output.txt

$ ./test
1.0
0.1000
0.112
0.112
2652 49490604

time_format_test.c

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

#define NSEC_PER_SEC 1000000000L
#define NSEC_PRECISION (NSEC_PER_SEC / 1000)

static void time_since_start(char *str, size_t size, struct timespec cur_time)
{
    int ret;
    long nsec, sec;

    nsec = cur_time.tv_nsec;
    sec = cur_time.tv_sec;
    if (nsec < 0) {
        nsec += NSEC_PER_SEC;
        sec--;
    }
    nsec = (nsec + (NSEC_PRECISION / 2)) / NSEC_PRECISION;
    ret = snprintf(str, size, "%ld.%ld", sec, nsec);
    if (ret < 0) {
        str[size - 1] = '\0';
    }
}

int main(int argc, char *argv[]) {
    static struct timespec cur_time;
    char time_since[16];
    cur_time.tv_sec = 1;
    cur_time.tv_nsec = 9;
    time_since_start(time_since, sizeof(time_since), cur_time);
    printf("%s\n", time_since); // prints 1.0
    cur_time.tv_sec = 0;
    cur_time.tv_nsec = 1000000009;
    time_since_start(time_since, sizeof(time_since), cur_time);
    printf("%s\n", time_since); // prints 0.1000
    cur_time.tv_sec = 0;
    cur_time.tv_nsec = 111500000;
    time_since_start(time_since, sizeof(time_since), cur_time);
    printf("%s\n", time_since); // prints 0.112
    cur_time.tv_sec = 0;
    cur_time.tv_nsec = 111550000;
    time_since_start(time_since, sizeof(time_since), cur_time);
    printf("%s\n", time_since); // prints 0.112
    clock_gettime(CLOCK_MONOTONIC, &cur_time);
    printf("%lld %ld\n", (long long)cur_time.tv_sec, cur_time.tv_nsec);
    return 0;
}