Accumulator as a Linux kernel module

This is a toy linux kernel module that implements a character device (123, 0) that keeps an integer initialized to 0 at module load. Writing an integer (as a string) to the device will add the written integer to the kept integer. Reading the device will give the current value of the kept integer as a string.

Setup

$ make
$ sudo insmod accumulator.ko
$ sudo mknod /dev/accumulator c 123 0
$ sudo chmod ga+w /dev/accumulator

Run

$ cat /dev/accumulator
0
$ echo 2 > /dev/accumulator
$ cat /dev/accumulator
2
$ echo 2 > /dev/accumulator
$ cat /dev/accumulator
4
$ echo -5 > /dev/accumulator
$ cat /dev/accumulator
-1