example of char device driver

--------------------
 char_dev.c
--------------------

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h> // required for various structures related to files liked fops.
#include <asm/uaccess.h> // required for copy_from and copy_to user functions
#include <linux/semaphore.h>
#include <linux/cdev.h>
static int Major;
dev_t dev_no,dev;
struct device
{
char array[100];
struct semaphore sem;
}char_dev;

int open(struct inode *inode, struct file *filp)
 {
        printk(KERN_INFO "Inside open \n");
        if(down_interruptible(&char_dev.sem)) {
        printk(KERN_INFO " could not hold semaphore");
        return -1;
  }
 return 0;
}

int release(struct inode *inode, struct file *filp)
 {
        printk (KERN_INFO "Inside close \n");
        printk(KERN_INFO "Releasing semaphore");
        up(&char_dev.sem);
        return 0;
}

ssize_t read(struct file *filp, char *buff, size_t count, loff_t *offp)
{
       unsigned long ret;
       printk("Inside read \n");
       ret = copy_to_user(buff, char_dev.array, count);
       return ret;
}

ssize_t write(struct file *filp, const char *buff, size_t count, loff_t *offp)
{
       unsigned long ret;
       printk(KERN_INFO "Inside write \n");
       ret = copy_from_user(char_dev.array, buff, count);
       return count;
}

struct file_operations fops = {
 read:  read,
 write:  write,
 open:   open,
 release: release
};

struct cdev *kernel_cdev;

int char_dev_init (void)
{
      int ret;
      kernel_cdev = cdev_alloc();
      kernel_cdev->ops = &fops;
      kernel_cdev->owner = THIS_MODULE;
      printk (" Inside init module\n");
      ret = alloc_chrdev_region( &dev_no , 0, 1,"chr_arr_dev");
      if (ret < 0) {
      printk("Major number allocation is failed\n");
      return ret;
 }

Major = MAJOR(dev_no);
dev = MKDEV(Major,0);
sema_init(&char_dev.sem,1);
printk (" The major number for your device is %d\n", Major);
ret = cdev_add( kernel_cdev,dev,1);
if(ret < 0 )
{
     printk(KERN_INFO "Unable to allocate cdev");
     return ret;
}
     return 0;
}

void char_dev_cleanup(void)
{
     printk(KERN_INFO " Inside cleanup_module\n");
     cdev_del(kernel_cdev);
     unregister_chrdev_region(Major, 1);
}

MODULE_LICENSE("GPL");
module_init(char_dev_init);
module_exit(char_dev_cleanup);
-------------
Makefile :
-------------

ifneq ($(KERNELRELEASE),)
   obj-m := char_dev.o
else
KERNELDIR ?= /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)
default:
        $(MAKE) -C $(KERNELDIR) M=$(PWD) modules
endif

----------------
creating a node:
----------------
mknod /dev/char_dev c 250 0



User Application :-
-------------------
main.c
------------------

#include <stdio.h>
#include <fcntl.h>

main ( )
 {
        int i,fd;
        char ch, write_buf[100], read_buf[100];
        fd = open("/dev/char_dev", O_RDWR);
        if (fd == -1)
        {
                printf("Error in opening file \n");
                exit(-1);
        }
        printf ("Press r to read from device or w to write the device ");
        scanf ("%c", &ch);

        switch (ch) {
                case 'w':
                       printf (" Enter the data to be written into device");
                        scanf (" %[^\n]", write_buf);
                        write(fd, write_buf, sizeof(write_buf));
                        break;
                case 'r':
                        read(fd, read_buf, sizeof(read_buf));
                        printf ("The data in the device is %s\n", read_buf);
                        break;
                default:
                        printf("Wrong choice \n");
                        break;
        }
        close(fd);
}
------------------------------
Compiling User Application :
------------------------------
$gcc -o main main.c

#./main
Press r to read from device or w to write the device : w
Enter the data to be written into device  : American Megatrends Inc.

#./main
Press r to read from device or w to write the device : r
The data in the device is : American Megatrends Inc.