从linux 内核2.6的某个版本之后，devfs不复存在，udev成为devfs的替代。相比devfs，udev有很多优势，在此就不罗嗦了，提醒一 点，udev是应用层的东东，不要试图在内核的配置选项里找到它;加入对udev的支持很简单，以作者所写的一个字符设备驱动为例，在驱动初始化的代码里调用class_create为该设备创建一个class，再为每个设备调用 class_device_create创建对应的设备。(不太明白什么是devfs,udev,对linux中的文件系统没有什么概念呢。)

大致用法如下：
struct class *myclass = class_create(THIS_MODULE, “my_device_driver”);
class_device_create(myclass, NULL, MKDEV(major_num, 0), NULL, “my_device”);
这样的module被加载时，udev daemon就会自动在/dev下创建my_device设备文件。

class_create()
-------------------------------------------------
linux-2.6.22/include/linux/device.h
struct class *class_create(struct module *owner, const char *name)
    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.
在/sys/class/下创建类目录

class_device_create()
-------------------------------------------------
linux-2.6.22/include/linux/device.h
struct class_device *class_device_create(struct class        *cls,
                                         struct class_device *parent,
                                         dev_t               devt,
                                         struct device       *device,
                                         const char          *fmt, ...)

class_device_create - creates a class device and registers it with sysfs
    @cls: pointer to the struct class that this device should be registered to.
    @parent: pointer to the parent struct class_device of this new device, if any.
    @devt: the dev_t for the char device to be added.
    @device: a pointer to a struct device that is assiociated with this class device.
    @fmt: string for the class device's name

 

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 <linux/module.h>
    #include <linux/kernel.h>
    #include <linux/init.h>
    #include <linux/fs.h>
    #include <linux/cdev.h>
    #include <linux/device.h>

    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");  //创建一个名称为i2c-dev的class
     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

class_device_create（）；
class_device_destroy（）；

在2.6.27中变为：
device_create（）
device_destroy（）