How to deal with JSON in Zephyr?

Question

Hi,

I want to parse and encode JSON data and send it over Bluetooth.

I couldn't find any proper example project.

I have the following struct and description:

struct sensor_data
{
	float voc;
	float co2;
	bool is_fan_running;
	bool is_lamps_on;
};

static const struct json_obj_descr sensor_data_descr[] = {
	JSON_OBJ_DESCR_PRIM(struct sensor_data, voc, JSON_TOK_FLOAT),
	JSON_OBJ_DESCR_PRIM(struct sensor_data, co2, JSON_TOK_FLOAT),
	JSON_OBJ_DESCR_PRIM(struct sensor_data, is_fan_running, JSON_TOK_TRUE),
	JSON_OBJ_DESCR_PRIM(struct sensor_data, is_lamps_on, JSON_TOK_TRUE),
};

I can parse it with the following code:

	char json_msg[] = "{\"voc\":10.45,\"co2\":25.56,\"is_fan_running\":true,\"is_lamps_on\":false}";
	struct sensor_data sensor_results;

	ret = json_obj_parse(json_msg, sizeof(json_msg),
						 sensor_data_descr,
						 ARRAY_SIZE(sensor_data_descr),
						 &sensor_results);

But I cannot encode a struct to json:

	char encoded_json[1024];
	struct sensor_data sensor_results;
	
	sensor_results.co2 = 45.0f;
	sensor_results.voc = 21.0f;
	sensor_results.is_fan_running = true;
	sensor_results.is_lamps_on = true;
	
	err = json_obj_encode_buf(sensor_data_descr, ARRAY_SIZE(sensor_data_descr), &sensor_results, encoded_json, 1024);
	LOG_INF("Encode err: %d", err);
	LOG_INF("Encode : %s", encoded_json);

It gives always:

[00:00:05.008,514] <inf> app: Encode err: -12

[00:00:05.008,544] <inf> app: Encode : {"voc":

How can I encode this struct to json string properly?

Thanks.