Writing a Simple Character Device driver in Linux

A Character device driver needs a major number and a minor number. The devices are registered in the Kernel and it lies either in the /dev/ or in the /proc folder.

The following example uses a char device driver with major number 222 and a minor number 0. The name of the device driver namely “new_device

It uses the following things

  • Open or register a device
  • close or unregister the device
  • Reading from the device (Kernel to the userspace)
  • Writing to the device (userlevel to the kernel space)

There are three files, Copy the following or download all the three files here

/* new_dev.c*/

#include “new_dev.h”

MODULE_DESCRIPTION(“A simple char device”);

static int r_init(void);
static void r_cleanup(void);


static int r_init(void)
printk(“<1>failed to register”);
return 0;
static void r_cleanup(void)
return ;

/*new_dev.h */

* my device header file
#ifndef _NEW_DEVICE_H
#define _NEW_DEVICE_H

#include <linux/fs.h>
#include <linux/sched.h>
#include <linux/errno.h>
#include <asm/current.h>
#include <asm/segment.h>
#include <asm/uaccess.h>

char my_data[80]=”hi from kernel”; /* our device */

int my_open(struct inode *inode,struct file *filep);
int my_release(struct inode *inode,struct file *filep);
ssize_t my_read(struct file *filep,char *buff,size_t count,loff_t *offp );
ssize_t my_write(struct file *filep,const char *buff,size_t count,loff_t *offp );
struct file_operations my_fops={
open: my_open,
read: my_read,
write: my_write,

int my_open(struct inode *inode,struct file *filep)
/*MOD_INC_USE_COUNT;*/ /* increments usage count of module */
return 0;

int my_release(struct inode *inode,struct file *filep)
/*MOD_DEC_USE_COUNT;*/ /* decrements usage count of module */
return 0;
ssize_t my_read(struct file *filep,char *buff,size_t count,loff_t *offp )
/* function to copy kernel space buffer to user space*/
if ( copy_to_user(buff,my_data,strlen(my_data)) != 0 )
printk( “Kernel -> userspace copy failed!n” );
return strlen(my_data);

ssize_t my_write(struct file *filep,const char *buff,size_t count,loff_t *offp )
/* function to copy user space buffer to kernel space*/
if ( copy_from_user(my_data,buff,count) != 0 )
printk( “Userspace -> kernel copy failed!n” );
return 0;


obj-m += new_dev.o

make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

How to compile

Put all the three files in the same folder and execute the following commands

  • make (to compile the module)
  • insmod new_dev.ko (To insert the module)

Once the module is inserted, do the following

mknod /dev/new_device c 222 0 (Command to make an entry in the /dev/, once the device is created, go and see the /dev/ folder for the entry new_device)

cat /dev/new_device (The message will be printed which is from the kernel, that is read operation)

echo “This is a write information to the kernel” > /dev/new_device (This command is to perform the write operation)

After checking the read and write operation, just remove the module

rmmod new_dev.ko

(Source: http://linuxgazette.net/125/mishra.html)

6 thoughts on “Writing a Simple Character Device driver in Linux”

  1. Sir, how do we Implement a simple character device to initiate a communication between 2 pc's using RS-232 serial port.

  2. Hi Sir ,
    Can you please help me i need to write a platform driver for I2C bus , please give me some tips how to write platform driver .

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