---
title: "The Plan 9 audio interface is composed of 4 files"
slug: the-plan-9-audio-interface-is-composed-of-4-files
type: evergreen
stime: 2026-05-25 @ 12:00AM
status: rough
certainty: likely
importance: 5
tags: [plan9, audio]
---

The Plan 9 audio interface is composed of 4 files:

```
; walk -fexp /dev/audio* /dev/volume
--rw-rw-rw /dev/audio
---w--w--w- /dev/audioctl
--r--r--r-- /dev/audiostat
--rw--rw--rw- /dev/volume
```

`/dev/audio` is the most important and is used to play (write) and record (read) audio:

```
; audio/mp3dec <file.mp3 >/dev/audio
; dd -conv swap </dev/audio | audio/mp3enc >rec.mp3
```

`/dev/audiostat` is for driver-specific status:

```
; cat /dev/audiostat
bufsize 1024 buffered   7168
codec 0 pin 22 inpin 23
[...]
pin 22 out jack ext front hpout green  aout 16,  aout 17
pin 23 in nothing sep rear micin pink
[...]
outpath aout 16  pin 22
inpath pin 23  ain 20
```

The output would then be different for every driver. Above is a demonstration.

`/dev/audioctl` is the counterpart for audiostat, and is used for driver-specific control:

```
# hda
; echo pin 22 >/dev/audioctl
# nusb/audio
; echo out off >/dev/audioctl
```

This allows you to send configuration messages to the driver.

`/dev/volume` controls the volume, sampling rate, delay, and format controls:

```
; cat /dev/volume
dev /dev/audio
mix 100 100
fmtout s16c2r44100
speed 44100
master 89
master 89
recgain 0 0
delay 1764
; echo master 50 >/dev/volume
; echo speed 48000 >/dev/volume
; echo delay 1920 >/dev/volume # 40 ms
```

All of the drivers implement this interface: AC97, SB16, Intel HDA, USB audio, MNT Reform, ... You can have more than one available (locally or not).

```
/dev/audio
/dev/audioU93a25
/dev/volume
/dev/volumeU93a25
```

## Related

- [The default format of Plan 9 audio streams as expected by /dev/audio is a 16-bit LE integer, stereo, sampled at 44.1 kHz](default-format-of-plan-9-audio-streams.md)
- [The default PCM format s161e predates plan 9](the-default-pcm-format-s161e-predates-plan-9.md)
- [12th International Workshop on Plan 9](12th-international-workshop-on-plan-9.md)