A little program that shows the time, ticking on the second, as exactly as the OS allows

thetime.c

/* cc -o thetime thetime.c -lrt */
#include <signal.h>
#include <time.h>

#include <stdio.h>

#define BUFSZ 128

int main() {
	timer_t timer;
	struct itimerspec itspec;
	sigset_t sigset;
	int signo;
	struct timespec now;
	struct tm now_tm;
	char buf[BUFSZ];
	int phase = 0;

	if(sigemptyset(&sigset)) {
		perror("sigemptyset");
		return 1;
	}
	if(sigaddset(&sigset, SIGALRM)) {
		perror("sigaddset");
		return 1;
	}
	if(sigprocmask(SIG_BLOCK, &sigset, NULL)) {
		perror("sigprocmask");
		return 1;
	}

	if(timer_create(CLOCK_REALTIME, NULL, &timer)) {
		perror("timer_create");
		return 1;
	}

	if(clock_gettime(CLOCK_REALTIME, &itspec.it_value)) {
		perror("clock_gettime");
		return 1;
	}
	itspec.it_interval.tv_sec = 1;
	itspec.it_interval.tv_nsec = 0;
	itspec.it_value.tv_sec++;
	itspec.it_value.tv_nsec = 0;

	if(timer_settime(timer, TIMER_ABSTIME, &itspec, NULL)) {
		perror("timer_settime");
		return 1;
	}

	while(1) {
		if(sigwait(&sigset, &signo)) {
			perror("sigwait");
			return 1;
		}

		if(signo != SIGALRM) continue;

		if(clock_gettime(CLOCK_REALTIME, &now)) {
			perror("clock_gettime");
			return 1;
		}
		localtime_r(&now.tv_sec, &now_tm);
		if(!strftime(buf, BUFSZ, "%FT%T", &now_tm)) {
			perror("strftime");
			return 1;
		}
		if(printf("\r\x1b[K%s.%09ld  %s", buf, now.tv_nsec, phase ? "TICK" : "TOCK") < 0) {
			perror("printf");
			return 1;
		}
		if(fflush(stdout)) {
			perror("fflush");
			return 1;
		}
		phase = !phase;
	}
}