#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/interrupt.h>
#include <linux/proc_fs.h>
#include <linux/sched.h>
#include <asm/uaccess.h>
#include <asm/io.h>
#include <linux/slab.h>

#define STRING_INIT {0, 0, 0}

struct string {
	char *data;
	size_t len;
	size_t space;
};

void add(struct string* s, const char *addition, size_t count){
	size_t space_left = s->space - s->len;
	size_t addition_len = strlen(addition);
	if(count && addition_len > count) // Thanks Anna, the second part of this was backwards
		addition_len = count;
	if(space_left < addition_len){
		s->data = krealloc(s->data, s->len + addition_len, GFP_KERNEL);
		s->space = s->len + addition_len;
	}
	strcpy(s->data + s->len, addition); // This could cause a buffer overflow!
	s->len += addition_len;
}

void reserve(struct string* s, size_t capacity){
	if(capacity < s->len)
		return;
	s->data = krealloc(s->data, capacity, GFP_KERNEL);
	s->space = capacity;
}

void set_length_by_null(struct string* s){
	s->len = strlen(s->data);
}

#define PROC_FILE_NAME "realloc_demo"
struct string contents = STRING_INIT;

ssize_t read_simple(struct file *filp,char *buf,size_t count,loff_t *offp ) 
{
	size_t copysize = count;
	size_t potential_copy = contents.len - *offp;
	printk("READ:  buf = %lx, count = %lu, *loff_t = %llu\n", (long unsigned int)buf, count, *offp);
	if(*offp >= contents.len)
		return 0;
	if(potential_copy < count)
		copysize = potential_copy;
	memcpy(buf, contents.data + *offp, copysize);
	*offp += copysize;
	return copysize;
}

unsigned int call_count = 0;
ssize_t write_simple(struct file *filp,const char *buf,size_t count,loff_t *offp)
{
	add(&contents, buf, count);
	printk("WRITE:  buf = %lx, count = %lu, *loff_t = %llu, contents.len = %lu\n", (long unsigned int)buf, count, *offp, contents.len);
	*offp += count;
	return count;
}

struct proc_ops proc_fops = {
	proc_read: read_simple,
	proc_write: write_simple
};

int init_module (void) {
	proc_create(PROC_FILE_NAME,0,NULL,&proc_fops);
	return 0;
}

void cleanup_module(void) {
	remove_proc_entry(PROC_FILE_NAME,NULL);
}

MODULE_LICENSE("GPL");