Ardour5.0 OSC read

How to read a value of a parameter of a plug-in without modifying it?
(Ardor 5.x Librazik). I use liblo for OSC

Not enough information to answer I think. I am assuming you have some application/program that uses liblo to send and receive OSC. I am thinking that you want to check the value of a plugin parameter by sending some OSC message to Ardour and expect a return message with that information. I do not know if you want random access to this plugin, or only as a part of the selected strip or something else all together. In the case of the selected strip, the available values are sent at the time the strip is selected or if the plugin parameters are banked. Of course if the value is changed feedback is sent too.

In the case of random access there are some messages that may work. I do not recall when these were added though, if it was before 5.12 or after.
/strip/plugin/list ssid - will list the plugins in a strip
/strip/plugin/descriptor ssid piid - will list all parameters with names, types, limits, etc. This does include the value as well. However the value is not a float from 0.0 to 1.0 but rather whatever limits and type the plugin happens to set the port at. So it is up to the controller to interpret what the value actually means. The selected strip feedback is always converted to a linear 0.0 to 1.0 controller value.

I might also add that the OSC code has changed a fair amount since Ardour 5.12. However, I do not think there is a change in this area. There are plans to allow subscribing to a parameter which would include polling for a value. But no time line on that as it is a big project and has not been even started.

thank you for your reply
Yes it is a very simple application that has an OSC server and listens to a plugin “dbmeter” Grame (language Faust).
This plugin has no output, only 64input and I would like to be able to retrieve the values ​​of the parameters.
The standalone version is available here

In that case filter some of what was written above. The selected strip functions do not include meter values at this time. However the /strip/plugin/descriptor ssid piid should. Note that it sends a series of messages, one for each parameter, so it should be possible to just look for just the parameter you are interested in and pull the value from that. See for the order of the information in each message.

In the same vein is it possible to retrieve the list of regions of a track and the parameters of these regions?

Absolutely not.

This is not an appropriate use of OSC. If you want to do stuff like this, you should be using a scripting language. Ardour has extremely deep support for Lua as a scripting language.

OSC is a control protocol. It is not a general purpose programming language, and it is not even designed to be a conversational communications protocol.