#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>

int main(int argc, char *argv[])
{
	int fd;
	unsigned char tally = 0;
	unsigned char next = 0;
	if (argc!=2) return -1;

	fd = open(argv[1], O_RDWR|O_APPEND);
	if (fd<0) return -2;

	while (read(fd, &next, 1)) {
		//printf("Tally: %X, next byte: %X\n", tally, next);
		tally ^= next;
	};

	printf("Final tally: %X\n", tally);

	write(fd, &tally, 1);

	close(fd);
	return 0;
}