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subasync ā€” Asynchronous subscription example

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

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

volatile MQTTClient_deliveryToken deliveredtoken;

void delivered(void *context, MQTTClient_deliveryToken dt)
{
    printf("Message with token value %d delivery confirmed0, dt);
    deliveredtoken = dt;
}

int msgarrvd(void *context, char *topicName, int topicLen, MQTTClient_message *message)
{
    printf("Message arrived0);
    printf("     topic: %s0, topicName);
    printf("   message: %.*s0, message->payloadlen, (char*)message->payload);
    MQTTClient_freeMessage(&message);
    MQTTClient_free(topicName);
    return 1;
}

void connlost(void *context, char *cause)
{
    printf("0onnection lost0);
    printf("     cause: %s0, cause);
}

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

    if ((rc = MQTTClient_create(&client, ADDRESS, CLIENTID,
        MQTTCLIENT_PERSISTENCE_NONE, NULL)) != MQTTCLIENT_SUCCESS)
    {
        printf("Failed to create client, return code %d0, rc);
        rc = EXIT_FAILURE;
        goto exit;
    }

    if ((rc = MQTTClient_setCallbacks(client, NULL, connlost, msgarrvd, delivered)) != MQTTCLIENT_SUCCESS)
    {
        printf("Failed to set callbacks, return code %d0, rc);
        rc = EXIT_FAILURE;
        goto destroy_exit;
    }

    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;
    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS)
    {
        printf("Failed to connect, return code %d0, rc);
        rc = EXIT_FAILURE;
        goto destroy_exit;
    }

    printf("Subscribing to topic %s0or client %s using QoS%d0n"
           "Press Q<Enter> to quit0n", TOPIC, CLIENTID, QOS);
    if ((rc = MQTTClient_subscribe(client, TOPIC, QOS)) != MQTTCLIENT_SUCCESS)
    {
        printf("Failed to subscribe, return code %d0, rc);
        rc = EXIT_FAILURE;
    }
    else
    {
        int ch;
        do
        {
                ch = getchar();
        } while (ch!='Q' && ch != 'q');

        if ((rc = MQTTClient_unsubscribe(client, TOPIC)) != MQTTCLIENT_SUCCESS)
        {
                printf("Failed to unsubscribe, return code %d0, rc);
                rc = EXIT_FAILURE;
        }
    }

    if ((rc = MQTTClient_disconnect(client, 10000)) != MQTTCLIENT_SUCCESS)
    {
        printf("Failed to disconnect, return code %d0, rc);
        rc = EXIT_FAILURE;
    }
destroy_exit:
    MQTTClient_destroy(&client);
exit:
    return rc;
}

Info

Tue Sep 14 2021 Paho MQTT C Client Library