Detect time change with timerfd_settime() in Linux.

timechange.c

#include <stdio.h>
#include <sys/timerfd.h>
#include <sys/timerfd.h>
#include <time.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h> /* Definition of uint64_t */

#ifndef TFD_TIMER_CANCEL_ON_SET
#  define TFD_TIMER_CANCEL_ON_SET (1 << 1)
#endif

#define TIME_T_MAX (time_t)((1UL << ((sizeof(time_t) << 3) - 1)) - 1)

#define handle_error(msg) \
  do { perror(msg); exit(EXIT_FAILURE); } while (0)

int
main(int argc, char *argv[])
{
  /* We only care for the cancellation event, hence we set the
   * timeout to the latest possible value. */
  struct itimerspec new_value = {
    .it_value.tv_sec = TIME_T_MAX,
  };
  uint64_t exp;
  int fd;
  int s;

  fd = timerfd_create(CLOCK_REALTIME, 0);
  if (fd == -1)
    handle_error("timerfd_create");

  if (timerfd_settime(fd, TFD_TIMER_ABSTIME|TFD_TIMER_CANCEL_ON_SET, &new_value, NULL) == -1)
    handle_error("timerfd_settime");

  while (1) {
    s = read(fd, &exp, sizeof(uint64_t));
    printf("time has been changed\n");
  }

  exit(EXIT_SUCCESS);
}