timechange.c

/* https://gist.github.com/gdamjan/9560411 */
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <sys/timerfd.h>
#include <time.h>
#include <unistd.h>

#define handle_error(msg)					\
	do { perror(msg); return 1; } while (0)

/* Missing defines in GLIBC <= 2.24 */
#ifndef TFD_TIMER_CANCEL_ON_SET
#define TFD_TIMER_CANCEL_ON_SET (1 << 1)
#endif
#ifndef TFD_SETTIME_FLAGS
#define TFD_SETTIME_FLAGS (TFD_TIMER_ABSTIME | TFD_TIMER_CANCEL_ON_SET)
#endif

static void msec2tspec(int msec, struct timespec *ts)
{
	if (msec) {
		ts->tv_sec  =  msec / 1000;
		ts->tv_nsec = (msec % 1000) * 1000000;
	} else {
		ts->tv_sec  = 0;
		ts->tv_nsec = 0;
	}
}

/* No cancel on reltime timers, only abstime, Linux <= 4.8 */
int main(int argc, char *argv[])
{
	int fd;
	ssize_t s;
	uint64_t exp;
	struct timeval tv;
	struct itimerspec time;

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

//	msec2tspec(5000, &time.it_value);
	gettimeofday(&tv, NULL);
	time.it_value.tv_sec = tv.tv_sec + 3600;
	time.it_value.tv_nsec = 0;
	msec2tspec(0, &time.it_interval);
	if (timerfd_settime(fd, TFD_SETTIME_FLAGS, &time, NULL) == -1)
		handle_error("timerfd_settime");

	s = read(fd, &exp, sizeof(uint64_t));
	if (s != sizeof(uint64_t))
		handle_error("read");

	return 0;
}