API aws_iot_mqtt_subscribe() don't support to define topic name in stack memory

[chunhuajiang]

Hi,
The API aws_iot_mqtt_subscribe() is used to subscribe some topic, and its 2nd parameter is the topic name, but I found that if I define topic name in stack, like this

void test(AWS_IoT_Client *client)
{
    char topic_name[128];
    IoT_Error_t rc = FAILURE;

    memset(topic_name, 0, sizeof(topic_name));
    strcpy(topic_name, "$aws/things/MyIoTButton/shadow/update/delta");
    rc = aws_iot_mqtt_subscribe(client, topic_name, strlen(topic_len), xxx, xxx);
    /// rc == SUCCESS, but xxx_cb will not be called when the topic is published at most time
}

Then, even thought its return code indicates SUCCESS, there is a great possibility that xxx_cb will not be called when the topic is published.

But If I define topic name like this,

void test(AWS_IoT_Client *client)
{
    aws_iot_mqtt_subscribe(client, "$aws/things/MyIoTButton/shadow/update/delta", strlen("$aws/things/MyIoTButton/shadow/update/delta"), xxx, xxx);
}

The xxx_cb will always be called when the topic is published.

I think its not reasonable,

[huguesBouvier]

Hi,

You are correct, this is an issue. Thank you very much for reporting it!
The problem comes from here:
in aws_iot_mqtt_client_subscribe.c, in _aws_iot_mqtt_internal_subscribe:
pClient->clientData.messageHandlers[indexOfFreeMessageHandler].topicName =
pTopicName;
It is passing a pointer fro stack to the message handler. That is what is causing the issue.
Regards,

[chunhuajiang]

yes, that's the root cause

[gordonwang0]

Hi @tidyjiang8,

We added a note in the documentation about stack parameters needing to remain in scope during an API call, #175. A true fix to this problem (copying parameters) isn't planned at this time. Closing issue.