The document describes how to create a simple character device driver for Linux. It involves writing C code for file operations like open, read, write and close. The code is compiled into a kernel module which is loaded and tested. Key steps include creating files for the driver code, adding a Makefile, building the kernel object, loading the module, creating a device file, and verifying the file operations by reading kernel logs. The module can then be unloaded after testing is completed.

1. CREATION OF SIMPLE CHARACTER DEVICE DRIVER - PC
Character device driver for PC
Create simple code for character device driver:
Create a new directory inside the source code and create files in this directory:
$ vi char_drv.c
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/fs.h>
int file_open (struct inode *pinode, struct file *pfile)
{
printk(KERN_INFO "Inside the %s functionn",__FUNCTION__);
return 0;
}
size_t file_read (struct file *pfile, char __user *buffer, size_t
length, loff_t *offset)
{
printk(KERN_INFO "Inside the %s functionn",__FUNCTION__);
return 0;
}
ssize_t file_write (struct file *pfile, const char __user *buffer,
size_t length, loff_t *offset)
{
printk(KERN_INFO "Inside the %s functionn",__FUNCTION__);
return length;
}
int file_close (struct inode *pinode, struct file *pfile)
{
printk(KERN_INFO "Inside the %s functionn",__FUNCTION__);
return 0;
}
/* To hold the file operations performed on the device */
struct file_operations fops = {
.owner = THIS_MODULE,
.open = file_open,
.read = file_read,
.write = file_write,
.release = file_close,
};
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2. CREATION OF SIMPLE CHARACTER DEVICE DRIVER - PC
int char_drv_init(void)
{
printk(KERN_INFO "Inside the %s functionn",__FUNCTION__);
/* Register with the kernel and indicate that we are
registering character device driver */
register_chrdev(240 /* Major Number */,
"char_drv" /* Name of the driver */,
&fops /* file operations */);
return 0;
}
void char_drv_exit(void)
{
printk(KERN_INFO "Inside the %s functionn",__FUNCTION__);
/* unregister the character device driver */
unregister_chrdev(240, "char_drv");
}
module_init(char_drv_init);
module_exit(char_drv_exit);
Create Makefile:
$ vi Makefile
obj-m += char_drv.o
Building the Driver:
$ make -c /lib/modules/$(uname -r)/build M=$PWD modules
after using the above command, you get kernel object file “char_dev.ko”.
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3. CREATION OF SIMPLE CHARACTER DEVICE DRIVER - PC
Inserting the char_drv module:
$ sudo insmod ./char_drv.ko
Now you can check the driver get loaded by using below command
$ lsmod | grep char
char_drv 12767 0
To check the major number we used for char_drv, found using the command:
$ cat /proc/devices
It will list all the character devices and block devices. First column is Major number
and second column is drivers.
240 char_drv
we sucessfully loaded our driver and is linked to the major number 240.
We are going to do device entry here:
$ sudo mknod -m 666 /dev/char_drv c 240 0
mknode ---> make node
666 ---> read/write permission
/dev/char_drv ---> name of the driver is char_drv
c ---> represent character driver
240 ---> Major Number
0 ---> Minor Number
$ ls -l /dev/char_drv
crw-rw-rw- 1 root root 240 , 0 Jun 27 15:20 /dev/char_drv
Verifying the File Operations: open, read, write, close:
Now open two terminal namely terminal1 and terminal2.
Terminal2 – commands used to check the functions name invoked at that time which
is display in terminal1.
Terminal1 - displays the kernel messages.
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4. CREATION OF SIMPLE CHARACTER DEVICE DRIVER - PC
Terminal 2 :
$ sudo insmod ./char_drv.ko ---------->
$ cat /dev/char_drv ----------->
“cat” command usually opens the file,
reads it then displays and at last close the
file.
$ echo “testing” > /dev/char_drv ------->
“echo” command usually opens the file
as /dev/char_drv, writes testing in to this
file and then closes the file.
$ sudo rmmod char_drv --------->
Terminal 1:
$ sudo tail -f /var/log/syslog
Inside the char_drv_init function
Inside the file_open function
Inside the file_read function
Inside the file_close function
Inside the file_open function
Inside the file_write function
Inside the file_close function
Inside the char_drv_exit function
Removing the Module:
To remove the module, use the command as below:
$ sudo rmmod char_drv
your module is successfully unloaded.
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