Skip to main content

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<linux/module.h>
#include<linux/init.h>
#include "new_dev.h"

MODULE_AUTHOR("PRADEEPKUMAR");
MODULE_DESCRIPTION("A simple char device");

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

module_init(r_init);
module_exit(r_cleanup);

static int r_init(void)
{
printk("<1>hi\n");
if(register_chrdev(222,"new_device",&my_fops)){
printk("<1>failed to register");
}
return 0;
}
static void r_cleanup(void)
{
printk("<1>bye\n");
unregister_chrdev(222,"new_device");
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,
release:my_release,
};

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;
}
#endif

Makefile

obj-m += new_dev.o

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

clean:
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)

Comments

  1. [...] This post was mentioned on Twitter by Pradeepkumar, Pradeepkumar. Pradeepkumar said: Writing a Simple Character Device driver in Linux http://goo.gl/fb/KO9U [...]

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

    ReplyDelete
  3. [...] http://www.pradeepkumar.org/2010/03/writing-a-simple-character-device-driver-in-linux.html [...]

    ReplyDelete
  4. 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 .

    ReplyDelete
  5. This was very helpful for me..thanks for the posting..

    ReplyDelete
  6. Thanks Sir, this was helpful to me

    ReplyDelete

Post a Comment

Popular posts from this blog

Installation of NS2 (ns-2.35) in Ubuntu 20.04

Installation of NS2 (ns-2.35) in Ubuntu 20.04 LTS Step 1: Install the basic libraries like      $] sudo apt install build-essential autoconf automake libxmu-dev Step 2: install gcc-4.8 and g++-4.8 open the file using sudo mode $] sudo nano /etc/apt/sources.list Include the following line deb http://in.archive.ubuntu.com/ubuntu bionic main universe $] sudo apt update $] sudo apt install gcc-4.8 g++-4.8 Step 3:  Unzip the ns2 packages to home folder $] tar zxvf ns-allinone-2.35.tar.gz $] cd ns-allinone-2.35/ns-2.35 Modify the following make files. ~ns-2.35/Makefile.in Change @CC@ to gcc-4.8 change @CXX@ to g++-4.8 ~nam-1.15/Makefile.in ~xgraph-12.2/Makefile.in ~otcl-1.14/Makefile.in Change in all places  @CC@ to gcc-4.8 @CPP@ or @CXX@ to g++-4.8 open the file: ~ns-2.35/linkstate/ls.h Change at the Line no 137  void eraseAll() { erase(baseMap::begin(), baseMap::end()); } to This void eraseAll() { this->erase(baseMap::begin(), baseMap::end()); } All changes made Step 4: Open a new termi

Installing ns-3.34 in Ubuntu 20.04

This post shows how to install ns 3.34 in Ubuntu 20.04 LTS Prerequisites: Fresh installation of Ubuntu Version 20.04 LTS  ns3.34 can be downloaded from here Follow the video link for complete step by step instructions on the installation.  This version fixes the compilation issues of vanet-routing-compare.cc (bug in ns3.33)  Issue the following commands after opening a terminal  $ sudo apt update $ sudo apt install g++ python3 python3-dev python-dev pkg-config sqlite3 python3-setuptools git qt5-default gir1.2-goocanvas-2.0 python3-gi python3-gi-cairo python3-pygraphviz gir1.2-gtk-3.0 ipython3 openmpi-bin openmpi-common openmpi-doc libopenmpi-dev autoconf cvs bzr unrar openmpi-bin openmpi-common openmpi-doc libopenmpi-dev tcpdump wireshark libxml2 libxml2-dev Unzip or untar the ns-allinone-3.34.tar.bz2 in the home folder (in my case its /home/pradeepkumar) $ cd ns-allinone-3.34/ $ ./build.py --enable-examples --enable-tests  Once the installation is completed, you may get an output show

Installation of ns3 in Windows 10 and Windows 11 OS using WSL (Windows Subsystem for Linux)

This post shows how to install ns-3.33 in Windows 10 through WSL (Windows Subsystem for Linux) This posts works for Windows 11 also (I have tested it on a Windows 11 ISO and it works the Same way as mentioned in the following post.) This post will work for ns-3.3x version. Prerequisites : Install Windows Subsystem for Linux with GUI: Please refer the following video  System Information: OS used: Windows 10 and WSL (Ubuntu 20.04) GUI: XServer for Windows NS3 Version: ns-3.33 See the following complete video on how to install ns3 in Windows 10 Step 0 : Open XLaunch Step 1 :  Open WSL using PowerShell and open it as Administrator Command:/  wsl $ xfce4-session The GUI of Ubuntu Opens within Windows 10 OS. Step 2 : Download ns3 from nsnam.org website through Mozilla Firefox browser Step 3: Open a Terminal  $ sudo apt update $ sudo apt install build-essential autoconf automake libxmu-dev python3-pygraphviz cvs mercurial bzr git cmake p7zip-full python3-matplotlib python-tk python3-dev qt5-q