我们在刚开始写Linux设备驱动程序的时候，很多时候都是利用mknod命令手动创建设备节点，实际上Linux内核为我们提供了一组函数，可以用来在模块加载的时候自动在/dev目录下创建相应设备节点，并在卸载模块时删除该节点，当然前提条件是用户空间移植了udev。

内核中定义了struct class结构体，顾名思义，一个struct class结构体类型变量对应一个类，内核同时提供了class_create(…)函数，可以用它来创建一个类，这个类存放于sysfs下面，一旦创建好了这个类，再调用device_create(…)函数来在/dev目录下创建相应的设备节点。这样，加载模块的时候，用户空间中的udev会自动响应device_create(…)函数，去/sysfs下寻找对应的类从而创建设备节点。

注意，在2.6较早的内核版本中，device_create(…)函数名称不同，是class_device_create(…)，所以在新的内核中编译以前的模块程序有时会报错，就是因为函数名称不同，而且里面的参数设置也有一些变化。

struct class和device_create(…) 以及device_create(…)都定义在/include/linux/device.h中，使用的时候一定要包含这个头文件，否则编译器会报错。

在2.6.26.6内核版本中，struct class定义在头文件include/linux/device.h中：

/*

      * device classes

      */

    struct class {

      const char        *name;

      struct module     *owner;

  nbsp;struct kset         subsys;

      struct list_head         devices;

      struct list_head         interfaces;

      struct kset              class_dirs;

      struct semaphore sem;    /* locks children, devices, interfaces */

      struct class_attribute   *class_attrs;

      struct device_attribute      *dev_attrs;

  int (*dev_uevent)(struct device *dev, struct kobj_uevent_env *env);

  void (*class_release)(struct class *class);

      void (*dev_release)(struct device *dev);

  int (*suspend)(struct device *dev, pm_message_t state);

      int (*resume)(struct device *dev);

};

class_create(…)在/drivers/base/class.c中实现：

     /**

    * class_create - create a struct class structure

    * @owner: pointer to the module that is to "own" this struct class

    * @name: pointer to a string for the name of this class.

    *

    * This is used to create a struct class pointer that can then be used

    * in calls to device_create().

    *

    * Note, the pointer created here is to be destroyed when finished by

    * making a call to class_destroy().

    */

   struct class *class_create(struct module *owner, const char *name)

   {

      struct class *cls;

      int retval;

      cls = kzalloc(sizeof(*cls), GFP_KERNEL);

      if (!cls) {

           retval = -ENOMEM;

           goto error;

      }

  cls->name = name;

      cls->owner = owner;

      cls->class_release = class_create_release;

  retval = class_register(cls);

      if (retval)

           goto error;

  return cls;

error:

      kfree(cls);

      return ERR_PTR(retval);

    }

第一个参数指定类的所有者是哪个模块，第二个参数指定类名。

在class.c中，还定义了class_destroy(…)函数，用于在模块卸载时删除类。

device_create(…)函数在/drivers/base/core.c中实现：

  /**

     * device_create - creates a device and registers it with sysfs

     * @class: pointer to the struct class that this device should be registered to

     * @parent: pointer to the parent struct device of this new device, if any

     * @devt: the dev_t for the char device to be added

     * @fmt: string for the device's name

     *

     * This function can be used by char device classes. A struct device

     * will be created in sysfs, registered to the specified class.

     *

     * A "dev" file will be created, showing the dev_t for the device, if

     * the dev_t is not 0,0.

     * If a pointer to a parent struct device is passed in, the newly created

     * struct device will be a child of that device in sysfs.

     * The pointer to the struct device will be returned from the call.

     * Any further sysfs files that might be required can be created using this

     * pointer.

     *

     * Note: the struct class passed to this function must have previously

     * been created with a call to class_create().

