libmosquittopp - sample client hangs on loop_stop() method - c++

Question

I'm trying to create a simple MQTT client for my home application and I'm using libmosquittopp (which is C++ version of libmosquitto).
There is not much of a documentation for this library, but I found 2 examples (here and here) that helped me to create a code for my "MQTTWrapper" class.
Here's my code:
MQTTWrapper.h :
#pragma once
#include <mosquittopp.h>
#include <string>
class MQTTWrapper : public mosqpp::mosquittopp
{
public:
MQTTWrapper(const char* id, const char* host_, int port_);
virtual ~MQTTWrapper();
void myPublish(std::string topic, std::string value);
private:
void on_connect(int rc);
void on_publish(int mid);
std::string host;
int port;
};
MQTTWrapper.cpp
#include "MQTTWrapper.h"
#include <iostream>
MQTTWrapper::MQTTWrapper(const char* id, const char* host_, int port_) :
mosquittopp(id), host(host_), port(port_)
{
mosqpp::lib_init();
int keepalive = 10;
if (username_pw_set("sampleuser", "samplepass") != MOSQ_ERR_SUCCESS) {
std::cout << "setting passwd failed" << std::endl;
}
connect_async(host.c_str(), port, keepalive);
if (loop_start() != MOSQ_ERR_SUCCESS) {
std::cout << "loop_start failed" << std::endl;
}
}
MQTTWrapper::~MQTTWrapper()
{
std::cout << "1" << std::endl;
if (loop_stop() != MOSQ_ERR_SUCCESS) {
std::cout << "loop_stop failed" << std::endl;
}
std::cout << "2" << std::endl;
mosqpp::lib_cleanup();
std::cout << "3" << std::endl;
}
void MQTTWrapper::on_connect(int rc)
{
std::cout << "Connected with code " << rc << "." << std::endl;
}
void MQTTWrapper::myPublish(std::string topic, std::string value) {
int ret = publish(NULL, topic.c_str(), value.size(), value.c_str(), 1, false);
if (ret != MOSQ_ERR_SUCCESS) {
std::cout << "Sending failed." << std::endl;
}
}
void MQTTWrapper::on_publish(int mid) {
std::cout << "Published message with id: " << mid << std::endl;
}
and my main():
#include <iostream>
#include <string>
#include "MQTTWrapper.h"
int main(int argc, char *argv[])
{
MQTTWrapper* mqtt;
mqtt = new MQTTWrapper("Lewiatan IoT", "my.cloudmqtt.host", 12345);
std::string value("Test123");
mqtt->myPublish("sensors/temp", value);
std::cout << "about to delete mqtt" << std::endl;
delete mqtt;
std::cout << "mqtt deleted" << std::endl;
return 0;
}
Sorry for so much code.
My problem is that when I compile it and execute - my application hangs infinitely (I only waited 9 minutes) in MQTTWrapper destructor on loop_stop() method.
Tested with libmosquittopp 1.4.8 (debian package) and then after removing it- with version 1.4.9 from github.
loop_start() and loop_stop(bool force=false) should start/stop a separate thread that handles messaging.
I have tested it with forced stop (loop_stop(true)) but this way my application stops and does not publish any data. loop_stop() on the other hand publishes the data but then halts.
console output (make && ./executable):
g++ -c MQTTWrapper.cpp
g++ -c main.cpp
g++ -o executable main.o MQTTWrapper.o -lmosquittopp
about to delete mqtt
1
Connected with code 0.
Published message with id: 1
(here it hangs infinitely...)
My question:
Why this loop_stop() hangs and how to fix it?
(any documentation/tutorial/example appreciated)

Answer 1

Try call disconnect() before loop_stop(). You should also bear in mind that you're effectively doing this:
connect_async();
loop_start();
loop_stop();
The client may not even have had chance to connect, nor the thread actually be started before you tell it to stop.
It's worth considering running actions in the callbacks:
on_connect -> call publish
on_publish -> call disconnect