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;
+}

problem-357.c

@@ -1,6 +1,7 @@
 #include <math.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include "primes.h"
 
 #define	MAX_NUM	100000000
@@ -13,6 +14,7 @@ int main(int argc, char **argv)
 	long sum = 0;
 	char *primes;
 
+	/*
 	primes = malloc((MAX_NUM+1)*sizeof(char));
 	for (i=1; i<MAX_NUM+1; i++) {
 		if (is_prime(i)) {
@@ -21,6 +23,12 @@ int main(int argc, char **argv)
 			primes[i] = FALSE;
 		}
 	}
+	*/
+	primes = eratosthenes(MAX_NUM);
+	if (primes == NULL) {
+		printf("Error");
+		return 1;
+	}
 
 	for(i=1; i<MAX_NUM; i++) {
 		match = TRUE;