my_dev.c

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#ifndef __KERNEL__
#define __KERNEL__
#endif

#ifndef MODULE
#define MODULE
#endif

#define __NO_VERSION__

#include <linux/kernel.h>
#include <linux/module.h>

#if CONFIG_MODVERSIONS == 1
#define MODVERSIONS
#include <linux/modversions.h>
#endif

#include <linux/fs.h>

#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif

/* Conditional compilation. LINUX_VERSION_CODE is 
 * the code (as per KERNEL_VERSION) of this version. */
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,2,0)
#include <asm/uaccess.h>  /* for put_user */
#endif


#define SUCCESS 0

#define DEVICE_NAME "kueng_char_dev"
#define BUF_LEN 50

static  int Device_Open =0;
static char Message[BUF_LEN];

static int Major;

static int mydev_open(struct inode *inode,struct file *file)
{
  static int counter = 0;
   if(Device_Open)
    return -EBUSY;
  
  Device_Open=1;    

  MOD_INC_USE_COUNT;
  return 0;
}

static int mydev_release(struct inode *inode,struct file *file)
{
  Device_Open=0;
  MOD_DEC_USE_COUNT;  
  return 0;
}

static ssize_t mydev_read(struct file *file,char *buffer, size_t length ,loff_t *f_pos)
{
  int bytes_read=0;
if(verify_area(VERIFY_WRITE,buffer,length)==-EFAULT)
    return -EFAULT; 

   copy_to_user(buffer,Message,length);
  return bytes_read;  
}

static ssize_t mydev_write(struct file *file, const char *buffer,size_t length,loff_t *f_pos)
{
  int i = BUF_LEN>length?BUF_LEN:length;
if(verify_area(VERIFY_READ,buffer,length)==-EFAULT)
    return -EFAULT;  
  copy_from_user(Message,buffer,i);
  return length;
}

struct  file_operations  Fops = { 
  release: mydev_release,
  open: mydev_open,
  read: mydev_read,
  write:mydev_write
};

int init_module(void)
{
  Major = register_chrdev(0,DEVICE_NAME,&Fops);
  if(Major<0){
    printk("%s device failed with %d\n","Sorry, registering the character",Major);
    return Major;
  }
    printk("%s The major device number is %d\n","Registeration is a success.",Major);
  return 0;
}

void cleanup_module(void)
{
  int ret;
  ret = unregister_chrdev(Major,DEVICE_NAME);
  if(ret<0)
    printk("Error in unregister_chrdev: %d\n",ret);
}
MODULE_LICENSE("GPL");
MODULE_AUTHOR("KUENG");