return something in main()

approximate-pi.c

@@ -20,4 +20,6 @@ double approximate_pi(unsigned int steps) {
 int main(int argc, char *argv[]) {
   // printf("sizeof(int) = %d\n", sizeof(int));
   printf("pi = %8.7f\n", approximate_pi(10000000));
+
+  return 0;
 }