mqtt客户端c代码订阅

以下是一个简单的MQTT客户端C代码，用于订阅主题：

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

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

volatile MQTTClient_deliveryToken deliveredtoken;

void delivered(void *context, MQTTClient_deliveryToken dt) {
    printf("Message with token value %d delivery confirmed\n", dt);
    deliveredtoken = dt;
}

int msgarrvd(void *context, char *topicName, int topicLen, MQTTClient_message *message) {
    printf("Message arrived\n");
    printf("  topic: %s\n", topicName);
    printf("  message: ");

    char* payloadptr = message->payload;
    for (int i = 0; i < message->payloadlen; i++) {
        putchar(*payloadptr++);
    }
    putchar('\n');
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
    return 1;
}

void connlost(void *context, char *cause) {
    printf("\nConnection lost\n");
    printf("  cause: %s\n", cause);
}

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

    MQTTClient_create(&client, ADDRESS, CLIENTID, MQTTCLIENT_PERSISTENCE_NONE, NULL);
    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;

    MQTTClient_setCallbacks(client, NULL, connlost, msgarrvd, delivered);

    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS) {
        printf("Failed to connect, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }
    printf("Subscribing to topic %s\nfor client %s using QoS%d\n\n", TOPIC, CLIENTID, QOS);
    MQTTClient_subscribe(client, TOPIC, QOS);

    printf("Press Q<Enter> to quit\n\n");
    while (getchar() != 'Q') {}

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

此代码使用Eclipse Paho MQTT C客户端库，它提供了一个简单的API来连接到MQTT代理并订阅主题。在此示例中，客户端连接到本地主机上的MQTT代理，并订阅名为“test/topic”的主题。当代理发布与此主题相关的消息时，客户端将调用msgarrvd函数来处理该消息。如果连接丢失，connlost函数将被调用。在主函数中，客户端将等待用户按下Q键来退出程序。