vpcm

vpcm is a kernel extension similar to SoundFlower (https://github.com/mattingalls/Soundflower) or BlackHole (https://github.com/ExistentialAudio/BlackHole). Unlike SoundFlower and BlackHole, it does not only create virtual sound devices in your computer's GUI, but also entries for virtual devices in your computer's /dev filesystem.

This is is useful in conjunction with existing sound tools from Linux available through Homebrew or MacPorts, or to send audio data over the network using a streaming tool like netcat.

Once vpcm has been installed, a new device entry /dev/vpcmctl exists in the /dev filesystem. Using the echo command, it is possible to send commands to the vpcm controller in order to create or delete virtual sound devices. Reading from /dev/vpcmctl will provide you with an overview of existing vpcm devices. Sending the create command will create a new vpcm:

$ echo create --rate=48000 --buffer-frames=1024 MyDevice >/dev/vpcmctl
$ cat /dev/vpcmctl
vpcm Virtual Audio Device, built Mar 27 2020 22:54:55
Number of device pairs: 1
"MyDevice" -> /dev/vpcm1
  CoreAudio clients: 0
  Device node state: closed
  Configuration:
	--playback
	--rate=48000
	--channels=2
	--buffer-frames=1024
	--latency-msec=0
	--format=float32le
	--overflow=zeros

In your computer's GUI, a new entry "MyDevice" will appear under audio devices. Choosing that for output, audio data will now be available to read from /dev/vpcm1. This will be raw data in the format described by the device options, and may be piped into a command line tool or written to disk:

$ cat /dev/vpcm1 > myaudio.raw

The following options are available when creating a device:

• --playback or --record to choose the direction into which the device operates.
• --rate=<sampling rate> to choose the sampling rate.
• --channels=<number of channels> for the number of playback or recording channels.
• --buffer-frames=<frames> for the device's internal buffer size in terms of audio frames.
• --latency-msec=<latency> for the device's nominal latency (used by the system when synchronizing audio and video).
• --format=<float32|s16> to choose the number format used.
• --overflow=<zeros|noise|discard> to specify what happens when the device is running out of data.
• --[no-]eof-on-idle determines whether a pipe or output file is closed as soon as the audio engine side of the device is idle.
• --raw to omit volume scaling and clipping operations on sample data.
• --posix-pipe will report EPIPE (broken pipe) to I/O requests if there is no active client on the GUI side. Some command line tools require this to work if data is piped to or from a vpcm device.

Besides the create command, a few other commands are available:

• delete <GUI name> deletes a device with given GUI name.
• name <GUI name> provides the device path of the device with the given GUI name on the next read from the vpcmctl device.
• describe <GUI name> provides the GUI device name, with all options, of the named device. Output will be available on the next read from the vpcmctl device.

Build

• Open the XCode project at Source/vpcm.xcodeproj/
• Choose Product->Build For->Running from the XCode menu
• The kext will be located at Build/InstallerRoot/Library/Extensions/vpcm.kext

Disable kext signing

To allow the kext to be loaded, disable kext signing as described here: https://github.com/sergeybratus/netfluke/blob/master/howto-disable-kext-signing.txt

Install

To install, copy the kext to /Library/Extensions (not /System/Library/Extensions). Reboot the machine for the installation to take effect.