Why is build_output_stream data length different every time?

[zhpixel517]

I am trying to build a network audio streaming application in rust and I have been using cpal to do it. I am going to have an mpsc thread that is in charge of buffering all the received samples from the network, and will then pass the received audio samples to the real-time thread inside the closure of build_output_stream. Something like this:

let stream = output_device.build_output_stream(
   &config.into(),
   move |data: &mut [f32], _: &_|  {

          data = thread_receiver.recv() // or something like that

   },
   move |err| {
        eprintln!("Output error: {:?}", err);
    },
    None 
).unwrap();

I am using this on a laptop running Ubuntu.

The tricky part is that the length of the data array as passed in from the closure is different every time, which kind of throws a wrench into my existing approach of buffering the received samples. If I were to put a println("{:?}", data.length); it would output a different number every time the closure runs.

Why does build_output_stream have this behavior, and what can I do about it?

[karasikq]

@zhpixel517 Hi, that's because you using ALSA as output backend. I'm pretty sure that SupportedBufferSize value inside your config is something like that: Range { min: 1, max: 4194304 }. And as result your data buffer always changes. I don't really know who produce this behavior cpal or ALSA, but when I use portaudio (c++) + ALSA everything was ok. Some solutions:

• receive only needed number of sample (like .recv(data.len())
• create external FIFO buffer, receive any number of samples, output only data.len()
• write optional pipewire backend for CPAL :)

Some observation: if you pass to config your specified buffer size(for example 2048), in most of cases(or always) data.len() will be LESS than 2048