paho.mqtt.cpp/examples/sync_reconnect.cpp

// sync_reconnect.cpp
//
// This is a Paho MQTT C++ client, sample application.
//
// It's a fairly contrived, but useful example of an MQTT data monitor and
// publisher, using the C++ synchronous client interface. A fairly common
// usage for MQTT applications to stay offline for much of the time and only
// connect to the broker when there is data to send.
//
// Since we don't have a universal sensor to use for this example, we simply
// use time itself as out input data. We periodically "sample" the time
// value, connect, send the value, disconnect, and then sleep. In this case
// we use the system clock, measuring the time with millisecond precision.
//
// The sample demonstrates:
//  - The synchronous client
//  - Connecting to an MQTT server/broker
//  - Periodically reconnecting to the broker
//  - Publishing messages using a `topic` object
//  - Using `connect_options` with builder classes
//

/*******************************************************************************
 * Copyright (c) 2019-2023 Frank Pagliughi <fpagliughi@mindspring.com>
 *
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v2.0
 * and Eclipse Distribution License v1.0 which accompany this distribution.
 *
 * The Eclipse Public License is available at
 *    http://www.eclipse.org/legal/epl-v20.html
 * and the Eclipse Distribution License is available at
 *   http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * Contributors:
 *    Frank Pagliughi - initial implementation and documentation
 *******************************************************************************/

#include <iostream>
#include <cstdlib>
#include <string>
#include <thread>
#include <atomic>
#include <chrono>
#include <cstring>
#include "mqtt/client.h"

using namespace std;
using namespace std::chrono;

const std::string DFLT_SERVER_ADDRESS { "mqtt://localhost:1883" };

// The QoS for sending data
const int QOS = 1;

// How often to sample the "data"
const auto SAMPLE_PERIOD = seconds(5);

// --------------------------------------------------------------------------
// Gets the current time as the number of milliseconds since the epoch:
// like a time_t with ms resolution.

uint64_t timestamp()
{
	auto now = system_clock::now();
	auto tse = now.time_since_epoch();
	auto msTm = duration_cast<milliseconds>(tse);
	return uint64_t(msTm.count());
}

// --------------------------------------------------------------------------

int main(int argc, char* argv[])
{
	// The server URI (address)
	string address = (argc > 1) ? string(argv[1]) : DFLT_SERVER_ADDRESS;

	// The amount of time to run (in sec). Zero means "run forever".
	uint64_t trun = (argc > 2) ? stoll(argv[2]) : 0LL;

	cout << "Initializing for server '" << address << "'..." << endl;

	mqtt::client cli(address, "");

	auto connOpts = mqtt::connect_options_builder()
		.clean_session()
		.finalize();

	cli.set_timeout(seconds(3));

	auto top = cli.get_topic("data/time", QOS);

	uint64_t	t = timestamp(),
				tstart = t;

	try {
		// We need to connect once before we can use reconnect()
		cli.connect(connOpts);

		while (true) {
			cout << "\nCollecting data..." << endl;

			// Collect some data
			t = timestamp();

			if (!cli.is_connected()) {
				cout << "Reconnecting..." << endl;
				cli.reconnect();
			}

			cout << "Publishing data: " << t << "..." << endl;
			top.publish(to_string(t));

			cout << "Disconnecting..." << endl;
			cli.disconnect();

			// Quit if it's past time
			if (trun > 0 && t >= (trun + tstart))
				break;

			cout << "Going to sleep." << endl;
			this_thread::sleep_for(SAMPLE_PERIOD);
		}
	}
	catch (const mqtt::exception& exc) {
		cerr << exc << endl;
		return 1;
	}

 	return 0;
}