My first kernel module

crapmod.c

#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/uaccess.h>


static ssize_t dev_read(struct file *fptr, char __user *buff, size_t count, loff_t *position);
static struct file_operations fops = {
  .owner = THIS_MODULE,
  .read = dev_read,
};

static int dev_major = 0;
static char* dev_name = "crap";

static int init(void)
{
  int major = register_chrdev (0, dev_name, &fops);
  if (major < 0)
    return -ENOSYS;
  printk( KERN_NOTICE "Got dynamically allocated major number %i", major );
  dev_major = major;

  return  0;
}

static void exit(void)
{
  if (dev_major != 0) {
    printk( KERN_NOTICE "Releasing dynamically allocated major number %i", dev_major );
    unregister_chrdev(dev_major, dev_name);
  }
  return;
}

static char* msg = "ABSOLUTE UNIT\n";
static int msg_size = sizeof("ABSOLUTE UNIT\n")-1;

static ssize_t dev_read(struct file *fptr, char __user *buff, size_t count, loff_t *position) {
  printk( KERN_NOTICE "Got read for size %i at offset %i", count, *position );
  if (*position >= msg_size)
    return 0; //Nothing left to read.

  int remaining = msg_size - *position;

  if (remaining <= count) {
    if(copy_to_user(buff, msg + *position, remaining) != 0)
      return -EFAULT;
    *position += remaining;
    return remaining;
  }

  // Must not fit into buffer.
  if(copy_to_user(buff, msg + *position, count) != 0)
    return -EFAULT;
  *position += count;
  return count;
}

MODULE_LICENSE("GPL");
module_init(init);
module_exit(exit);

Makefile

obj-m := crapmod.o
module_name-objs := crapmod.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

load: all
	sudo insmod ./crapmod.ko

unload:
	sudo rmmod crapmod