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briankendall / proxy-audio-device

A virtual audio driver for macOS to sends all audio to another output
The Unlicense
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Play through unsupported in AudioServerPlugin? #6

Closed gchilds closed 3 years ago

gchilds commented 3 years ago

Building play through into the audio server plugin is incredibly convenient... but isn't HAL access from the plugin unsupported?

From AudioServerPlugin.h:

An AudioServerPlugIn operates in a limited environment. First and foremost, an AudioServerPlugIn
may not make any calls to the client HAL API in the CoreAudio.framework. This will result in
undefined (but generally bad) behavior.

This document says "HAL" includes anything in AudioHardware.h, e.g. AudioDeviceStart: https://developer.apple.com/library/archive/documentation/MusicAudio/Conceptual/CoreAudioOverview/WhatsinCoreAudio/WhatsinCoreAudio.html So technically proxy-audio-device should expect this "generally bad, undefined" behaviour. Yet it in practice it seems to work fine. Is the only danger the occasional deadlock?

briankendall commented 3 years ago

Yes, Proxy Audio Device is breaking some rules and doing some hacky things. I went out of my way to have any calls to the HAL API happen in a separate thread, with my thinking being that it's a similar circumstance to calling them from another process. So far it seems to prevent deadlocks and hasn't caused any issues. Hopefully it stays that way. If something goes wrong then what happens is coreaudiod deadlocks, which of course is not great, but not the end of the world either. I did it several times in the course of developing the plugin.

The benefit is that this doesn't require a separate process to do the actual proxying of audio from one device to another. And doing so would require another buffer (I think), introducing further latency, and I wanted to keep latency as low as possible.

(If you want to see a total hack, check out the way the configuration app passes data to the driver. I know I could've done it the right way by having an intermediate launch daemon, but it seemed like so much work and boilerplate just to be able to pass a simple configuration to the driver.)

gchilds commented 3 years ago

The strings? I saw! I think the whole thing is fantastic! Best AudioServerPlugin I’ve seen yet.