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)