using floating point in CC2538 based RE-Mote board for CoAP Server with Contiki OS

Question

I am using RE-Mote Development Board along with Contiki-OS. I am interfacing it with Adafruit BNO055 Absolute Orientation Sensor.

I want particularly:

1. the Accelerometer Data in floating point precision.
2. the Euler Angles in floating point precision.

I did some digging in and found out that the printf in Contiki is Board dependent and not many boards have floating point printing implementation. ^source

However this might become critical for creating a CoAP Resource because when returning data the code uses snprintf.

The snippet:

/*GET Method*/

RESOURCE(res_bno055,
    "title=\"BNO055 Euler\";rt=\"bno055\"",
    res_get_handler,
    NULL,
    NULL,
    NULL);

static void res_get_handler(void *request, void *response, uint8_t *buffer,
uint16_t preferred_size, int32_t *offset) {

   adafruit_bno055_vector_t euler_data = getVector(VECTOR_EULER);
   double eu_x = euler_data.x;
   double eu_y = euler_data.y;
   double eu_z = euler_data.z;

   unsigned int accept = -1;

   REST.get_header_accept(request, &accept);

   if(accept == -1 || accept == REST.type.TEXT_PLAIN) {
        /*PLAIN TEXT Response*/
        REST.set_header_content_type(response, REST.type.TEXT_PLAIN);
        // >>>>>>>>>>will snprintf create a problem? <<<<<<<<<<<<<<<<<
        snprintf((char *)buffer, REST_MAX_CHUNK_SIZE, "%lf, %lf, %lf",eu_x,eu_y, eu_z,);
        REST.set_response_payload(response, (uint8_t *)buffer, strlen((char *)buffer));
   } else if (accept == REST.type.APPLICATION_JSON) {
        /*Return JSON REPONSE*/
        REST.set_header_content_type(response, REST.type.APPLICATION_JSON);
        // >>>>>>>>>>>>>>>same question here.. <<<<<<<<<<<<<<<<<<<<<<<<<<<<
        snprintf((char *)buffer, REST_MAX_CHUNK_SIZE, "{'bno055_eu':{ 'X':%f, 'Y':%f, 'Z':%f}}",
        eu_x,  eu_y, eu_z);
        REST.set_response_payload(response, buffer, strlen((char *)buffer));
   } else {
        REST.set_response_status(response, REST.status.NOT_ACCEPTABLE);
        const char *msg = "Only text/plain and application/json";
        REST.set_response_payload(response, msg, strlen(msg));
   }
}

I have tried the above mentioned Resource Code and setup a CoAP Server but I get

plain-text response:

, , ,

json response:

{'bno055_eu: { 'X': , 'Y': , 'Z': }}

the struct:

typedef struct {
    double x;
    double y;
    double z;
}adafruit_bno055_vector_t

What would be an optimal way to create a GET CoAP Resource with floating point response?

Answer 1

@eugene-nikolaev I am just playing by the examples available. I am not sure what you mean but I think set_payload_response() function might not take the values of double. If you can give me a hint as to how to proceed I can try it out.

I am not very experienced in C so I cannot' show you a good snippet. But you're casting your buffer to (uint8_t) and for sure set_payload_response acquires binary payload.

It would be like (I note again - it could be not too correct):

REST.set_response_payload(response, (uint8_t *) &euler_data, sizeof(adafruit_bno055_vector_t));

But it will work only for your else branch.

In a classical meaning of CoAP I am used to send a binary payload or CBOR-encoded payload, and parse it on the other side in the similar cases. It all depends on what are your CoAP peers and what do you want to achieve.

UPD: Regarding the plaintext/json branches - I'd advise you to check which range/precision of values your module provides. Maybe there is no big point to use double as @Lundin said.

Also, do you really need plain-text and json format?