https://devzone.nordicsemi.com/f/nordic-q-a/73882/transfer-object-through-different-c-filesMy project starts to grow and I would like to separate different parts of code into modules. Each module will present one .c file together with its .h file. In some .c files, I have to have the instance of the timer_id or struct which is defined in .hen-USTelligent Community 13Tue, 13 Apr 2021 13:42:00 GMThttps://devzone.nordicsemi.com/thread/304580?ContentTypeID=1Tue, 13 Apr 2021 13:42:00 GMT137ad170-7792-4731-bb38-c0d22fbe4515:71637d33-c569-493c-bef0-eb2427b75749Einar Thorsrud<p>Hi,</p> <p>As you note the app timer is created using the APP_TIMER_DEF that does a few things, including making a&nbsp;app_timer_id_t instance with the name you provide to&nbsp;APP_TIMER_DEF This is the same regardless of which app timer implementation you use, but I show both here from SDK 17.0.2 for reference:</p> <p><pre class="ui-code" data-mode="c_cpp">#ifdef APP_TIMER_V2 /** * @brief app_timer control block */ typedef struct { nrf_sortlist_item_t list_item; /**&lt; Token used by sortlist. */ uint64_t end_val; /**&lt; RTC counter value when timer expires. */ uint32_t repeat_period; /**&lt; Repeat period (0 if single shot mode). */ app_timer_timeout_handler_t handler; /**&lt; User handler. */ void * p_context; /**&lt; User context. */ NRF_LOG_INSTANCE_PTR_DECLARE(p_log) /**&lt; Pointer to instance of the logger object (Conditionally compiled). */ volatile bool active; /**&lt; Flag indicating that timer is active. */ } app_timer_t; /**@brief Timer ID type. * Never declare a variable of this type, but use the macro @ref APP_TIMER_DEF instead.*/ typedef app_timer_t * app_timer_id_t; #define _APP_TIMER_DEF(timer_id) \ NRF_LOG_INSTANCE_REGISTER(APP_TIMER_LOG_NAME, timer_id, \ APP_TIMER_CONFIG_INFO_COLOR, \ APP_TIMER_CONFIG_DEBUG_COLOR, \ APP_TIMER_CONFIG_INITIAL_LOG_LEVEL, \ APP_TIMER_CONFIG_LOG_ENABLED ? \ APP_TIMER_CONFIG_LOG_LEVEL : NRF_LOG_SEVERITY_NONE); \ static app_timer_t CONCAT_2(timer_id,_data) = { \ .active = false, \ NRF_LOG_INSTANCE_PTR_INIT(p_log, APP_TIMER_LOG_NAME, timer_id) \ }; \ static const app_timer_id_t timer_id = &amp;CONCAT_2(timer_id,_data) #else //APP_TIMER_V2 typedef struct app_timer_t { uint32_t data[CEIL_DIV(APP_TIMER_NODE_SIZE, sizeof(uint32_t))]; } app_timer_t; /**@brief Timer ID type. * Never declare a variable of this type, but use the macro @ref APP_TIMER_DEF instead.*/ typedef app_timer_t * app_timer_id_t; #define _APP_TIMER_DEF(timer_id) \ static app_timer_t CONCAT_2(timer_id,_data) = { {0} }; \ static const app_timer_id_t timer_id = &amp;CONCAT_2(timer_id,_data) #endif</pre></p> <p>So you could declare the app_timer_id_t instance as extern if it was not for the fact that it is static (meaning you have to modify the SDK code if you do it).</p> <p>While the above is possible it is perhaps not so clean as it interferes with the SDK code, so a better solution could perhaps be to use the app_timer only in one file, either in your main.c or another file. And if you need to access it, use access functions instead of the app_timer API, so that the actual app_timer API is only called from one file. This would be a mater of preference and how you like to organize your C code more than specific to the nRF5 SDK per se.</p><div style="clear:both;"></div>