C语言实现MQTT客户端

在C语言中，实现MQTT（Message Queuing Telemetry Transport）客户端通常涉及到使用专门的库，如Paho MQTT Client、mosquitto_c等，它们提供了API来处理连接、订阅、发布消息和断开连接等操作。

以下是使用Paho MQTT客户端库的一个基本示例：

#include <stdio.h>
#include "paho-mqtt3.c"

#define MQTT_BROKER "your_broker_address"
#define MQTT_PORT 1883
#define MQTT_TOPIC "your_topic"
#define MQTT_CLIENT_ID "client_id"

MQTTClient client;
int result;

void on_connect(int unused_client, char *unused_user_data, int reason_code) {
    if (reason_code == 0) {
        printf("Connected to broker\n");
        result = mqttSubscribe(client, MQTT_TOPIC, 0); // Subscribe to topic
    } else {
        fprintf(stderr, "Connection failed: %d\n", reason_code);
    }
}

void on_message(MQTTClient *c, const MQTTMessage *msg) {
    printf("Received message: topic=%s payload=%.*s\n",
           msg->topic, (int)strlen(msg->payload), msg->payload);
}

int main() {
    mqttSetup(&client);

    result = mqttConnect(&client, MQTT_BROKER, MQTT_PORT, MQTT_CLIENT_ID, "", 60);
    if (result != MQTT_SUCCESS) {
        printf("Connection failed: %d\n", result);
        return -1;
    }

    mqttSetCallback(&client, on_connect, NULL, on_message);

    while (!mqttLoopOnce(&client));
    mqttDisconnect(&client);

    mqttCleanUp(&client);

    return 0;
}

在这个例子中，`on_connect`函数负责连接成功后的操作，`on_message`则处理接收到的消息。你需要替换`MQTT_BROKER`、`MQTT_PORT`、`MQTT_TOPIC`和`MQTT_CLIENT_ID`为你实际使用的值。