Console tick clock

console_tick_clock.c

/**
 * Console tick clock by Joel Lehtonen, joel.lehtonen ät iki.fi
 * Public Domain
 *
 * Clock which outputs a character every second. The time displayed
 * matches the wall clock at every newline.
 *
 * This may experience leap second issue. Not tested with that. Also,
 * sorry about the magic numbers. This is just a funny console clock,
 * not a proof of work :-)
 */

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

#define GUARD(x) { if ((x)==-1) err(1,"Unable to initialize clock"); }

void init_clock(int *fd, time_t *now);

void init_clock(int *fd, time_t *now)
{
	struct timespec start;
	struct itimerspec spec;

	GUARD(clock_gettime(CLOCK_REALTIME, &start));
	*now = start.tv_sec/20*20; // Multiple of date string length

	/* Define a timer which starts exactly on the edge of next
	 * second and has interval of one second. */
	spec.it_value.tv_sec = *now+1;
	spec.it_value.tv_nsec = 0;
	spec.it_interval.tv_sec = 1;
	spec.it_interval.tv_nsec = 0;

	GUARD(*fd = timerfd_create(CLOCK_REALTIME, 0));
	GUARD(timerfd_settime(*fd, TFD_TIMER_ABSTIME, &spec, NULL));
}

int main(int argc, char *argv[])
{
	int fd;
	time_t now;
	uint64_t ticks=0;
	char date_buf[21];
	date_buf[0] = '\0';
	const char *date_p = date_buf;

	init_clock(&fd, &now);

	while (1) {
		if (ticks == 0) {
			// We have "processed" all seconds, wait for more
			ssize_t s = read(fd, &ticks, sizeof(uint64_t));
			if (s != sizeof(uint64_t)) {
				errx(2,"Timer reading error");
			}
		}
		ticks--;
		now++;

		if (*date_p == '\0') {
			time_t future = now+19;
			// Fill in date buffer
			size_t len = strftime(date_buf, sizeof(date_buf),
					      "%F %H:%M:%S\n", localtime(&future));
			if (len != 20 ) errx(3,"Date formatting error");
			date_p = date_buf;
		}

		putchar(*date_p++);
		fflush(stdout);
	}
}