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

void over_write_last_byte(char *path, unsigned char byte);

int main(int argc, char *argv[]) {
    if (argc <= 2) {
        fprintf(stderr, "Usage: %s <filename> <byte>\n", argv[0]);
        return 1;
    }
    unsigned char byte = strtoul(argv[2], NULL, 0);
    over_write_last_byte(argv[1],byte);
    return 0;
}

// Given a file path and a byte, 
// modify the file so the final byte is replaced with a given value
// If the file is empty do nothing
// If the file can't be opened call perror and exit
void over_write_last_byte(char *path, unsigned char byte) {
    int f = open(path, O_WRONLY);

    if (lseek(f, -1, SEEK_END) >= 0) {
        write(f, &byte, 1);
    }

    close(f);
}