51单片机使用mqtt协议连接云平台程序c语言

51单片机使用MQTT协议连接云平台需要使用MQTT客户端库，常用的有Paho MQTT、EMQTT等。

以下是一个简单的示例程序，使用Paho MQTT库连接到阿里云IoT平台：

#include <stdio.h>
#include <string.h>
#include "paho_mqtt.h"

#define MQTT_HOST       "xxxxxx.iot-as-mqtt.cn-shanghai.aliyuncs.com"
#define MQTT_PORT       1883
#define MQTT_USERNAME   "xxxxxx&clientId=1234567890"
#define MQTT_PASSWORD   "xxxxxxxxxxxxxxxxxxxxxxxxxxxx"
#define MQTT_TOPIC      "/sys/xxxxxx/xxxxxx/thing/event/property/post"

void on_message_received(mqtt_message_data_t *md) {
    printf("Message received:\n");
    printf("Topic: %s\n", md->topic_name);
    printf("Payload: %s\n", (char *)md->message->payload);
}

int main() {
    // 初始化MQTT客户端
    mqtt_client_t client;
    mqtt_init(&client, MQTT_HOST, MQTT_PORT, on_message_received);

    // 设置MQTT连接信息
    mqtt_connect_data_t connect_data = mqtt_connect_data_initializer;
    connect_data.username = MQTT_USERNAME;
    connect_data.password = MQTT_PASSWORD;
    connect_data.client_id = "1234567890";
    connect_data.keep_alive_interval = 60;

    // 连接MQTT服务器
    mqtt_connect(&client, &connect_data);

    // 发布消息
    mqtt_message_t message;
    char payload[] = "{\"temperature\":25.0,\"humidity\":50.0}";
    message.payload = payload;
    message.payload_len = strlen(payload);
    message.qos = MQTT_QOS_0;
    message.retained = false;
    mqtt_publish(&client, MQTT_TOPIC, &message);

    // 等待消息
    while (true) {
        mqtt_yield(&client, 1000);
    }

    // 断开MQTT连接
    mqtt_disconnect(&client);

    return 0;
}

注意替换MQTT_HOST、MQTT_USERNAME、MQTT_PASSWORD和MQTT_TOPIC等参数为自己的实际值。此示例中，MQTT客户端会连接到阿里云IoT平台，并发布一条JSON格式的消息到指定的主题上。可根据自己的需求修改代码。