#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <string.h>

#define DEVICE_NAME "nulll"

static int capacity = 0;
static int used = 0;
module_param(capacity, int, S_IRUGO);

static int major_number;

static ssize_t device_write( struct file *, const char *, size_t, loff_t * );

static struct file_operations fops = {
        .write = device_write
};

static int __init test_init (void) {
    major_number = register_chrdev(0, DEVICE_NAME, &fops);

    if (major_number < 0) {
        printk("Register failed with %d\n", major_number);
        return major_number;
    }
    return 0;
}

static void __exit test_exit (void) {
    unregister_chrdev( major_number, DEVICE_NAME );
}

module_init( test_init );
module_exit( test_exit );

static ssize_t device_write (struct file *filp, const char *buff, size_t len, loff_t * off) {
    if (capacity != 0 && (used + len) > capacity) return ENOSPC;
    capacity += len;
    return 0;
}