Sieve Primes in C

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

int main ()

{
    int i, j, x;
    char *sieve = NULL;
    int  *primes = NULL;

    printf("\nInput highest number: ");
    scanf ("%d", & x);

    // X is our input and size is now x.
    sieve = malloc (sizeof (*sieve) * x);
    printf ("Debug: sieve is %p\n", sieve);

    for (i = 2; i < x; i++) {
        sieve[i] = 1;
    }
    for (i = 2; i < sqrt (x); i++) {
        if (sieve[i] == 1){
            for (j =i; i * j < x; j++) {
                sieve[i * j] = 0;
            }
        }
    }
    // Print out what the sieve contains.
    for (i = 2, j = 0; i < x; i++) {
        if (sieve[i]) {
            printf ("%3d. Prime number = %d\n", ++j, i);
        }
    }
    // Fetch primes from the sieve into a compact array.
    for (i = 2, j = 0; i < x; i++) {
        if (sieve[i]) {
            primes = realloc (primes, ++j * sizeof (*primes));
            primes [j - 1] = i;
        }
    }
    printf ("Debug: primes is %p\n", primes);

    // Print out the list of primes.
    for (i = 0; i < j; i++) {
        printf ("%3d. Prime number = %d\n", i + 1, primes[i]);
    }
    primes = realloc (primes, (size_t) 0);
    printf ("Debug: primes is %p\n", primes);
    free (sieve);

    return 0;
}