Thread CoAP Response Handler - How to get POST response code

[Rob C]

My project is based on the Thread cloud coap client example.  I would like to know if my CoAP POSTs to the cloud are successful.

I added a "post_response_handler" to the SendRequest:

        //error = otCoapSendRequest(p_instance, p_request, &message_info, NULL, NULL);
        error = otCoapSendRequest(p_instance, p_request, &message_info, post_response_handler, NULL); //added response handler

The post_response_handler is firing after a successful post, but I can't figure out how to extract the coap response code (typedef enum otCoapCode) from the message.  Here's what I have so far.  Any help would be greatly appreciated.

static void post_response_handler(        void                * p_context,
                                          otCoapHeader        * p_header,
                                          otMessage           * p_message,
                                          const otMessageInfo * p_message_info,
                                          otError               result)
{
    (void)p_context;
    (void)p_header;
    
    // restore the polling period back to initial slow value
    UNUSED_VARIABLE(poll_period_restore(thread_ot_instance_get()));

    UNUSED_VARIABLE(otMessageRead(p_message,
                                      otMessageGetOffset(p_message),
                                      &???????.???????,
                                      sizeof(?????????.???????))
    );
    
    NRF_LOG_INFO("CoAP code: %d\r\n", &????????.???????);
    
}

[Hung Bui]

Hi Rob,

Have you checked the result value ? It should be 0 if it's success response. 

Do you have OT_COAP_TYPE_CONFIRMABLE type in your header when you send the request ? 

 

[Rob C]

Thanks for your reply.

Yes, result is returning 0.  Yes, I have set OT_COAP_TYPE_CONFIRMABLE.

Using the result=0 may be an acceptable short-term workaround, but I was hoping to be able to access the specific response code for further context and troubleshooting.  I was assuming this detail, along with the token, etc. would be returned so that I could get a reliable confirmation that a specific message was successfully delivered.

Does this make sense?

[Lukasz Duda]

Hello Rob,

CoAP Response code is embedded to CoAP Header of the message. You can easily pull it out using following function:

otCoapCode otCoapHeaderGetCode(const otCoapHeader *aHeader)

As you can see, variable of otCoapHeader type is already provided to the application response handler.

Specifically for your example, the last line of the code would look like this:

NRF_LOG_INFO("CoAP code: %d\r\n", otCoapHeaderGetCode(p_header));

You can read more about this function here.

[Rob C]

Thank you Lukasz!  This works perfectly.

Your first sentence was the piece of understanding I was missing -- that the response code is appended to the header of the outgoing message when the CoAP transaction is completed.

This will allow me to track success of individual messages, if necessary, using a combination of otCoapHeaderGetCode() and otCoapHeaderGetToken().

For others following along, reference the CoAP cheat sheet at https://github.com/markushx/coap-cheatsheet for meanings of response codes.