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subscribe ā€” Subscription example

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

#if !defined(_WIN32)
#include <unistd.h>
#else
#include <windows.h>
#endif

#if defined(_WRS_KERNEL)
#include <OsWrapper.h>
#endif

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

int disc_finished = 0;
int subscribed = 0;
int finished = 0;

void connlost(void *context, char *cause)
{
        MQTTAsync client = (MQTTAsync)context;
        MQTTAsync_connectOptions conn_opts = MQTTAsync_connectOptions_initializer;
        int rc;

        printf("0onnection lost0);
        if (cause)
                printf("     cause: %s0, cause);

        printf("Reconnecting0);
        conn_opts.keepAliveInterval = 20;
        conn_opts.cleansession = 1;
        if ((rc = MQTTAsync_connect(client, &conn_opts)) != MQTTASYNC_SUCCESS)
        {
                printf("Failed to start connect, return code %d0, rc);
                finished = 1;
        }
}


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

void onDisconnectFailure(void* context, MQTTAsync_failureData* response)
{
        printf("Disconnect failed, rc %d0, response->code);
        disc_finished = 1;
}

void onDisconnect(void* context, MQTTAsync_successData* response)
{
        printf("Successful disconnection0);
        disc_finished = 1;
}

void onSubscribe(void* context, MQTTAsync_successData* response)
{
        printf("Subscribe succeeded0);
        subscribed = 1;
}

void onSubscribeFailure(void* context, MQTTAsync_failureData* response)
{
        printf("Subscribe failed, rc %d0, response->code);
        finished = 1;
}


void onConnectFailure(void* context, MQTTAsync_failureData* response)
{
        printf("Connect failed, rc %d0, response->code);
        finished = 1;
}


void onConnect(void* context, MQTTAsync_successData* response)
{
        MQTTAsync client = (MQTTAsync)context;
        MQTTAsync_responseOptions opts = MQTTAsync_responseOptions_initializer;
        int rc;

        printf("Successful connection0);

        printf("Subscribing to topic %s0or client %s using QoS%d0n"
           "Press Q<Enter> to quit0n", TOPIC, CLIENTID, QOS);
        opts.onSuccess = onSubscribe;
        opts.onFailure = onSubscribeFailure;
        opts.context = client;
        if ((rc = MQTTAsync_subscribe(client, TOPIC, QOS, &opts)) != MQTTASYNC_SUCCESS)
        {
                printf("Failed to start subscribe, return code %d0, rc);
                finished = 1;
        }
}


int main(int argc, char* argv[])
{
        MQTTAsync client;
        MQTTAsync_connectOptions conn_opts = MQTTAsync_connectOptions_initializer;
        MQTTAsync_disconnectOptions disc_opts = MQTTAsync_disconnectOptions_initializer;
        int rc;
        int ch;

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

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

        conn_opts.keepAliveInterval = 20;
        conn_opts.cleansession = 1;
        conn_opts.onSuccess = onConnect;
        conn_opts.onFailure = onConnectFailure;
        conn_opts.context = client;
        if ((rc = MQTTAsync_connect(client, &conn_opts)) != MQTTASYNC_SUCCESS)
        {
                printf("Failed to start connect, return code %d0, rc);
                rc = EXIT_FAILURE;
                goto destroy_exit;
        }

        while (!subscribed && !finished)
                #if defined(_WIN32)
                        Sleep(100);
                #else
                        usleep(10000L);
                #endif

        if (finished)
                goto exit;

        do
        {
                ch = getchar();
        } while (ch!='Q' && ch != 'q');

        disc_opts.onSuccess = onDisconnect;
        disc_opts.onFailure = onDisconnectFailure;
        if ((rc = MQTTAsync_disconnect(client, &disc_opts)) != MQTTASYNC_SUCCESS)
        {
                printf("Failed to start disconnect, return code %d0, rc);
                rc = EXIT_FAILURE;
                goto destroy_exit;
        }
        while (!disc_finished)
        {
                #if defined(_WIN32)
                        Sleep(100);
                #else
                        usleep(10000L);
                #endif
        }

destroy_exit:
        MQTTAsync_destroy(&client);
exit:
        return rc;
}

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Tue Sep 14 2021 Paho Asynchronous MQTT C Client Library