#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>
#include <time.h>
#include <err.h>
#include <sys/timerfd.h>

static void
print_elapsed_time(void)
{
	static struct timespec start;
	struct timespec curr;
	static int first_call = 1;
	int secs, nsecs;

	if (first_call) {
		first_call = 0;
		if (clock_gettime(CLOCK_MONOTONIC, &start) == -1)
			err(1, "clock_gettime");
	}

	if (clock_gettime(CLOCK_MONOTONIC, &curr) == -1)
		err(1, "clock_gettime");

	secs = curr.tv_sec - start.tv_sec;
	nsecs = curr.tv_nsec - start.tv_nsec;
	if (nsecs < 0) {
		secs--;
		nsecs += 1000000000;
	}
	printf("%d.%03d: ", secs, (nsecs + 500000) / 1000000);
}

int
main(int argc, char *argv[])
{
	struct itimerspec new_value;
	int max_exp, fd;
	struct timespec now;
	uint64_t exp, tot_exp;
	ssize_t s;

	if ((argc != 2) && (argc != 4)) {
		fprintf(stderr, "%s init-secs [interval-secs max-exp]\n", argv[0]);
		return 1;
	}

	if (clock_gettime(CLOCK_MONOTONIC, &now) == -1)
		err(1, "clock_gettime");

	new_value.it_value.tv_sec = now.tv_sec + atoi(argv[1]);
	new_value.it_value.tv_nsec = now.tv_nsec;
	if (argc == 2) {
		new_value.it_interval.tv_sec = 0;
		max_exp = 1;
	} else {
		new_value.it_interval.tv_sec = atoi(argv[2]);
		max_exp = atoi(argv[3]);
	}
	new_value.it_interval.tv_nsec = 0;

	fd = timerfd_create(CLOCK_MONOTONIC, 0);
	if (fd == -1)
		err(1, "timerfd_create");

	if (timerfd_settime(fd, TFD_TIMER_ABSTIME, &new_value, NULL) == -1)
		err(1, "timerfd_settime");

	print_elapsed_time();
	printf("timer started\n");

	for (tot_exp = 0; tot_exp < (uint64_t)max_exp;) {
		s = read(fd, &exp, sizeof(uint64_t));
		if (s != sizeof(uint64_t))
			err(1, "read");

		tot_exp += exp;
		print_elapsed_time();
		printf("read: %llu; total=%llu\n",
		       (unsigned long long)exp,
		       (unsigned long long)tot_exp);
	}

	return 0;
}