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  作者：武汉华嵌嵌入式培训  长沙中心        讲师： 周龙

Step 1:  申请设备号（主要是申请主设备号）

  有两种方式:

⑴静态申请

 通过下面这个函数实现：

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

/* 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.*/

这种方式主要用于，驱动开发者事先知道该驱动主设备号的情况。

⑵动态申请

通过下面这个函数实现：

int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,      const char *name)

/* 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.*/

这种方式由系统动态分配一个设备号，返回的设备号保存在参数dev中。

Step 2 :注册字符设备

在linux 内核中用struct cdev表示一个字符设备。

字符设备的注册与注销分别通过下面的两个函数来实现：

int cdev_add(struct cdev *p, dev_t dev, unsigned count)；

/**

 * cdev_add() - add a char device to the system

 * @p: the cdev structure for the device

 * @dev: the first device number for which this device is responsible

 * @count: the number of consecutive minor numbers corresponding to this

 *         device

 *

 * cdev_add() adds the device represented by @p to the system, making it

 * live immediately.  A negative error code is returned on failure.

 */

void cdev_del(struct cdev *p)；

不过，在注册一个字符设备之前，要调用下面这个函数来初始化struct cdev结构体:

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

/**

 * 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().

 */

另外，struct cdev结构体变量可以声明为一个指针，内核提供了一个函数来申请：

struct cdev *cdev_alloc(void)；

step 3:创建设备节点

有两种方法：

一是通过mknod命令来创建。如：

  mknod /dev/yourname c major minor

其中“yourname”可以是任意符合unix下路径名的名字，不一定要是你代码里定义的驱动或设备的名字；c 表示创建字符设备节点，major是你成功申请的主设备号，minor是次设备号，这个可以是任意的（在次设备号范围内）

另外一种方法是通过udev自动生成。这种方法需要在你的代码里创建一个设备类，然后在这个设备类的基础上，创建一个设备；另外应用程序需要跑一个udevd的后台程序。

struct class*  class_create(owner, name);

struct device *device_create(struct class *class, struct device *parent,

   dev_t devt, void *drvdata, const char *fmt, ...)

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