作者：刘洪涛，华清远见嵌入式培训中心讲师。

在Linux2.6内核中，devfs被认为是过时的方法，并最终被抛弃，udev取代了它。Devfs的一个很重要的特点就是可以动态创建设备结点。那我们现在如何通过udev和sys文件系统动态创建设备结点呢？

下面通过一个实例，说明udev、sys动态创建设备结点的方法。注意代码中红色的部分是为了实现动态创建设备结点添加的。

#include <linux/module.h>

#include <linux/kernel.h>

#include <linux/init.h>

#include <linux/fs.h>

#include <linux/cdev.h>

#include <asm/uaccess.h>

#include <linux/devICe.h>

MODULE_LICENSE ("GPL");

int hello_major = 252;

int hello_minor = 0;

int number_of_devices = 1;

char data[50]="foobar not equal to barfoo";

struct cdev cdev;

dev_t dev = 0;

staTIc int hello_open (struct inode *inode, struct file *file)

{

printk (KERN_INFO "Hey! device opened\n");

return 0;

}

static int hello_release (struct inode *inode, struct file *file)

{

printk (KERN_INFO "Hmmm... device closed\n");

return 0;

}

ssize_t hello_read (struct file *filp, char *buff, size_t count, loff_t *offp)

{

ssize_t result = 0;

if (copy_to_user (buff, data, sizeof(data)-1))

result = -EFAULT;

else

printk (KERN_INFO "wrote %d bytes\n", count);

return result;

}

ssize_t hello_write (struct file *filp, const char *buf, size_t count, loff_t *f_pos)

{

ssize_t ret = 0;

printk (KERN_INFO "Writing %d bytes\n", count);

if (count>127) return -ENOMEM;

if (count<0) return -EINVAL;

if (copy_from_user (data, buf, count)) {

ret = -EFAULT;

}

else {

data[127]="\0";

printk (KERN_INFO"Received: %s\n", data);

ret = count;

}

return ret;

}

struct file_operations hello_fops = {

. wner = THIS_MODULE,

. pen = hello_open,

. release = hello_release,

. read = hello_read,

. write = hello_write

};

struct class *my_class;

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);

/* creating your own class */

my_class =class_create(THIS_MODULE, "farsight_class");//add by lht

if(IS_ERR(my_class)) {

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

return ;

}

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

class_device_create(my_class,NULL, devno, NULL,"farsight_dev");

// device_create(my_class,NULL, devno,"farsight_dev");

}

static int __init hello_2_init (void)

{

int result;

dev = MKDEV (hello_major, hello_minor);

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

if (result<0) {

printk (KERN_WARNING "hello: can"t get major number %d\n", hello_major);

return result;

}

char_reg_setup_cdev ();

printk (KERN_INFO "char device registered\n");

return 0;

}

static void __exit hello_2_exit (void)

{

dev_t devno = MKDEV (hello_major, hello_minor);

cdev_del (&cdev);

unregister_chrdev_region (devno, number_of_devices);

class_device_destroy(my_class, devno);

class_destroy(my_class);

}

module_init (hello_2_init);

module_exit (hello_2_exit);v

在编译了驱动后，可以查看/dev/farsight_dev设备结点，和 /sys/class/farsight_class/farsight_dev/ 本代码的测试环境是Ubantu7.04，内核版本是2.6.20-15-generi。在不同版本的内核中，有些系统函数的参数可能不太一样。

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