Info about bt_conn_ref and bt_conn_unref?

[Andrea Verdecchia]

Maybe my question is trivial, but I want to understand the utility of bt_conn_ref and bt_conn_unref and when they need to be used. In the documentation I didn't get the concept of connection's reference count. Can anyone give me clarification about it?

[Håkon Alseth]

Hi,

 

Maybe my question is trivial, but I want to understand the utility of bt_conn_ref and bt_conn_unref and when they need to be used. In the documentation I didn't get the concept of connection's reference count. Can anyone give me clarification about it?

The functions are small, and hold the count the users of a connection:

https://github.com/nrfconnect/sdk-zephyr/blob/v3.4.99-ncs1-rc1/subsys/bluetooth/host/conn.c#L1285-L1323

 

When a connection is initiated, the application shall reference the connection via "bt_conn_ref()", to signal to any other threads/services that use the link that there are other users of the specific connection.

Similarly, bt_conn_unref() is called when the link is disconnected, so that each thread using the connection can prepare their services for disconnection.

 

Kind regards,

Håkon

[Andrea Verdecchia]

Håkon Alseth said:

the application shall reference the connection via "bt_conn_ref()"

Do you mean calling bt_conn_ref() is mandatory when a connection is initialized? My doubts about the use of theese functions stem from my intention to add central multilink feature to my application. In the zephyr's example the application uses only the bt_conn_unref(), the bt_conn_ref() is never called. Why?

[Håkon Alseth]

Hi,

 

That sample is made by the zephyr community, and I do not know the details of why it is setup in the way it is, but I can state that even though the application directly does not call bt_conn_ref(), it is called via the bt_conn_le_create() function and returned to the application here:

https://github.com/nrfconnect/sdk-zephyr/blob/v3.4.99-ncs1-rc1/samples/bluetooth/central_multilink/src/central_multilink.c#L83C6-L83C21

 

I agree that this is not easy to unwind, especially when it is called in-directly in other APIs like above.

 

Kind regards,

Håkon

[Andrea Verdecchia]

Håkon Alseth said:

I agree that this is not easy to unwind, especially when it is called in-directly in other APIs like above.

Thanks for your point. In my application, a multilink central device, I use a scan filter with auto-connect and discovery manager and I had to use the bt_conn_ref() in the discovery complete callbak in ortder to everything work, but I don't understant if I'm using it in the proper way.

[Håkon Alseth]

If you have assertions enabled, you should be able to detect "double unref" scenarios:

https://github.com/nrfconnect/sdk-zephyr/blob/v3.4.99-ncs1-rc1/subsys/bluetooth/host/conn.c#L1317

 

Kind regards,

Håkon