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           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。