Hello MQTT C


In this tutorial, you will learn how to use MQTT client in C with Cumulocity using pre-defined messages (called "static templates").


In order to follow this tutorial, check the following prerequisites:

  • You have a valid tenant, a user, and a password in order to access Cumulocity.
  • Verify that you have a gcc compiler installed:

    gcc --version
    gcc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609
    Copyright (C) 2015 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions.  There is NO
  • Download, compile and install the MQTT C Paho Client, you will find more details about Paho on this website. Paho website

Develop the "Hello, MQTT world!" client

To develop a very simple "Hello, world!" MQTT client for Cumulocity, you need to

  • Create the application.
  • Build and run the application.

Create application

Create a source file, for example "hello_mqtt.c" with the following content:

#include "stdlib.h"
#include "string.h"
#include "unistd.h"
#include "MQTTClient.h"

#define ADDRESS     "<<serverUrl>>"
#define CLIENTID    "<<clientId>>"

void publish(MQTTClient client, char* topic, char* payload) {
    MQTTClient_message pubmsg = MQTTClient_message_initializer;
    pubmsg.payload = payload;
    pubmsg.payloadlen = strlen(pubmsg.payload);
    pubmsg.qos = 2;
    pubmsg.retained = 0;
    MQTTClient_deliveryToken token;
    MQTTClient_publishMessage(client, topic, &pubmsg, &token);
    MQTTClient_waitForCompletion(client, token, 1000L);
    printf("Message '%s' with delivery token %d delivered\n", payload, token);

int on_message(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    char* payload = message->payload;
    printf("Received operation %s\n", payload);
    return 1;

int main(int argc, char* argv[]) {
    MQTTClient client;
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    conn_opts.username = "<<tenant>>/<<username>>";
    conn_opts.password = "<<password>>";

    MQTTClient_setCallbacks(client, NULL, NULL, on_message, NULL);

    int rc;
    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to connect, return code %d\n", rc);
    //create device
    publish(client, "s/us", "100,C MQTT,c8y_MQTTDevice");
    //set hardware information
    publish(client, "s/us", "110,S123456789,MQTT test model,Rev0.1");
    //listen for operation
    MQTTClient_subscribe(client, "s/ds", 0);

    for (;;) {
        //send temperature measurement
        publish(client, "s/us", "211,25");
    MQTTClient_disconnect(client, 1000);
    return rc;

Replace "<<clientId>>", "<<serverUrl>>", "<<tenant>>", "<<username>>", "<<password>>" with your data.

The Cumulocity MQTT protocol supports both unsecured TCP and also secured SSL connections (e.g. tcp://mqtt.cumulocity.com:1883 or ssl://mqtt.cumulocity.com:8883), so as the "<<serverUrl>>" you can pick the one which fits for you. When using SSL remember to configure MQTTClient_SSLOptions and set it in the MQTTClient_connectOptions.

What does the code in "main" do?

  • Configure an MQTT connection.
  • Register a on_message callback function which will print incoming messages.
  • Connect with Cumulocity via an MQTT protocol.
  • Create a new device with aC MQTT name and ac8y_MQTTDevice type.
  • Update the device hardware information by putting a S123456789 serial, a MQTT test model model and a Rev0.1 revision.
  • Subscribe to the static operation templates for the device - this will result in a on_message method call every time new operation is created.
  • Send temperature measurement every 3 seconds.

What does the code in "publish" do?

  • Create a new MQTT message and set a payload.
  • Publish message via MQTT protocol.
  • Wait maximum 1 second for a message delivered ACK from the server.

Note that the subscription is established after the device creation, otherwise if there is no device for a given clientId the server will not accept it.

Build and run

To build:

gcc hello_mqtt.c -o hello_mqtt -lpaho-mqtt3c

To run:

Message '100,C MQTT,c8y_MQTTDevice' with delivery token 1 delivered

After starting the application you should see a new device in the Cumulocity application in the device list. Additionally, if there will be a new operation created for this device, (for example c8y_Restart) information about it will be printed to the console.

Improve the agent

Now that you have done your first step, check out the Section Hello MQTT to learn more about Cumulocity MQTT and improve your application.