1 #include <stdio.h>
2 #include <stdint.h>
3 #include <fcntl.h>
4 #include <unistd.h>
5 #include <netinet/in.h>
6
7 #define BB_SIZE (2*512)
8
main(int argc,char ** argv)9 int main(int argc, char **argv)
10 {
11 int fd;
12 unsigned int i;
13 uint32_t sum;
14 uint8_t bootblock[BB_SIZE];
15 uint32_t *p = (uint32_t *)bootblock;
16 fd = open(argv[1], O_RDWR);
17 if (fd < 0) {
18 return 1;
19 }
20 if (read(fd, bootblock, BB_SIZE) < BB_SIZE) {
21 perror("read");
22 return 1;
23 }
24 if (ntohl(p[0]) != 'DOS\0') {
25 fprintf(stderr, "bad bootblock signature!\n");
26 return 1;
27 }
28 p[1] = 0;
29 for (sum = 0, i = 0; i < (BB_SIZE)/sizeof(uint32_t); i++) {
30 uint32_t old = sum;
31 // big endian
32 sum += ntohl(*p++);
33 // overflow
34 if (sum < old)
35 sum++;
36 }
37 sum = ~sum;
38 //fprintf(stderr, "checksum: 0x%lx\n", (long)sum);
39 // big endian
40 ((uint32_t *)bootblock)[1] = htonl(sum);
41 lseek(fd, 0LL, SEEK_SET);
42 write(fd, bootblock, BB_SIZE);
43 return 0;
44 }
45