Callback functions are used by the µC/MQTT-client stack. Their purpose is to notify the upper application of certain activities occurring in the µC/MQTT-client core.
Each particular callback for every event is set with MQTTc_ConnSetParam and is optional. The p_arg parameter passed in every callback is also set via MQTTc_ConnSetParam with the MQTTc_PARAM_TYPE_CALLBACK_ARG_PTR type.
On Complete Callbacks
The following callbacks are of type MQTTc_CMPL_CALLBACK. Even though they are optional, they are necessary to know that a particular message has been completely sent, in order to re-use it for another message. The generic complete callback may be used to emulate all other callbacks in this category.
On Error Callback
This callback is of type MQTTc_ERR_CALLBACK. Even though it is optional, it is recommended to have it, to know if a problem occurs.
On PUBLISH message received callback
This callback is of type MQTTc_PUBLISH_RX_CALLBACK. This callback is required only if the application expects to receive data from the broker. If not, it will never be called.