @dev.  Returns zero or a negative error code.

  */

 int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,

             const char *name)

 {

     struct char_device_struct *cd;

     cd = __register_chrdev_region(, baseminor, count, name);

     if (IS_ERR(cd))

         return PTR_ERR(cd);

     *dev = MKDEV(cd->major, cd->baseminor);

     return ;

 }

 /**

  * unregister_chrdev_region() - return a range of device numbers

  * @from: the first in the range of numbers to unregister

  * @count: the number of device numbers to unregister

  *

  * This function will unregister a range of @count device numbers,

  * starting with @from.  The caller should normally be the one who

  * allocated those numbers in the first place...

  */

 void unregister_chrdev_region(dev_t from, unsigned count)

 {

     dev_t to = from + count;

     dev_t n, next;

     for (n = from; n < to; n = next) {

         next = MKDEV(MAJOR(n)+, );

         if (next > to)

             next = to;

         kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n));

     }

 }