try to print time on every second on realtime clock

main.c

#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <time.h>

const int sec_in_nsec = 1000000000;

#define ITERMAX     10000

typedef long long nanotime_t;

// nanotime -- get time in nanoseconds
static inline nanotime_t
nanotime(clockid_t clk_id)
{
    struct timespec ts;
    clock_gettime(clk_id, &ts);
    return ts.tv_sec * sec_in_nsec + ts.tv_nsec;
}

static nanotime_t
time_for_realtime()
{
    nanotime_t t0, t1;
    int i;
    t0 = nanotime(CLOCK_MONOTONIC);
    for (i = 0; i < ITERMAX; i++) {
        nanotime(CLOCK_REALTIME);
    }
    t1 = nanotime(CLOCK_MONOTONIC);
    return (t1 - t0) / ITERMAX;
}

static nanotime_t
time_for_fprintf()
{
    nanotime_t t0, t1;
    long t;
    int i;
    t = 123;
    t0 = nanotime(CLOCK_MONOTONIC);
    for (i = 0; i < ITERMAX; i++) {
        fprintf(stderr, "%09ld\n", t);
    }
    t1 = nanotime(CLOCK_MONOTONIC);
    return (t1 - t0) / ITERMAX;
}

int run_timer() {
    nanotime_t avg = time_for_realtime();
    nanotime_t tp = time_for_fprintf();
    fprintf(stderr, "avg=%lld\n", avg);
    fprintf(stderr, "tp=%lld\n", tp);

    for (;;) {
        struct timespec ts;
        int ret = clock_gettime(CLOCK_REALTIME, &ts);
        if (ret < 0) {
            fprintf(stderr, "error in clock_gettime: %s\n", strerror(errno));
            return 1;
        }

        // print just nanosecond for simplicity;
        fprintf(stderr, "%09ld\n", ts.tv_nsec);

        ts.tv_sec = 0;
        ts.tv_nsec = sec_in_nsec - ts.tv_nsec - avg - tp;
        if (ts.tv_nsec < 0) {
            ts.tv_nsec += sec_in_nsec;
        }
        ret = nanosleep(&ts, NULL);
        if (ret < 0) {
            fprintf(stderr, "error in nanosleep: %s\n", strerror(errno));
            return 1;
        }
    }
}

int main(int argc, char **argv) {
    return run_timer();
}