#include <stdio.h>
#include <fcntl.h>
#include <sys/types.h>

int main(int argc, char **argv)
{
  int fd;
  int ret;
  char buf[512];

  if (argc != 2)
    errx(1, "usage: %s device", argv[0]);
  printf("opening raw device %s\n", argv[1]);
  fd = open(argv[1], O_RDWR);
  if (fd < 0)
    err(1, "open");
  ret = read(fd, buf, sizeof(buf));
  if (ret != sizeof(buf))
    err(1, "read");
  printf("magic code: 0x%02hhx%02hhx\n", buf[510], buf[511]);
  if (*(unsigned short *)&buf[510] != htons(0x55aa))
    errx(1, "magic code");
  if (memcmp(&buf[0x03], "OpenBSD", 7) ||
      memcmp(&buf[0x2b], "UNIX", 4) ||
      memcmp(&buf[0x30], "LABEL\0UFS 4.4\0", 14))
    errx(1, "not an OpenBSD filesystem");
  if (*(unsigned int *)&buf[0x56] != htonl(0xb402cd16) ||
      buf[0x5a] != 0x0c ||
      *(unsigned int *)&buf[0x5c] != htonl(0xa8037403) ||
      buf[0x60] != 0x4e ||
      *(unsigned short *)&buf[0x61] != htons(0x30f6))
    errx(1, "bad opcode");
  printf("old force_chs value: 0x%02hhx\n", buf[0x5b]);
  if (buf[0x5b] != 0x00)
    errx(1, "force_chs value seems to already be set... exit!");
  printf("set it to 0x01...\n");
  buf[0x5b] = 0x01;
  printf("writing new 512-bytes block...\n");
  ret = lseek(fd, 0, SEEK_SET);
  if (ret == -1)
    err(1, "lseek");
  ret = write(fd, buf, sizeof(buf));
  if (ret != sizeof(buf))
    err(1, "write");
  printf("success!\n");
}