用c语言实现MQTT协议通信华为云IOT

在C语言中实现MQTT协议通信华为云IOT可以使用MQTT C语言客户端库，以下是具体的实现步骤：

1. 首先需要在华为云IOT上创建设备并获取设备证书以及设备ID和物联网平台的服务地址。

2. 下载MQTT C语言客户端库，例如Eclipse Paho MQTT C库，该库可以在Linux、Windows和Mac OS等平台上使用。下载地址：https://github.com/eclipse/paho.mqtt.c

3. 在代码中引入相关头文件：

#include "MQTTClient.h"

4. 创建MQTT客户端并初始化MQTT连接参数：

MQTTClient client;
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;

5. 设置连接参数，包括设备证书、设备ID、服务地址、连接超时时间等：

char* address = "ssl://XXXX.iot-mqtts.cn-north-4.myhuaweicloud.com:8883";
char* clientID = "XXXX";
char* username = "XXXX";
char* password = "XXXX";
char* root_ca = "XXXX.pem";

conn_opts.keepAliveInterval = 20;
conn_opts.cleansession = 1;
conn_opts.username = username;
conn_opts.password = password;
conn_opts.ssl = &ssl_opts;

其中，`ssl://XXXX.iot-mqtts.cn-north-4.myhuaweicloud.com:8883`为华为云IOT物联网平台的服务地址，`clientID`为设备ID，`username`和`password`为设备证书的用户名和密码，`root_ca`为根证书。

6. 创建MQTT连接：

MQTTClient_create(&client, address, clientID, MQTTCLIENT_PERSISTENCE_NONE, NULL);
if (MQTTClient_connect(client, &conn_opts) != MQTTCLIENT_SUCCESS) {
    printf("Failed to connect\n");
    return -1;
}

7. 发布消息：

MQTTClient_message pubmsg = MQTTClient_message_initializer;
pubmsg.payload = "Hello world!";
pubmsg.payloadlen = strlen("Hello world!");
pubmsg.qos = QOS;
MQTTClient_publishMessage(client, TOPIC, &pubmsg, &token);

其中，`TOPIC`为MQTT主题，`QOS`为消息发布的质量等级。

8. 订阅主题：

MQTTClient_subscribe(client, TOPIC, QOS);

9. 接收消息：

void messageArrived(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    printf("Message arrived\n");
    printf("  topic: %s\n", topicName);
    printf("  message: ");
    fwrite(message->payload, 1, message->payloadlen, stdout);
    printf("\n");
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
}

10. 断开MQTT连接：

MQTTClient_disconnect(client, 10000);
MQTTClient_destroy(&client);

完整的代码示例如下：

#include "MQTTClient.h"
#include <string.h>

#define QOS 1
#define TIMEOUT 10000L

void messageArrived(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    printf("Message arrived\n");
    printf("  topic: %s\n", topicName);
    printf("  message: ");
    fwrite(message->payload, 1, message->payloadlen, stdout);
    printf("\n");
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
}

int main(int argc, char* argv[]) {
    MQTTClient client;
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    MQTTClient_SSLOptions ssl_opts = MQTTClient_SSLOptions_initializer;
    char* address = "ssl://XXXX.iot-mqtts.cn-north-4.myhuaweicloud.com:8883";
    char* clientID = "XXXX";
    char* username = "XXXX";
    char* password = "XXXX";
    char* root_ca = "XXXX.pem";
    char* topic = "XXXX";

    MQTTClient_message pubmsg = MQTTClient_message_initializer;
    MQTTClient_deliveryToken token;
    int rc;

    MQTTClient_create(&client, address, clientID, MQTTCLIENT_PERSISTENCE_NONE, NULL);
    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;
    conn_opts.username = username;
    conn_opts.password = password;
    ssl_opts.enableServerCertAuth = 1;
    ssl_opts.trustStore = root_ca;
    conn_opts.ssl = &ssl_opts;
    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to connect, return code %d\n", rc);
        return -1;
    }

    MQTTClient_subscribe(client, topic, QOS);
    MQTTClient_setCallbacks(client, NULL, NULL, messageArrived, NULL);

    pubmsg.payload = "Hello world!";
    pubmsg.payloadlen = strlen("Hello world!");
    pubmsg.qos = QOS;
    pubmsg.retained = 0;
    MQTTClient_publishMessage(client, topic, &pubmsg, &token);
    printf("Waiting for up to %d seconds for publication of %s\n"
           "on topic %s for client with ClientID: %s\n",
           (int)(TIMEOUT/1000), "Hello world!", topic, clientID);
    rc = MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf("Message with delivery token %d delivered\n", token);

    MQTTClient_disconnect(client, 10000);
    MQTTClient_destroy(&client);
    return rc;
}

注意，在使用该代码前需要将其中的`XXXX`替换为对应的设备证书、设备ID和华为云IOT服务地址等信息。