Add sieve of eratosthenes

primes.h

@@ -1,3 +1,8 @@
+#include <stdlib.h>
+
+#define	TRUE	1
+#define FALSE	0
+
 int is_prime(long num)
 {
 	int i = 5;
@@ -19,3 +24,25 @@ int is_prime(long num)
 
 	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;
+}