linker command failed with exit code 1

[kaihirota]

I'm using UE4.26 on MacOS Monterey 12.2.1, with JetBrains Rider IDE.
The broker that I've set up is RabbitMQ hosted on AWS MQ.
I get the following error when I attempt to build by clicking on build button in the IDE:

Undefined symbols for architecture x86_64:
  "USpacesMarkerManager::rc", referenced from:
      USpacesMarkerManager::connect_callback(mosquitto*, void*, int) in SpacesMarkerManager.cpp.o
      USpacesMarkerManager::BeginSubscribe() in SpacesMarkerManager.cpp.o
ld: symbol(s) not found for architecture x86_64
0>clang: Error  : linker command failed with exit code 1 (use -v to see invocation)
Rebuild failed at 9:35:20 PM 

Code:

auto USpacesMarkerManager::connect_callback(struct mosquitto* mosq, void* obj, int result) -> void
{
	UE_LOG(LogTemp, Warning, TEXT("RabbitMQ Connection established: %d"), result);
	ResponseCode = result;
}

auto USpacesMarkerManager::message_callback(struct mosquitto* mosq, void* obj,
                                                  const struct mosquitto_message* message) -> void
{
	bool match = 0;
	FString Payload = FString((char*) message->payload);
	FString Topic = FString(message->topic);
	UE_LOG(LogTemp, Warning, TEXT("RabbitMQ Message Received: Payload: %s, Topic: %s"), *Payload, *Topic);
	
	mosquitto_topic_matches_sub("marker.dynamic.*", message->topic, &match);
	if (match) {
		UE_LOG(LogTemp, Warning, TEXT("Topic: %s Data: %s"), *Topic, *Payload);
	}

}

void USpacesMarkerManager::BeginSubscribe()
{
	mosquitto_lib_init();
	mosquitto* MqttClient = mosquitto_new("UE", false, 0);
	
	if (MqttClient)
	{
		mosquitto_connect_callback_set(MqttClient, &USpacesMarkerManager::connect_callback);
		mosquitto_message_callback_set(MqttClient, &USpacesMarkerManager::message_callback);
		
		ResponseCode = mosquitto_connect(
			MqttClient,
			"xxxxxxxx:xxxxxxxx*@xxx.ap-southeast-2.amazonaws.com",
			5671,
			60
		);
		mosquitto_subscribe(MqttClient, NULL, "*", 0);
		
		while(RunStatus){
			ResponseCode = mosquitto_loop(MqttClient, -1, 1);
			if(RunStatus && ResponseCode){
				UE_LOG(LogTemp, Warning, TEXT("RabbitMQ Connection Error"));
				sleep(10);
				mosquitto_reconnect(MqttClient);
			}
		}
		mosquitto_destroy(MqttClient);
	}
	mosquitto_lib_cleanup();
}

I would appreciate any insight on how I can resolve this build issue. TIA!

[kaihirota]

I found out the issue was caused by

mosquitto_connect_callback_set(MqttClient, &USpacesMarkerManager::connect_callback);

in particular, making any reference to static class variable caused the linker error, like using ResponseCode or RunStatus.

So my question is, how do you use mosquitto_loop if you can't use any references to variables outside of the callback function scope? Or if I'm mistaken, I would be grateful if someone could show me how you use reference to variables defined outside of the callback function scope. I want to use some kind of variable to control whether the subscription should continue or halt.