It is better to announce we received a null pointer which is different from empty string.

Chapter1/ex1-13-vertical.c

@@ -8,10 +8,11 @@
 #define IN 1
 #define OUT 0
 
-unsigned len(const char *);
+int len(const char *);
 
 main()
 {
+	int word_count = 0;
         /*
         int c, state;
         state = OUT;
@@ -31,12 +32,14 @@ main()
 
         printf("%s length is %d.\n", "Hello", len("Hello"));
         printf("%s length is %d.\n", "", len(""));
+        printf("%s length is %d.\n", NULL, len(NULL));
 
 }
-unsigned len(const char *str)
+
+int len(const char *str)
 {
         if (str == NULL)
-                return 0;
+                return -1;
 
         int length = 0;