#include <stdlib.h>

#define	TRUE	1
#define FALSE	0

int is_prime(long num)
{
	int i = 5;
	int w = 2;

	if (num == 1)
		return 0;
	if (num == 2 || num == 3)
		return 1;
	if (!(num%2) || !(num%3))
		return 0;

	while ((i*i) <= num) {
		if (!(num%i))
			return 0;
		i += w;
		w = 6 - w;
	}

	return 1;
}

char* eratosthenes(long num)
{
	long i, j;
	char* primes;

	primes = (char*)malloc((num+1)*sizeof(char));
	if (primes == NULL) {
		return NULL;
	}

	memset(primes, TRUE, num);
	primes[1] = FALSE;

	for (i=2; i<=num; i++)
		if (primes[i])
			/* Mark factors as composite */
			for (j=i*i; j<num; j+=i)
				primes[j] = FALSE;

	return primes;
}