primes.c

#include <stdio.h>
#include <math.h>

#define CHECK_UP_TO 100

int main() {
    _Bool notPrime[CHECK_UP_TO] = { 1, 1 };

    for (int i = 0; i < sqrt(CHECK_UP_TO); i++) {
        if (notPrime[i] == 0) {
            int j = 2 * i;

            while (j < CHECK_UP_TO) {
                notPrime[j] = 1;
                j += i;
            }
        }
    }

    for (int i = 0; i < CHECK_UP_TO; i++) if (notPrime[i] == 0) printf("%d ", i);   // print

    return 0;
}

// sieve of eratosthenes implementation by jdtech3