Sample MQTT program - simple send and receive

Dependencies:   C12832 MQTT

Dependents:   MQTT_G_SENSOR

This program and the MQTT libraries it uses are part of the EclipseTM Paho project; specifically the embedded client.

This example and API are working, but are still in progress. Please give us your feedback.

HelloMQTT is an example of using the MQTT API. The MQTT API is portable across network interface stacks. MQTT is designed to be used with TCP/IP, but any transport with similar characteristics should be suitable.

HelloMQTT uses the EthernetInterface to show how this works. The MQTT library contains an MQTTEthernet.h header, which is a wrapper around the mbed ethernet interface. To use the MQTT API with ethernet, include the following two headers:

#include "MQTTEthernet.h"
#include "MQTTClient.h"

then instantiate an MQTT client like this:

MQTTEthernet ipstack = MQTTEthernet();             
MQTT::Client<MQTTEthernet, Countdown> client = MQTT::Client<MQTTEthernet, Countdown>(ipstack);

Countdown is a timer class supplied in the MQTT library that is specific to mbed, so you should not need to change that.

If you want to use the MQTT API with a different network stack, create an interface header similar to MQTTEthernet.h. The class you create must have the following two methods at a minimum:

    int read(char* buffer, int len, int timeout);
    int write(char* buffer, int len, int timeout);

where the timeout is in milliseconds, and the return value is the number of characters read from or written to the network, or -1 on failure.


Jan Jongboom
3 months ago

File content as of revision 20:49c9daf2b0ff:

 * Copyright (c) 2014, 2015 IBM Corp.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * and Eclipse Distribution License v1.0 which accompany this distribution.
 * The Eclipse Public License is available at
 * and the Eclipse Distribution License is available at
 * Contributors:
 *    Ian Craggs - initial API and implementation and/or initial documentation
 *    Ian Craggs - make sure QoS2 processing works, and add device headers

  This is a sample program to illustrate the use of the MQTT Client library
  on the mbed platform.  The Client class requires two classes which mediate
  access to system interfaces for networking and timing.  As long as these two
  classes provide the required public programming interfaces, it does not matter
  what facilities they use underneath. In this program, they use the mbed
  system libraries.


 // change this to 0 to output messages to serial instead of LCD
#define USE_LCD 1

#include "C12832.h"

// the actual pins are defined in mbed_app.json and can be overridden per target

#define logMessage lcd.cls();lcd.printf


#define logMessage printf



#include "easy-connect.h"
#include "MQTTNetwork.h"
#include "MQTTmbed.h"
#include "MQTTClient.h"

int arrivedcount = 0;

void messageArrived(MQTT::MessageData& md)
    MQTT::Message &message = md.message;
    logMessage("Message arrived: qos %d, retained %d, dup %d, packetid %d\r\n", message.qos, message.retained, message.dup,;
    logMessage("Payload %.*s\r\n", message.payloadlen, (char*)message.payload);

int main(int argc, char* argv[])
    float version = 0.6;
    char* topic = "mbed-sample";

    logMessage("HelloMQTT: version is %.2f\r\n", version);

    NetworkInterface* network = easy_connect(true);
    if (!network) {
        return -1;

    MQTTNetwork mqttNetwork(network);

    MQTT::Client<MQTTNetwork, Countdown> client = MQTT::Client<MQTTNetwork, Countdown>(mqttNetwork);

    const char* hostname = "";
    int port = 1883;
    logMessage("Connecting to %s:%d\r\n", hostname, port);
    int rc = mqttNetwork.connect(hostname, port);
    if (rc != 0)
        logMessage("rc from TCP connect is %d\r\n", rc);

    MQTTPacket_connectData data = MQTTPacket_connectData_initializer;
    data.MQTTVersion = 3;
    data.clientID.cstring = "mbed-sample";
    data.username.cstring = "testuser";
    data.password.cstring = "testpassword";
    if ((rc = client.connect(data)) != 0)
        logMessage("rc from MQTT connect is %d\r\n", rc);

    if ((rc = client.subscribe(topic, MQTT::QOS2, messageArrived)) != 0)
        logMessage("rc from MQTT subscribe is %d\r\n", rc);

    MQTT::Message message;

    // QoS 0
    char buf[100];
    sprintf(buf, "Hello World!  QoS 0 message from app version %f\r\n", version);
    message.qos = MQTT::QOS0;
    message.retained = false;
    message.dup = false;
    message.payload = (void*)buf;
    message.payloadlen = strlen(buf)+1;
    rc = client.publish(topic, message);
    while (arrivedcount < 1)

    // QoS 1
    sprintf(buf, "Hello World!  QoS 1 message from app version %f\r\n", version);
    message.qos = MQTT::QOS1;
    message.payloadlen = strlen(buf)+1;
    rc = client.publish(topic, message);
    while (arrivedcount < 2)

    // QoS 2
    sprintf(buf, "Hello World!  QoS 2 message from app version %f\r\n", version);
    message.qos = MQTT::QOS2;
    message.payloadlen = strlen(buf)+1;
    rc = client.publish(topic, message);
    while (arrivedcount < 3)

    if ((rc = client.unsubscribe(topic)) != 0)
        logMessage("rc from unsubscribe was %d\r\n", rc);

    if ((rc = client.disconnect()) != 0)
        logMessage("rc from disconnect was %d\r\n", rc);


    logMessage("Version %.2f: finish %d msgs\r\n", version, arrivedcount);

    return 0;