#include <stdio.h>
#include <stdint.h>
#include <fcntl.h>
#include <unistd.h>
#include <netinet/in.h>

#define BB_SIZE (2*512)

int main(int argc, char **argv)
{
        int fd;
        unsigned int i;
        uint32_t sum;
        uint8_t bootblock[BB_SIZE];
        uint32_t *p = (uint32_t *)bootblock;
        fd = open(argv[1], O_RDWR);
        if (fd < 0) {
                return 1;
        }
        if (read(fd, bootblock, BB_SIZE) < BB_SIZE) {
                perror("read");
                return 1;
        }
        if (ntohl(p[0]) != 'DOS\0') {
                fprintf(stderr, "bad bootblock signature!\n");
                return 1;
        }
        p[1] = 0;
        for (sum = 0, i = 0; i < (BB_SIZE)/sizeof(uint32_t); i++) {
                uint32_t old = sum;
                // big endian
                sum += ntohl(*p++);
                // overflow
                if (sum < old)
                        sum++;
        }
        sum = ~sum;
        //fprintf(stderr, "checksum: 0x%lx\n", (long)sum);
        // big endian
        ((uint32_t *)bootblock)[1] = htonl(sum);
        lseek(fd, 0LL, SEEK_SET);
        write(fd, bootblock, BB_SIZE);
        return 0;
}