function pop is done

stack.c

@@ -22,18 +22,20 @@ return newNode;
 // freed by caller.)
 StackNode *pop(StackNode *stack)
 {
+    if (stack == NULL)
+        return NULL;
 
+    StackNode *next = stack->next;
+    free(stack);
+    return next;
 }
 
 // Returns the data of the top element.
 void *top(StackNode *stack)
 {
 if (stack == NULL)
-    return NULL;
-
-StackNode *next = stack->next;
-free(stack);
-return next;
+  return NULL;
+return stack->data;
 }
 
 // Clears stack and releases all memory.