C语言代码实现mqtt服务器连接

要实现MQTT服务器的连接，你可以通过使用MQTT客户端库来编写C语言代码。以下是一个简单的示例：

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

#define ADDRESS     "tcp://localhost:1883"
#define CLIENTID    "ExampleClientSub"
#define TOPIC       "test"
#define QOS         1
#define TIMEOUT     10000L

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

    do
    {
        rc = MQTTClient_receive(client, &msg, TIMEOUT);

        if (msg)
        {
            printf("Received message: %.*s\n", msg->payloadlen, (char*)msg->payload);
            MQTTClient_freeMessage(&msg);
            MQTTClient_free(msg);
        }
    } while (rc != MQTTCLIENT_SUCCESS);

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

    return rc;
}

在这个示例中，我们使用了Eclipse Paho MQTT C客户端库来实现MQTT服务器的连接和订阅。在代码中，我们首先定义了连接的地址、客户端ID、订阅的主题、QoS等参数。然后，我们创建了一个MQTT客户端，并设置连接选项。通过调用MQTTClient_connect()函数来连接MQTT服务器并订阅指定的主题。最后，我们通过循环调用MQTTClient_receive()函数来接收服务器发送的消息，并在控制台上输出。最后，我们通过调用MQTTClient_disconnect()函数来断开客户端与服务器的连接，并销毁MQTT客户端。