Very basic char device driver

hello.c

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/semaphore.h>
#include <asm/uaccess.h>


#define DEVICE_NAME "solidusdevice"


//this is our device
struct fake_device {
  char data[100];
  struct semaphore sem;
}virtual_device;


//
struct cdev* mcdev; // m stands for my
int major_number;
int ret;

dev_t dev_num;  // holds major number given by kernel



int device_open(struct inode* inode, struct file* filp)
{
  if (down_interruptible(&virtual_device.sem) != 0)
    {
      printk(KERN_ALERT "Could not lock the device");
      return -1;
    }
  printk(KERN_INFO "Opened device");
  return 0;
}


ssize_t device_read(struct file* filp, char* bufStoreData, size_t bufCount, loff_t* curOffset )
{
  printk(KERN_INFO "Reading from device");
  ret = copy_to_user(bufStoreData, virtual_device.data, bufCount);
  return ret;
}


ssize_t device_write(struct file* filp, const char* bufStoreData, size_t bufCount, loff_t* curOffset )
{
  printk(KERN_INFO "Writing to device");
  ret = copy_from_user(bufStoreData, virtual_device.data, bufCount);
  return ret;
}

int device_close(struct inode* inode, struct file* filp)
{
  up(&virtual_device.sem);
  printk(KERN_INFO "Device Closed");
  return 0;
}


struct file_operations fops = {
  .owner   =  THIS_MODULE,
  .open    =  device_open,
  .release =  device_close,
  .write   =  device_write,
  .read    =  device_read
};



static int driver_entry()
{
  ret = alloc_chrdev_region(&dev_num, 0,1,DEVICE_NAME);
  if(ret < 0)
    {
      printk(KERN_ALERT "Failed to allocate a major number\n" );
      return ret;
    }
  major_number = MAJOR(dev_num);
  printk(KERN_ALERT "Major number is: %d", major_number);

  mcdev         = cdev_alloc();
  mcdev-> ops   = &fops;
  mcdev-> owner = THIS_MODULE;

  ret = cdev_add(mcdev, dev_num, 1);
  if (ret < 0)
    {
      printk(KERN_ALERT "Unable to add the device to kernel");
      return ret;
    }

  sema_init(&virtual_device.sem, 1);
  return 0;
}



static void driver_exit()
{
  cdev_del(mcdev);
  unregister_chrdev_region(dev_num,1);
  printk(KERN_ALERT "Unloaded module");
}



module_init(driver_entry);
module_exit(driver_exit);