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

uint8_t sector[512];

int main(int argc, char **argv)
{
        int fd, i;
        uint16_t sum;
        uint8_t *p = sector;
        fd = open(argv[1], O_RDWR);
        if (fd < 0) {
                return 1;
        }
        if (read(fd, sector, 512-2) < 512-2) {
                perror("read");
                return 1;
        }
        for (sum = 0, i = 0; i < (512-2)/2; i++) {
                uint16_t v;
                v = *p++ << 8;
                v += *p++;
                sum += v;
        }
        sum = 0x1234 - sum /*+ 1*/;
        //sum = 0xaa55;
        // big endian
        *p++ = (uint8_t)(sum >> 8);
        *p++ = (uint8_t)sum;
        //lseek(fd, 0LL, SEEK_SET);
        write(fd, &sector[512-2], 2);
        close(fd);
        return 0;
}