https://devzone.nordicsemi.com/f/nordic-q-a/62062/stray-in-programHello all, I&#39;m still learning C at this point of stage. What I ended up was following a tutorial online on simple BLE project controlling the on-board LED. The tutorial can be found here: https://www.novelbits.io/smart-ble-lightbulb-application-nrf52en-USTelligent Community 13Thu, 04 Jun 2020 12:42:26 GMThttps://devzone.nordicsemi.com/thread/253289?ContentTypeID=1Thu, 04 Jun 2020 12:42:26 GMT137ad170-7792-4731-bb38-c0d22fbe4515:b2b118c6-153d-4d2d-9280-024464d1cd82Marjeris Romero<p>Hi,</p> <p>Yes, your understanding of &#39;##&#39; operator is right, also known as concatenation, and is supported by GCC compiler: <a href="https://gcc.gnu.org/onlinedocs/cpp/Concatenation.html">https://gcc.gnu.org/onlinedocs/cpp/Concatenation.html</a></p> <p>But I think I see the reason you are getting the error message, try to get rid of the semi-colon &#39;;&#39; after #define BLE_LED_SERVICE_DEF(_name)</p> <p>When you use the semi-colon after that line you are ending the defining there, and not defining NRF_SDH_BLE_OBSERVER.</p> <p>Best regards,</p> <p>Marjeris</p><div style="clear:both;"></div>