Bluetooth in Windows: service discovery SDP
lazarus / 34-Windows

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Graham1 · 2021-10-12 22:51

In an earlier post I found how to look up BT devices using
BluetoothAPIs.dll and Winsock2, and then how to connect, read and
write to a specific port on a found device:

https://forum.lazarus.freepascal.org/index.php/topic,53862.msg401002.h
tml#msg401002

Now I'm trying to find how to discover what ports and services are
available for the devices rather than just connecting to a hardcoded
one. In the Python library BlueZ they have a function find_service
that does what I want but I can't find the equivalent in the Windows
API:

https://pybluez.readthedocs.io/en/latest/api/find_service.html

I couldn't find anything about it in the BT API documents at:

https://docs.microsoft.com/en-us/windows/win32/api/bluetoothapis/

For Winsock I think it might be related to the lpafpProtocols in the
WSAQuerySet but I'm not sure how to set and access it:

https://docs.microsoft.com/en-us/windows/win32/api/winsock2/

I tried looking in the JWA source in
C:\Pascal\fpc\3.2.0\source\packages\winunits-jedi\src but wasn't sure
what to look for.

Does anybody have an example of how to do this, or a Pascal version of
the Python function mentioned above?

Thanks!

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Graham1 · 2021-10-14 04:20

I'm making some progress. I've found that for each device I locate in
the initial loop of WSALookupServiceBegin and WSALookupServiceNext I
need to perform a secondary Lookup loop for the RFCOMM_Protocol_UUID
and the device's BT address. So I can now list the RFCOMM services for
the devices and I can see the 'Serial Port' service line I'm after.

But how to I get to the channel number?

My flags include LUP_RETURN_ADDR OR LUP_RETURN_BLOB but the port
number in the address just shows 0. I think that is intended as the
COM port number for the connecting device rather than the channel for
the BT device itself.

I think it must be in the Blob data but I'm not sure how to access
that. Can anyone point me in the right direction?

Thanks!