BLE GATT characteristic variable length (Zephyr)

[Nick_RA]

I have a characteristic whose value is an array of char, which can from 1 to 20 bytes long. Using SDK17 I can set a vlen attribute and use the function update_characteristic() but I can't see a way to do the same in NCS/Zephyr.

So, how can I change the value *and length* of a characteristic?

These are all read-only, no notify, and created with the macros created using the macros BT_GATT_SERVICE_DEFINE BT_GATT_PRIMARY_SERVICE & BT_GATT_CHARACTERISTIC

Currently using NCS2.9.0 with a nRF52832DK

[Hung Bui]

Hi Nick, 
You will receive a call back when the client do a read. 
I would suggest to take a look at our Academy course for BLE here: https://academy.nordicsemi.com/courses/bluetooth-low-energy-fundamentals/lessons/lesson-4-bluetooth-le-data-exchange/topic/blefund-lesson-4-exercise-1/

At step 5 it discusses about implementing the callback. 

[Nick_RA]

Hi

I already have the callback:

static ssize_t char_foo_read(struct bt_conn *conn, const struct bt_gatt_attr *attr,void *buf, uint16_t len, uint16_t offset)
{
    const char *value = attr->user_data;
    return bt_gatt_attr_read(conn, attr, buf, len, offset, value,len);
}

The intention here is to update the user_data before a connection is made. It's part of a configuration set by the customer via a uart CLI.

Or... are you saying I can ignore the actual characteristic data in the softdevice, and just return whatever I want to the central device whenever a connection is made? That doesn't seem right to me.

[Hung Bui]

Nick_RA said:

and just return whatever I want to the central device whenever a connection is made

It's actually true. You can take a look here at section 4 where I discussed a little bit about this approach in Zephyr and compare it to the old softdevice: 
https://devzone.nordicsemi.com/guides/nrf-connect-sdk-guides/b/software/posts/building-a-ble-application-on-ncs-comparing-and-contrasting-to-softdevice-based-ble-applications#mcetoc_1elnnim8t3
(The blog is a little bit old and may not have most up to date API, please follow the academy course) 

[Nick_RA]

That would explain why I couldn't find anything! It works fine, thanks.