Can't get C++ client to connect

[freezerdev]

I've been tasked with evaluating IoT Core to see if it will work for our needs. I downloaded and tried the 'pubsub' Python client sample which is able to connect and transmit to the server. Next I downloaded the C++ client and compiled with 'basic_pubsub' sample. I ran the executable with the same parameters as the Python code (endpoint, certificate, key, topic, etc.), but the client will not connect and transmit. The error code is "AWS_ERROR_MQTT_UNEXPECTED_HANGUP" Trying to debug the code, I think it is connecting but then the server immediately disconnects. The exact output is...

Connecting...
Connection failed with error libaws-c-mqtt: AWS_ERROR_MQTT_UNEXPECTED_HANGUP, The connection was closed unexpectedly.

What am I doing wrong, why won't the C++ pubsub sample work like the Python pubsub sample?

[jmklix]

Please take a look at our FAQ, and let me know if you still have any questions:

I keep getting AWS_ERROR_MQTT_UNEXPECTED_HANGUP

This could be many different things but it most likely is a policy issue. Start with using a super permissive IAM policy called AWSIOTFullAccess which looks like this:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "iot:*"
            ],
            "Resource": "*"
        }
    ]
}

After getting it working make sure to only allow the actions and resources that you need. More info about IoT IAM policies can be found here.

[freezerdev]

I figured it out once I enabled logging on the server. The Python sample uses the client Id of "basicPubSub" whereas the C++ sample uses "test-" followed by a GUID. Checking the policy on the server I see...

{
	"Effect": "Allow",
	"Action": "iot:Connect",
	"Resource": [
		"arn:aws:iot:us-east-1:XXX:client/sdk-java",
		"arn:aws:iot:us-east-1:XXX:client/basicPubSub",
		"arn:aws:iot:us-east-1:XXX:client/sdk-nodejs-*"
	]
}

If I either add a fourth connection string, or use the command line of the C++ app to override the default clientId then the C++ client is able to connect.

You should consider changing the default clientId in the C++ code to something that works out of the box, or add another connection string to a default IoT Core server.

[jmklix]

Thanks for the feedback, I'm glad that you where able to get it working. If you haven't already, you should check out the mqtt5 pubsub. It has better error messages along with many other features and improvements