How to use libmosquitto to make a request and get response using MQTT v5? - c++

Question

I'm trying to use libmosquitto to make a request (publish to a 'test/topic' topic) and I want to get a response based on the client (sender) id. So that means the client will publish to 'test/topic' and it will automatically subscribe 'test/topic/<client_id>'
The server has already subscribed on 'test/topic' and when it becomes the message, it will send a response (publish) to 'test/topic/<client_id>', which the client subscribed to receive that response in the first place.
The challenge here is how do I get the <client_id>, right. I already done this in python and js, where the client will send metadata or properties in the payload, which the server can unpack to get the client_id. However, I'm using C++ now and it's frustrating because I can't figure out how to get these properties.
Here is an example of how to do this in python. I just want to do the same with c++
I'm using the libmosquitto as I mentionned. I don't even have an example to show because I didn't find how to do this. There is literally no example on how to do this with the mosquitto c++ lib (which is confusing since mosquitto is a famous lib I guess).
I hope someone had a similar problem or can post an example for c++ and mosquitto lib. Thanks in advance.

Answer 1

When in doubt, look at the tests:
const char *my_client_id = ...;
mosquitto_property *proplist = NULL;
mosquitto_property_add_string_pair(&proplist, MQTT_PROP_USER_PROPERTY, "client_id", my_client_id);
mosquitto_publish_v5(mosq, &sent_mid, "test/topic", strlen("message"), "message", 0, false, proplist);
mosquitto_property_free_all(&proplist);
Since you asked in the comments, you can retrieve these properties from published messages by first setting an on_message callback using mosquitto_message_v5_callback_set and the implementing it like so:
void on_message(struct mosquitto *mosq, void *obj, const struct mosquitto_message *message, const mosquitto_property *props) {
std::string topic{message->topic};
if (topic == "test/topic") {
const char *client_id = nullptr;
mosquitto_property_read_string_pair(props, MQTT_PROP_USER_PROPERTY, nullptr, &client_id, false);
if (client_id) {
/* client_id contains a client id. */
}
}