RE: Meaning of context argument in event handlers

Rafig — Fri, 18 Sep 2020 13:22:07 GMT

Thanks a lot!

RE: Meaning of context argument in event handlers

haakonsh — Thu, 17 Sep 2020 11:35:37 GMT

From main.c NUS client:

BLE_NUS_C_DEF(m_ble_nus_c);                                             /**< BLE Nordic UART Service (NUS) client instance. */

From ble_nus_c.h:

/**@brief   Macro for defining a ble_nus_c instance.
 *
 * @param   _name   Name of the instance.
 * @hideinitializer
 */
#define BLE_NUS_C_DEF(_name)                                                                        \
static ble_nus_c_t _name;                                                                           \
NRF_SDH_BLE_OBSERVER(_name ## _obs,                                                                 \
                     BLE_NUS_C_BLE_OBSERVER_PRIO,                                                   \
                     ble_nus_c_on_ble_evt, &_name)

From nrf_sdh_ble.h:

/**@brief   Macro for registering @ref nrf_sdh_soc_evt_observer_t. Modules that want to be
 *          notified about SoC events must register the handler using this macro.
 *
 * @details This macro places the observer in a section named "sdh_soc_observers".
 *
 * @param[in]   _name       Observer name.
 * @param[in]   _prio       Priority of the observer event handler.
 *                          The smaller the number, the higher the priority.
 * @param[in]   _handler    BLE event handler.
 * @param[in]   _context    Parameter to the event handler.
 * @hideinitializer
 */
#define NRF_SDH_BLE_OBSERVER(_name, _prio, _handler, _context)                                      \
STATIC_ASSERT(NRF_SDH_BLE_ENABLED, "NRF_SDH_BLE_ENABLED not set!");                                 \
STATIC_ASSERT(_prio < NRF_SDH_BLE_OBSERVER_PRIO_LEVELS, "Priority level unavailable.");             \
NRF_SECTION_SET_ITEM_REGISTER(sdh_ble_observers, _prio, static nrf_sdh_ble_evt_observer_t _name) =  \
{                                                                                                   \
    .handler   = _handler,                                                                          \
    .p_context = _context                                                                           \
}

BLE_NUS_C_DEF declares a ble_nus_c_t called m_ble_nus_c that is then passed to NRF_SDH_BLE_OBSERVER as both the name of the 'observer' and the address of the context pointer.

Each time the context pointer in ble_nus_c_on_ble_evt is dereferenced as a ble_nus_c_t it is the m_ble_nus_c struct in main that is dereferenced.