mqtt客户端c代码订阅

以下是一个简单的C语言MQTT客户端代码，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       "example/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)
{
    int i;
    char* payloadptr;

    printf("Message arrived\n");
    printf("     topic: %s\n", topicName);
    printf("   message: ");

    payloadptr = message->payload;
    for(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);
    }

    MQTTClient_subscribe(client, TOPIC, QOS);

    printf("Subscribed to topic %s\n", TOPIC);
    printf("Press any key to exit\n");

    getchar();

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

在这个例子中，客户端订阅了名为“example/topic”的主题，并在消息到达时打印了消息内容。客户端在按下任意键后断开连接。