MQTT C语言实现

MQTT（Message Queuing Telemetry Transport）是一种轻量级的发布/订阅消息协议，专为物联网(IoT)设备设计，因为它对带宽和计算资源的需求非常低。在C语言中实现MQTT，通常会使用像Paho MQTT客户端库这样的第三方库，它是由Eclipse基金会维护的，提供了易于使用的API。

Paho MQTT库提供了一系列函数，如连接到MQTT服务器、发送和接收主题（topic）的消息、处理回调等。以下是使用C语言简单连接和发布消息的基本步骤：

#include "paho-mqtt.h"

MQTTClient client;
const char *host = "your_mqtt_broker_address";
int port = 1883; // 默认端口
const char *client_id = "your_client_id";

void on_connect(client_data_ptr unused, MQTTConnectReturnCode reconnect_code) {
    if (reconnect_code == MQTT_CONNACK_ACCEPTED) {
        printf("Connected to MQTT broker.\n");
        MQTTSubscribe(client, topic_to_subscribe, 0); // 设置订阅的主题
    } else {
        printf("Connection failed: %d\n", reconnect_code);
    }
}

void on_message(client_data_ptr unused, const MQTTMessage *message) {
    printf("Received message on topic '%s': %s\n", message->topic, (char *)message->payload);
}

int main() {
    MQTTNewClient(&client, "mqtt_c_library");
    MQTTSetCallback(client, on_connect, on_message, NULL);

    MQTTConnectOptions options = {MQTTOptionsDefault};
    options.client_id = client_id;

    MQTTConnect(client, host, port, &options);

    MQTTMessage publish_message;
    publish_message.payload = "Hello MQTT!";
    publish_message.payloadlen = strlen(publish_message.payload);
    publish_message.qos = 0; // QoS 0表示非确认

    MQTTPublish(client, topic_to_publish, publish_message.qos, publish_message.payload, publish_message.payloadlen, -1); // 发布消息

    MQTTDeleteClient(client);
    return 0;
}