Gray coded binary clock in C

gray_binary_clock.c

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

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

void init_clock(int *fd, time_t *now);
void put_binary(uint32_t x);

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

	GUARD(clock_gettime(CLOCK_REALTIME, &start));
	*now = start.tv_sec;

	/* 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));
}

void put_binary(uint32_t x)
{
	int i=32;
	while (i--) {
		putchar(x >> i & 1 ? '1' : '0');
	}
}

int main(int argc, char *argv[])
{
	int fd;
	time_t now;
	uint64_t ticks=0;

	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++;

		char humantime[30];
		if (strftime(humantime,sizeof(humantime),"%F %H:%M:%S / ",localtime(&now)) == 0) {
			errx(3,"Invalid date formatting");
		}
		fputs(humantime,stdout);

		time_t gray = now >> 1 ^ now;
		put_binary(gray);
		putchar('\n');
	}
}