Time complexity simple functions

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

double timeTaken(int start, int end) {
    return ((end - start) / CLOCKS_PER_SEC);
}

// O(n^2)
int f1(int x) {
    int res = 0;
    int a = 1;
    for(int i = 1; i <= x; ++i) {
        res += i;
        for(int j = 1; j <= x; j++) {
            a *= 1;
        }
    }
    return res;
}

// O(n)
int f2(int x) {
    int res = 0;
    for(int i = 1; i <= x; ++i) {
        res += i;
    }
    return res;
}

// O(1)
int f3(int x) {
    return (int) (x * (x + 1) / 2.0);
}

int main(void) {

    clock_t start;
    clock_t end;

    start = clock();
    printf("f1(): %d in time: %f\n", f1(100), timeTaken(start, end));
    end = clock();

    start = clock();
    printf("f2(): %d in time: %f\n", f2(100), timeTaken(start, end));
    end = clock();

    start = clock();
    printf("f3(): %d in time: %f\n", f3(100), timeTaken(start, end));
    end = clock();

    return 0;
}