Eclipse Paho MQTT Client: How to check for existing connection?

Question

On the Eclipse Paho MQTT website, the developers provide a client example (http://www.eclipse.org/paho/files/mqttdoc/MQTTClient/html/pubsync.html) that does the following:

Create a client object with the specified parameters
Connect the client with the specified connection options
Publish a MQTT message
Disconnect the client
Destroy the client object

This works well if all you want is to publish one single message.

In my code, I have a function that contains pretty much the same code as in the aforementioned example, however, the function is called repeatedly from main() as I need to publish a large number of messages one after another. The problem is, if I use the code exactly as in the example, every time my function is called a new connection is created and shortly after destroyed. This happens again and again and again as long as the function is called repeatedly, causing a huge overhead.

Is there a way to check whether a client object has already been created, and if so, don't do it again but use the existing one?

In my understanding, the MQTTClient_isConnected() function is supposed to do that: https://www.eclipse.org/paho/files/mqttdoc/MQTTClient/html/_m_q_t_t_client_8h.html#ad9e40bdb7149ee3e5d075db7f51a735f But if I try it like this, I get a Segmentation fault:

if (!MQTTClient_isConnected(client)) {
    MQTTClient_create(&client, mqtt.addr, CLIENT_ID, MQTTCLIENT_PERSISTENCE_NONE, NULL);
    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;
    conn_opts.username = TOKEN;

    if (MQTTClient_connect(client, &conn_opts) != MQTTCLIENT_SUCCESS) {
        printf("\n==> Connection to MQTT Broker failed.\n");
        MQTTClient_destroy(&client);
        exit(EXIT_FAILURE);
    }
}

[EDIT]

Here is a simple demo code that better illustrates what I'm trying to accomplish:

#include <stdio.h>
#include <MQTTClient.h>

MQTTClient client;

void publish_MQTT() {
    MQTTClient_connectOptions conn_opts =  MQTTClient_connectOptions_initializer;
    MQTTClient_message pubmsg = MQTTClient_message_initializer;
    MQTTClient_deliveryToken token;
    char *payload = (char *)calloc(1024, sizeof(char));

    strcpy(payload, "hello");

    printf("DEBUG_BEFORE >> MQTTClient_isConnected(client) = %d\n", MQTTClient_isConnected(client)); // DEBUG OUTPUT

    if (!MQTTClient_isConnected(client)) {
        MQTTClient_create(&client, addr, CLIENT_ID, MQTTCLIENT_PERSISTENCE_NONE, NULL);
        conn_opts.keepAliveInterval = 20;
        conn_opts.cleansession = 1;
        conn_opts.username = TOKEN;

        if (MQTTClient_connect(client, &conn_opts) != MQTTCLIENT_SUCCESS) {
            fprintf(stderr, RED "\n==> Connection to MQTT Broker failed.\n" RESET_CL);
            MQTTClient_destroy(&client);
            free(payload);
            exit(EXIT_FAILURE);
        }
    }

    printf("DEBUG_AFTER >> MQTTClient_isConnected(client) = %d\n", MQTTClient_isConnected(client)); // DEBUG OUTPUT

    pubmsg.payload = payload;
    pubmsg.payloadlen = strlen(payload);
    pubmsg.qos = QOS;
    pubmsg.retained = 0;

    MQTTClient_publishMessage(client, TOPIC, &pubmsg, &token);
    MQTTClient_waitForCompletion(client, token, TIMEOUT);

    //MQTTClient_disconnect(client, 10000);
    //MQTTClient_destroy(&client);
    free(payload);
}

int main(void) {
    for (i=0; i<1000; i++) {
        publish_MQTT();
    }

    return 0;
}

Please ignore the fact that the addr parameter is never specified (in my real code it is) or that it is pretty useless specifying a message in the publish_MQTT() function (in my real code, data is passed from main() to that function).

ci7i2en4 ci7i2en4 · Accepted Answer · 2018-05-27T15:19:52

I figured it out: Apparently, there is absolutely nothing wrong with the example codes in the original posting.

It turns out I was appending the port of the MQTT server to the addr parameter again and again (in a section of the code not shown here as I didn't suspect the source of the error to be there), every time the publish_MQTT() function was called. This made the addr char string grow and eventually exceed the specified length, thus causing the SegFault.

This way everything works just as intended:

printf("\nADDR = %s\n\n", addr); // DEBUG OUTPUT

if (!MQTTClient_isConnected(client)) {
    strcat(strcat(addr, ":"), pt); // This line needed to be placed here, not before that if block

    MQTTClient_create(&client, addr, CLIENT_ID, MQTTCLIENT_PERSISTENCE_NONE, NULL);
    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;
    conn_opts.username = TOKEN;

    if (MQTTClient_connect(client, &conn_opts) != MQTTCLIENT_SUCCESS) {
        printf("\n==> Connection to MQTT Broker failed.\n");
        MQTTClient_destroy(&client);
        free(payload);
        exit(EXIT_FAILURE);
    }
}

Eclipse Paho MQTT Client: How to check for existing connection?

2 Answers