Re: Help regarding IO configuration of multiple lines

[Kent Gibson <warthog618@gmail.com>]

On Tue, Mar 31, 2026 at 03:23:16PM +0200, Mathis Joussemet wrote:
> Hi Kent,
> 
> Sorry for the late reply, was up until now in exam hell.
> It indeed works! I can now drive 2 LEDs at the same time!
> 
> I just have 2 new questions:
> - I am now using a button to toggle one of the two LEDs by checking the
> buttons value using
> gpiod_line_request_get_value(request, offset[2])!=0)
> in a if statement.

I don't see any obvious reason that wouldn't read the button, assuming it is
the third line in your offset array, and the condition is tested at the time
you are pushing the button.

Have you tried the equivalent with the gpiod tools (gpioget) or some of the
example code (e.g. get_line_value) to confirm your hardware setup is working
as expected?

> I've setup the correct input bias, but when I press on
> it, the LED doesn't toggle. Checking with a scope still shows the voltage
> jumping to 3.3v when pressed and 0v at other times. Is there something I'm
> missing?

That being the input voltage?  For an input pulled down when the button is
not pressed and pulled high when it is pressed?
That is what I would expect to see.

> - I'm trying to switch from an snapshot based line change reporting to an
> event one. however when using a watch line event before/after the request,
> the program still blocks. How should i code it to prevent blocking while
> still having infos like output values types when there is a change?
> 

You can use edge events in a polling scenario by testing if the request
fd is ready to read using poll() or select() or similar.
It will become ready when an event is available.
You can then read the event as usual and the read will not block.
So similar to the async_watch_line_value example, but with a 0 timeout in
the poll() and checking the ret:

  ret = poll(&pollfd, 1, 0);
  if (ret > 0) {
    // read event...
  }

and doing whatever else you want in the polling loop.

Cheers,
Kent.