writing driver - "Failed to register device class" - c

Question

I am trying to write a character device driver based on this tutorial.
But when trying to insert my module I get the following error:
$ sudo insmod modules/main/main.ko
insmod: ERROR: could not insert module modules/main/main.ko: Operation not permitted
$dmesg
[ 1085.409026] Failed to register device class
This is the corresponding code snippet:
#ifndef __KERNEL__
#define __KERNEL__
#endif
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/device.h>
#define CLASS_NAME "my_class"
#define DEV_NAME "my_device"
static int dev_open(struct inode *, struct file *);
static int dev_release(struct inode *, struct file *);
static ssize_t dev_read(struct file *, char *, size_t, loff_t *);
static ssize_t dev_write(struct file *, const char *, size_t, loff_t *);
static int myCharClass;
static int myCharDevice;
int majorNumber;
static struct file_operations fops =
{
.open = dev_open,
.read = dev_read,
.write = dev_write,
.release = dev_release
};
int init_module(void)
{
printk("Hello world - DAO \n");
majorNumber = register_chrdev(0, DEV_NAME, &fops);
if (majorNumber < 0)
{
printk("Error when registering character device driver.\n");
return -1;
}
myCharClass = class_create(THIS_MODULE, CLASS_NAME);
if (IS_ERR(myCharClass))
{
unregister_chrdev(majorNumber, DEV_NAME);
printk(KERN_ALERT "Failed to register device class\n");
return -1;
}
myCharDevice = device_create(myCharClass, NULL, MKDEV(majorNumber, 0), NULL, DEV_NAME);
if (IS_ERR(myCharDevice))
{
class_destroy(myCharClass);
unregister_chrdev(majorNumber, DEV_NAME);
printk(KERN_ALERT "Failed to create the device\n");
return -1;
}
return 0;
}
void cleanup_module(void)
{
printk("<1> Ciao - DAO\n");
}
static int dev_open(struct inode *p_inode, struct file *p_file)
{
printk("Opened device.\n");
return 0;
}
static ssize_t dev_read(struct file *filep, char *buffer, size_t len, loff_t *offset)
{
printk("Reading device.\n");
return 0;
}
static ssize_t dev_write(struct file *filep, const char *buffer, size_t len, loff_t *offset)
{
printk("Writing device\n");
return 0;
}
static int dev_release(struct inode *inodep, struct file *filep)
{
printk("Releasing device.\n");
return 0;
}
MODULE_LICENSE("GPL");
Does anybody know what I am doing incorrectly?
I had a look at this post, but it didn't really help me, because I didn't really what the issue is.
Thanks