Implementing MQTT in C
IoT and IIoT developers can use C to implement MQTT, a communication protocol designed for efficient machine-to-machine data exchange in IoT systems. Using an MQTT C Client Library like Eclipse Paho C Client Library, developers can write applications for real-time communications using less code and data compared to other IoT communication protocols.
C is a widely used programming language created in the 1970s. It’s commonly used in operating systems, device drivers, and protocol stacks. Developers can use a C MQTT client to connect to an MQTT broker, publish data, and subscribe to topics to receive data.
How to Use an MQTT C Client Library?
MQTT C Client Libraries allow developers to write C applications to connect to a broker, publish messages, and subscribe to topics. There are a few different options for working with MQTT using C. This article will focus on the Eclipse Paho C Client Library. It supports MQTT v3 and MQTT v5, WebSocket Transport, and SSL/TLS. It offers both synchronous and asynchronous APIs. To get started with the Paho C Client Library, clone the repo and use the following code to install it.
git clone https://git.eclipse.org/r/paho/org.eclipse.paho.mqtt.c
make
sudo make installMake sure you include the header file in your application.
#include "MQTTClient.h"
Then you can create a client object. To do this, you need to begin by configuring an MQTT broker, either locally or using HiveMQ Cloud Serverless, and then use the broker URL to instantiate a client.
MQTTClient client;
rc = MQTTClient_create(&client, url, clientid, MQTTCLIENT_PERSISTENCE_NONE, NULL);How to Connect to an MQTT Broker in C?
MQTT works because of brokers. Devices only need to open one connection with a broker to share data with numerous other devices. The HiveMQ MQTT broker can support up to 200 million connections. HiveMQ has a hosted option, HiveMQ Cloud Serverless, as well as an open-source option, HiveMQ CE.
Once you’ve created a client instance, you can connect to a broker. The following code is an example of connecting with authentication credentials.
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
conn_opts.keepAliveInterval = 10;
conn_opts.cleansession = 1;
conn_opts.username = "username";
conn_opts.password = "password";
rc = MQTTClient_connect(client, conn_opts);After you’ve connected to a broker, you can publish messages and subscribe to topics to share data between IoT devices.
Code Examples of Using MQTT in C
Publishing MQTT Messages in C
char* payload = "a payload";
int payloadlen = strlen(payload);
int qos = 1;
int retained = 0;
MQTTClient_deliveryToken dt;
rc = MQTTClient_publish(client, topicName, payloadlen, payload, qos, retained, &dt);Subscribing MQTT Messages in C
const char* topic = "mytopic";
int qos = 2;
rc = MQTTClient_subscribe(client, topic, qos);You can find the above code examples and more information in our earlier blog Paho MQTT C Client.
MQTT Security Best Practices While Using C MQTT Broker
Security is especially important in IoT use cases where devices have limited memory and computing capabilities. Fortunately, there are steps you can take to keep your C MQTT applications secure. Using a secure network or VPN adds security to your applications. You can also use the popular security standard SSL/TLS for transport-level encryption. It encrypts your data while it is being transmitted and verifies identity on both sides. You can also implement security practices at the application level by using a client identifier, username, and password.
To use TLS/SSL, you need to add the SSL options data to the connect options:
MQTTClient_SSLOptions ssl_opts = MQTTClient_SSLOptions_initializer;
ssl_opts.enableServerCertAuth = 0;
conn_opts.ssl = &ssl_opts;Use Case Examples for MQTT in C
Developers can use C to implement MQTT for IoT use cases such as remote monitoring, home automation, telemetry, and sensor networks. Using MQTT with C can allow you to build applications to share data between devices in real time with limited bandwidth. For example, you could write a C application to subscribe to humidity data published to an MQTT broker and automate an application to send an alert if a greenhouse’s humidity falls below a set threshold.
Conclusion
Developers can use C with MQTT to build efficient IoT applications and work with real-time data. When deciding how to build an application, consider your specific requirements, constraints, and project objectives to make an informed choice about using MQTT in your C projects. Click here to learn more about MQTT Client libraries.
FAQs on Implementing MQTT in C
Read our blog Implementing MQTT in Java to learn how to set up an MQTT client and a broker connection in Java
Read our blog Implementing MQTT in Python to learn how to set up an MQTT client and a broker connection in Python.
Read our blog Implementing MQTT in C# to learn how to set up an MQTT client and a broker connection in C#.
Read our blog Implementing MQTT in Javascript to learn how to set up an MQTT client and a broker connection in Javascript.
HiveMQ Team
Team HiveMQ shares deep expertise in MQTT, Industrial AI, IoT data streaming, Unified Namespace (UNS), and Industrial IoT protocols. Our blogs explore real-world challenges, practical deployment guidance, and best practices for building modern, reliable, and a secure data backbone on the HiveMQ platform, along with thought leadership shaping the future of the connected world.
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