Предыдущий коммент не мой, но...

#include <stdbool.h>
#include <stdio.h>

#define size 8190

int main() {
        int count;

        puts("10 iterations");
        for (int iter = 10; iter > 0; iter--) {
                count = 0;
                bool flags[size] = { [0 ... size-1] = true };
                for (int i = 0; i < size; i++) {
                        if (flags[i]) {
                                int prime = 2 * i + 3;
                                int k = i + prime;
                                for (; k < size; k += prime) {
                                        flags[k] = false;
                                }
                                count++;
                        }
                }
        }
        printf ("\n%d primes\n", count);
        return 0;
}

Да, это C99.