Wireless Sensor Networks (WSN) are considered to be a key enabler towards the Internet of Things (IoT) concept and one of its main underlying technologies. Applications built on top of IoT infrastructures such as WSNs share a set of common functions that concern not only data management such as data collection, dissemination or aggregation, but also classical networking functions, such as routing or addressing. Even though application deployment is becoming more feasible, it can still be a complex process especially in the presence of vertical solutions. WSNs should, therefore, be more flexible in order to facilitate the development of efficient applications. A promising networking paradigm which can help achieve such a requirement is the Software-Defined Networking (SDN) concept which decouples the network data plane from the control plane. To allow multiple applications to share and reuse common networking functions, we propose to implement them as SDN reusable building blocks and to instantiate and customize them through SDN rules. On the other hand, we provide a rigorous definition of these functions and of their relationships in order to make them reusable to efficiently develop the applications. We consider the smart city as a typical WSN/IoT domain of applications. Our approach aims at facilitating the development, use and management of such functions in a WSN and thus of IoT applications.