C语言实现对mqtt服务器的连接

要在C语言中实现与MQTT服务器的连接，需要使用MQTT客户端库。以下是使用Eclipse Paho MQTT C客户端库连接到MQTT服务器的示例代码：

#include "MQTTClient.h"

#define ADDRESS     "tcp://mqtt.eclipse.org:1883"
#define CLIENTID    "ExampleClientSub"
#define TOPIC       "MQTT Examples"
#define QOS         1
#define TIMEOUT     10000L

int main(int argc, char* argv[])
{
    MQTTClient client;
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    MQTTClient_message msg = 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;

    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS)
    {
        printf("Failed to connect, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }

    if ((rc = MQTTClient_subscribe(client, TOPIC, QOS)) != MQTTCLIENT_SUCCESS)
    {
        printf("Failed to subscribe, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }

    printf("Subscribed to topic \"%s\"\n", TOPIC);

    while (1)
    {
        MQTTClient_messageArrived(client, TOPIC, &msg);
        printf("Message arrived! Topic: %s Message: %.*s\n", TOPIC, msg.payloadlen, (char*)msg.payload);
        MQTTClient_freeMessage(&msg);
        MQTTClient_publishMessage(client, TOPIC, &msg, &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), msg.payload, 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;
}

在此示例中，我们使用Eclipse Paho MQTT C客户端库连接到MQTT服务器。我们使用了经典的Eclipse MQTT服务器。您可以根据需要更改服务器的地址和端口号。

我们订阅一个名为“MQTT Examples”的主题，并在30秒内等待接收消息。如果接收到消息，将在控制台上打印出来，并发送一个响应消息。