cf.c

#define _GNU_SOURCE
#include <time.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

double f_freq;

double
getsec(void)
{
    struct timespec ts;
    clock_gettime(CLOCK_MONOTONIC, &ts);

    return ts.tv_sec + ts.tv_nsec/1000000000.0;
}

int main()
{
    double t0, t1;

    FILE *a = fopen("a.txt", "wb");
    fclose(a);

    for (int i=0; i<4; i++) {
        int nloop = 4096;

        t0 = getsec();

        for (int li=0; li<nloop; li++) {
            int fd = open("a.txt", O_RDWR);
            close(fd);
        }

        t1 = getsec();

        printf("%f[usec]\n", ((t1-t0)/nloop)*1000000.0);
    }
}