main.c

#include <assert.h>
#include <errno.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>
#include <bits/time.h>
#include <sys/time.h>
#include <time.h>
#include <sys/timerfd.h>

#define ARRAY_LENGTH(x) (sizeof(x) / sizeof((x)[0]))

#define BUF_SIZE 1024
static char buf[BUF_SIZE];

static char time_format[] = "%a %b %d %I:%M:%S %p %Z %Y\n";

static int setup_timerfd(int timer_fd, struct timespec *now) {
	assert(timer_fd > 0);
	assert(now);

	int r;

	struct itimerspec tspec = {
		.it_value = {.tv_sec = now->tv_sec + 1, .tv_nsec = 0},
		.it_interval = {.tv_sec = 1, .tv_nsec = 0}
	};

	r = timerfd_settime(
		timer_fd,
		TFD_TIMER_ABSTIME | TFD_TIMER_CANCEL_ON_SET,
		&tspec,
		0
	);
	assert(!r);

	return timer_fd;
}

static ssize_t write_time_fmt(const struct timespec *t, char *p, size_t p_len) {
	assert(t);
	assert(p);

	struct tm *gm = localtime((const time_t *)t);
	assert(gm);

	ssize_t n = strftime(p, p_len, time_format, gm);
	assert(n > 0);

	return n;
}

static void run(int timer_fd, struct timespec *now) {
	assert(timer_fd > 0);
	assert(now);

	int r;

	while (1) {
		char read_buf[8];

		ssize_t n = write_time_fmt(now, buf, BUF_SIZE);
		r = write(1, buf, n);
		assert(r == n);

		n = read(timer_fd, read_buf, 8);
		if (n != 8) {
			assert(errno == ECANCELED);
			return;
		}

		uint64_t dt = *((uint64_t *)read_buf);
		now->tv_sec += dt;
	}
}

int main(void) {
	int r;
	
	int timer_fd = timerfd_create(CLOCK_REALTIME, 0);
	assert(timer_fd > 0);

	while (1) {
		struct timespec now;
		r = clock_gettime(CLOCK_REALTIME, &now);
		assert(!r);

		now.tv_nsec = 0;

		setup_timerfd(timer_fd, &now);

		run(timer_fd, &now);
	}

	r = close(timer_fd);
	assert(!r);

	return EXIT_SUCCESS;
}