understand static and extern

ch4/main.c

@@ -0,0 +1,19 @@
+#include <stdio.h>
+#include "helper.h"
+
+void increment();
+
+int main() {
+  printf("hello world\n");
+
+  increment();
+  increment();
+
+  printf("sum within main: %d\n", sum);
+
+  sum++;
+
+  printf("sum within main: %d\n", sum);
+
+  return 0;
+}