settai.c

/* settai -- update kernel's TAI-UTC offset from tzdata
 * klg, Oct 2018 */
#define _POSIX_C_SOURCE 200809L
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <sys/timex.h>

static void set_utctz(const char *tz) {
  if (setenv("TZ", tz, 1) < 0) {
    perror("setenv");
    exit(EXIT_FAILURE);
  }
  tzset();
  if (tzname[0][0] != 'U' || tzname[0][1] != 'T' ||
    tzname[0][2] != 'C' || tzname[0][3] != 0) {
    fprintf(stderr, "%s: Zone not set\n", tz);
    exit(EXIT_FAILURE);
  }
}

static int get_tzdtai(time_t t) {
  struct tm *tm;
  time_t tai;
  set_utctz("posix/UTC");
  if ((tm = gmtime(&t)) == NULL) {
    perror("gmtime");
    exit(EXIT_FAILURE);
  }
  set_utctz("right/UTC");
  if ((tai = mktime(tm)) == (time_t)-1) {
    perror("mktime");
    exit(EXIT_FAILURE);
  }
  return tai - t + 10;
}

int main(int argc, char **argv) {
  struct timex tx;

  if (argc > 1) {
    tx.constant = atoi(argv[1]);
  }
  else {
    tx.modes = 0;
    switch (ntp_adjtime(&tx)) {
      case TIME_OOP:
        fprintf(stderr, "Leap second in progress\n");
        exit(EXIT_FAILURE);
      case -1:
        perror("ntp_adjtime");
        exit(EXIT_FAILURE);
    }
    tx.constant = get_tzdtai(tx.time.tv_sec);
  }

  tx.modes = MOD_TAI;
  if (ntp_adjtime(&tx) == -1) {
    perror("ntp_adjtime");
    exit(EXIT_FAILURE);
  }

  return 0;
}