I have simpe MQTT client application from hivemq site:

    MQTTClient client;
    MQTTClient_create(&client, "ssl://xxx.s1.eu.hivemq.cloud:8883", "Client_1",MQTTCLIENT_PERSISTENCE_NONE, NULL);
    int i = MQTTClient_setCallbacks(client, NULL, NULL, messageArrived, NULL);

int MQTTClient_setCallbacks(MQTTClient handle, void* context, MQTTClient_connectionLost* cl, MQTTClient_messageArrived* ma, MQTTClient_deliveryComplete* dc)
{
	int rc = MQTTCLIENT_SUCCESS;
	MQTTClients* m = handle;

	FUNC_ENTRY;
	Thread_lock_mutex(mqttclient_mutex);

	if (m == NULL || ma == NULL || m->c->connect_state != NOT_IN_PROGRESS)
...
}