Read and notification issues with gattlib BLE?

Question

I am writing a Linux application using the gattlib library in python3 to send and receive user inputted data between a BlueSnap DB9 BLE adapter and my Linux device. I have been able to successfully send a String of data to the adapter from my device and seen the output on the adapter's terminal, but I am having issues receiving data from the adapter.

I am following this example for reading and writing data using the gattlib library. I can write data using the write_cmd and write_by_handle functions but I am unable to read data or enable notifications with gattlib using any of the read functions mentioned there. Notifications don't appear to be enabled when using gattlib because the on_notification function I overwrote doesn't print out the print statement I added there.

I have determined that the handles for writing and reading data are 0x0043 and 0x0046, respectively. Here are the UUIDs for writing and reading that serialio provided to me: UUIDs.

When using bluetoothctl, after selecting the characteristic, I am able to write data to the adapter. Only after enabling notifications on bluetoothctl, only then am I able to read data as well. Once I disable notifications, attempting to read manually prints out all 0s instead of the data I want to read. What is the proper way to select a characteristic and enable notifications using gattlib in python3?

UPDATE: I was able to get notifications enabled. I ran hcidump on both bluetoothctl and my python code and determined the handle I had used for enabling notifications was incorrect. The correct handle for enabling notifications is 0x0047. Once I realized this mistake, I ran enable_notifications using the correct handle and set both parameters to True and was able to enable notifications and see incoming data on my device's terminal as I typed it on my adapter's terminal.

There is an example on how to receive notifications using gattlib: https://github.com/oscaracena/pygattlib/blob/master/examples/receive_notification.py — Michael Kotzjan, Aug 04 '21 at 05:48
I ran this example but it is unclear to me how this is supposed to work because when running this example, it only specifies a MAC address and not a specific characteristic that you can enable a notification on. Like I said for `bluetoothctl`, I had to select a characteristic first and enable notifications for that characteristic before being able to receive any data. I was trying to mimic what I did on `bluetoothctl`, but on a python3 application. — spage, Aug 04 '21 at 17:23

ukBaz · Answer 1 · 2021-08-05T06:06:23.797

Not using gattlib but here is an example of using the Python3 D-Bus bindings and the GLib event loop to read, write and get notifications from GATT characteristics.

from time import sleep
import pydbus
from gi.repository import GLib

# Setup of device specific values
dev_id = 'DE:82:35:E7:43:BE'
adapter_path = '/org/bluez/hci0'
device_path = f"{adapter_path}/dev_{dev_id.replace(':', '_')}"
temp_reading_uuid = 'e95d9250-251d-470a-a062-fa1922dfa9a8'
temp_period_uuid = 'e95d1b25-251d-470a-a062-fa1922dfa9a8'

# Setup DBus informaton for adapter and remote device
bus = pydbus.SystemBus()
mngr = bus.get('org.bluez', '/')
adapter = bus.get('org.bluez', adapter_path)
device = bus.get('org.bluez', device_path)
# Connect to device (needs to have already been paired via bluetoothctl)
device.Connect()

# wait for GATT services to be discovered
while not device.ServicesResolved:
    sleep(0.5)

# Some helper functions
def get_characteristic_path(device_path, uuid):
    """Find DBus path for UUID on a device"""
    mng_objs = mngr.GetManagedObjects()
    for path in mng_objs:
        chr_uuid = mng_objs[path].get('org.bluez.GattCharacteristic1', {}).get('UUID')
        if path.startswith(device_path) and chr_uuid == uuid:
           return path

def as_int(value):
    """Create integer from bytes"""
    return int.from_bytes(value, byteorder='little')

# Get a couple of characteristics on the device we are connected to
temp_reading_path = get_characteristic_path(device._path, temp_reading_uuid)
temp_period_path = get_characteristic_path(device._path, temp_period_uuid)
temp = bus.get('org.bluez', temp_reading_path)
period = bus.get('org.bluez', temp_period_path)
# Read value of characteristics
print(temp.ReadValue({}))
# [0]
print(period.ReadValue({}))
# [232, 3]
print(as_int(period.ReadValue({})))
# 1000

# Write a new value to one of the characteristics
new_value = int(1500).to_bytes(2, byteorder='little')
period.WriteValue(new_value, {})

# Enable eventloop for notifications
def temp_handler(iface, prop_changed, prop_removed):
    """Notify event handler for temperature"""
    if 'Value' in prop_changed:
        print(f"Temp value: {as_int(prop_changed['Value'])} \u00B0C")

mainloop = GLib.MainLoop()
temp.onPropertiesChanged = temp_handler
temp.StartNotify()
try:
    mainloop.run()
except KeyboardInterrupt:
    mainloop.quit()
    temp.StopNotify()
    device.Disconnect()

Read and notification issues with gattlib BLE?

1 Answers1