     */

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

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

    {

         va_list vargs;

         struct device *dev;

     va_start(vargs, fmt);

         dev = device_create_vargs(class, parent, devt, NULL, fmt, vargs);

         va_end(vargs);

         return dev;

    }

第一个参数指定所要创建的设备所从属的类，第二个参数是这个设备的父设备，如果没有就指定为NULL，第三个参数是设备号，第四个参数是设备名称，第五个参数是从设备号。

下面以一个简单字符设备驱动来展示如何使用这几个函数

 #include

    #include

    #include

    #include

    #include

    #include

MODULE_LICENSE ("GPL");

int hello_major = 555;

    int hello_minor = 0;

    int number_of_devices = 1;

struct cdev cdev;

    dev_t dev = 0;

struct file_operations hello_fops = {

      .owner = THIS_MODULE

    };

static void char_reg_setup_cdev (void)

    {

       int error, devno = MKDEV (hello_major, hello_minor);

       cdev_init (&cdev, &hello_fops);

       cdev.owner = THIS_MODULE;

       cdev.ops = &hello_fops;

       error = cdev_add (&cdev, devno , 1);

       if (error)

           printk (KERN_NOTICE "Error %d adding char_reg_setup_cdev", error);

}

struct class *my_class;

static int __init hello_2_init (void)

    {

       int result;

       dev = MKDEV (hello_major, hello_minor);

       result = register_chrdev_region (dev, number_of_devices, "hello");

       if (result<0) {

           printk (KERN_WARNING "hello: can't get major number %d/n", hello_major);

           return result;

     }

 char_reg_setup_cdev ();

 /* create your own class under /sysfs */

     my_class = class_create(THIS_MODULE, "my_class");

     if(IS_ERR(my_class))

     {

          printk("Err: failed in creating class./n");

          return -1;

      }

  /* register your own device in sysfs, and this will cause udev to create corresponding device node */

      device_create( my_class, NULL, MKDEV(hello_major, 0), "hello" "%d", 0 );

  printk (KERN_INFO "Registered character driver/n");

      return 0;

    }

static void __exit hello_2_exit (void)

    {

       dev_t devno = MKDEV (hello_major, hello_minor);

       cdev_del (&cdev);

   device_destroy(my_class, MKDEV(adc_major, 0));         //delete device node under /dev

       class_destroy(my_class);                               //delete class created by us

   unregister_chrdev_region (devno, number_of_devices);

   printk (KERN_INFO "char driver cleaned up/n");

    }

module_init (hello_2_init);

    module_exit (hello_2_exit);

这样，模块加载后，就能在/dev目录下找到hello0这个设备节点了。

例子2

drivers/i2c/i2c-dev.c

/*

 * module load/unload record keeping

 */

static int __init i2c_dev_init(void)

{

 int res;

 printk(KERN_INFO "i2c /dev entries driver/n");

 res = register_chrdev(I2C_MAJOR, "i2c", &i2cdev_fops);

 if (res)

  goto out;

 i2c_dev_class = class_create(THIS_MODULE, "i2c-dev");

 if (IS_ERR(i2c_dev_class)) {

  res = PTR_ERR(i2c_dev_class);

  goto out_unreg_chrdev;

 }

 res = i2c_add_driver(&i2cdev_driver);

 if (res)

  goto out_unreg_class;

 return 0;

out_unreg_class:

 class_destroy(i2c_dev_class);

out_unreg_chrdev:

 unregister_chrdev(I2C_MAJOR, "i2c");

out:

 printk(KERN_ERR "%s: Driver Initialisation failed/n", __FILE__);

 return res;

}

在 i2c_dev_init创建类i2c_dev_class

i2c_dev_class = class_create(THIS_MODULE, "i2c-dev");

static int i2cdev_attach_adapter(struct i2c_adapter *adap)

{

 struct i2c_dev *i2c_dev;

 int res;

 i2c_dev = get_free_i2c_dev(adap);

 if (IS_ERR(i2c_dev))

  return PTR_ERR(i2c_dev);

 /* register this i2c device with the driver core */

 i2c_dev->dev = device_create(i2c_dev_class, &adap->dev,

         MKDEV(I2C_MAJOR, adap->nr), NULL,

         "i2c-%d", adap->nr);

 if (IS_ERR(i2c_dev->dev)) {

  res = PTR_ERR(i2c_dev->dev);

  goto error;

 }

 res = device_create_file(i2c_dev->dev, &dev_attr_name);

 if (res)

  goto error_destroy;

 pr_debug("i2c-dev: adapter [%s] registered as minor %d/n",

   adap->name, adap->nr);

 return 0;

error_destroy:

 device_destroy(i2c_dev_class, MKDEV(I2C_MAJOR, adap->nr));

error:

 return_i2c_dev(i2c_dev);

 return res;

}

在i2cdev_attach_adapter调用device_create(i2c_dev_class, &adap->dev,

         MKDEV(I2C_MAJOR, adap->nr), NULL,

         "i2c-%d", adap->nr);

这样在dev目录就产生i2c-0 或i2c-1节点