MQTTClient_createOptions - Man Page

char struct_id [4]
int struct_version
int MQTTVersion

The eyecatcher for this structure. must be MQCO.

The version number of this structure. Must be 0

Whether the MQTT version is 3.1, 3.1.1, or 5. To use V5, this must be set. MQTT V5 has to be chosen here, because during the create call the message persistence is initialized, and we want to know whether the format of any persisted messages is appropriate for the MQTT version we are going to connect with. Selecting 3.1 or 3.1.1 and attempting to read 5.0 persisted messages will result in an error on create.