function top is done

stack.c

@@ -28,7 +28,12 @@ StackNode *pop(StackNode *stack)
 // Returns the data of the top element.
 void *top(StackNode *stack)
 {
+if (stack == NULL)
+    return NULL;
 
+StackNode *next = stack->next;
+free(stack);
+return next;
 }
 
 // Clears stack and releases all memory.