Bluetooth LE Windows 10 using WinRT C++ code works if device NOT paired. Fails with unreachable if device is paired..

[Ev1lC]

The following code snippet works if the Bluetooth LE device is NOT paired. After pairing the same device, the call to WriteClientCharacteristicConfigurationDescriptorAsync() fails with UNREACHABLE. No PIN code required for pairing.

Can anyone please help? The code will become part of a library with Java JNI bindings, so needs to be in C++. That's why I'm using the WinRT stuff. I'm using VS 2017 target SDK 10.0.18362.0 running on Windows 10 1903.

#include "pch.h"

#include <combaseapi.h>

using namespace winrt;
using namespace Windows::Foundation;
using namespace winrt::Windows::Foundation;
using namespace Windows::Storage::Streams;
using namespace Windows::Devices::Bluetooth;
using namespace Windows::Foundation::Collections;
using namespace Windows::Devices::Bluetooth::Advertisement;
using namespace Windows::Devices::Bluetooth::GenericAttributeProfile;

#define NU_SERVICE			"{6E400001-B5A3-F393-E0A9-E50E24DCCA9E}"
#define TX_CHARACTERISTIC	"{6E400002-B5A3-F393-E0A9-E50E24DCCA9E}"
#define RX_CHARACTERISTIC	"{6E400003-B5A3-F393-E0A9-E50E24DCCA9E}"

std::wstring guidToString(GUID uuid) {
	std::wstring guid;
	WCHAR* wszUuid = NULL;
	if(::UuidToString(&uuid, (RPC_WSTR*) &wszUuid) == RPC_S_OK) {
		guid = wszUuid;
		::RpcStringFree((RPC_WSTR*) &wszUuid);
	}
	return guid;
}

void str2ba(const char *straddr, unsigned long long *btaddr) {
	int i;
	unsigned int aaddr[6];
	unsigned long long tmpaddr = 0;

	if (sscanf_s(straddr, "%02x:%02x:%02x:%02x:%02x:%02x", &aaddr[0], &aaddr[1], &aaddr[2], &aaddr[3], &aaddr[4], &aaddr[5]) != 6) {
		return;
	}
	*btaddr = 0;
	for (i = 0; i < 6; i++) {
		tmpaddr = (unsigned long long) (aaddr[i] & 0xff);
		*btaddr = ((*btaddr) << 8) + tmpaddr;
	}
}

IAsyncAction OpenDevice(unsigned long long deviceAddress) {
	auto device = co_await BluetoothLEDevice::FromBluetoothAddressAsync(deviceAddress);

	std::wcout << std::hex <<
		"Device Information: " << std::endl <<
		"\tName     :" << device.Name().c_str() << std::endl <<
		"\tAddress  :" << device.BluetoothAddress() << std::endl <<
		"\tStatus   :" << (device.ConnectionStatus() == BluetoothConnectionStatus::Connected ? "Connected" : "Disconnected") << std::endl <<
		"\tDeviceId :" << device.DeviceId().c_str()  << std::endl <<
		std::endl;

	GUID nusGUID, txcGUID, rxcGUID;
	CLSIDFromString(TEXT(NU_SERVICE), &nusGUID);
	CLSIDFromString(TEXT(TX_CHARACTERISTIC), &txcGUID);
	CLSIDFromString(TEXT(RX_CHARACTERISTIC), &rxcGUID);

	auto services = co_await device.GetGattServicesAsync();//BluetoothCacheMode::Cached);
	for(GenericAttributeProfile::GattDeviceService const & s : services.Services()) {
		std::wcout << std::hex << "\t\tService - Guid: [" << guidToString(s.Uuid()) << "]" << std::endl;

		auto characteristics = co_await s.GetCharacteristicsAsync();
		for(GenericAttributeProfile::GattCharacteristic const & c : characteristics.Characteristics()) {
			std::wcout << std::hex << "\t\tCharacteristic - Guid: [" << guidToString(c.Uuid()) << "]" << std::endl;

			if(c.CharacteristicProperties() == GattCharacteristicProperties::Notify) {
				printf("Notify supported\n");

				GattCommunicationStatus status = co_await c.WriteClientCharacteristicConfigurationDescriptorAsync(GattClientCharacteristicConfigurationDescriptorValue::Notify);
				switch (status) {
				case GattCommunicationStatus::AccessDenied:
					printf("access denied\n");
					break;
				case GattCommunicationStatus::ProtocolError:
					printf("protocol error\n");
					break;
				case GattCommunicationStatus::Unreachable:
					printf("unreachable\n");
					break;
				case GattCommunicationStatus::Success:
					c.ValueChanged([](GattCharacteristic const& charateristic, GattValueChangedEventArgs const& args) {
						std::wcout << std::hex <<
							"\t\tNotified GattCharacteristic - Guid: [" << guidToString(charateristic.Uuid()) << "]" << std::endl;
					});

					//
					// Code to write to BT device ommitted.... Sleep 5 secs then exit.
					//
					Sleep(5000);
				}
			}
		}
	}

	device.Close();
}

int main() {
	init_apartment();

	// Connect to a specific Bluetooth device.
	unsigned long long deviceAddress;
	str2ba("db:e7:df:00:52:32", &deviceAddress);
	printf("device = %lld\n", deviceAddress);

	try {
		OpenDevice(deviceAddress).get();
	} catch (const std::exception& e) {
		std::cout << e.what() << std::endl;
		return FALSE;
	}
	return TRUE;
}

[awneil]

Ev1lC said:

Can anyone please help?

Probably needs to be someone on an appropriate  Windows developer forum?

See: 

devzone.nordicsemi.com/.../194086

[Ev1lC]

Yes. I've tried that. I was hoping somebody else had had this issue and figured out a clever way round it.

Our software needs to work with paired devices. I'm going to have to implement a nasty workaround - get the user to pair the device, make a note of it in a "cache file", then unpair it to get the thing connecting. Next time round I can offer the user a union of paired devices and devices from the cache file.

I think the Windows BLE code is broken, but their Bluetooth team is small and unable / unwilling to address this despite it being a repeatable bug. I guess that's why people use UART dongles with Windows!