Daemon Dancing in the Dark

Teaching OSS to Share

Say you're doing some programming, or trying out an older audio program, and you get some form of the following cryptic error:

unable to open `/dev/dsp', Device or resource busy

What does it mean and what can you do about it?

As to what it means - basically, an application somewhere isn't being very nice and sharing, like we have all be taught to do. Or it wants to hog the sound device all to itself and another app won't let it go. /dev/dsp is the device pointer for the default "dsp" or digital sound processor and, as audio remains a bit of an Achilles heel for Linux, there can be problems in the sandbox.

So what you need to do is to figure out which app currently has locked, so maybe you can quit that application to allow our other one to access it. You might think of using lsof or fuser to figure it out. So you check:

$ lsof /dev/dsp
$ fuser /dev/dsp

But unfortunately, it won't show you anything. I think this is because /dev/dsp is just an affectation, which allows Linux to point wherever it really wants to.

In this case, it is the /dev/snd/* files. It is here you want to see who is grabbing things:

$ ls /dev/snd
controlC0  hwC0D2    pcmC0D0p  pcmC0D1p  pcmC1D0c  seq
controlC1  pcmC0D0c  pcmC0D1c  pcmC0D2c  pcmC1D0p  timer
$ lsof /dev/snd/*
COMMAND    PID     USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
VirtualBo 4083 jdarnold  mem    CHR 116,10          4395 /dev/snd/pcmC0D0c
VirtualBo 4083 jdarnold  mem    CHR  116,9          4390 /dev/snd/pcmC0D0p
VirtualBo 4083 jdarnold   40r   CHR  116,2      0t0 4102 /dev/snd/timer
VirtualBo 4083 jdarnold   42u   CHR 116,10      0t0 4395 /dev/snd/pcmC0D0c
VirtualBo 4083 jdarnold   43u   CHR 116,12      0t0 4411 /dev/snd/controlC0
VirtualBo 4083 jdarnold   44r   CHR  116,2      0t0 4102 /dev/snd/timer
VirtualBo 4083 jdarnold   45u   CHR  116,9      0t0 4390 /dev/snd/pcmC0D0p
VirtualBo 4083 jdarnold   46u   CHR 116,12      0t0 4411 /dev/snd/controlC0
kmix      4155 jdarnold   12u   CHR 116,12      0t0 4411 /dev/snd/controlC0
kmix      4155 jdarnold   13u   CHR  116,5      0t0 4560 /dev/snd/controlC1
$ fuser /dev/snd/*
/dev/snd/controlC0:   4155  4800
/dev/snd/controlC1:   4155
/dev/snd/pcmC0D0c:    4800m
/dev/snd/pcmC0D0p:    4800m
/dev/snd/timer:       4800
$ ps -c 4800
  PID CLS PRI TTY      STAT   TIME COMMAND
 4800 TS   19 ?        SLl    0:12 /usr/lib/virtualbox/VirtualBox -comment Debia

Pretty conclusive proof that it is VirtualBox using the device. So let's see what it looks like after I quit out of VirtualBox:

$ lsof /dev/snd/*
COMMAND  PID     USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
kmix    4155 jdarnold   12u   CHR 116,12      0t0 4411 /dev/snd/controlC0
kmix    4155 jdarnold   13u   CHR  116,5      0t0 4560 /dev/snd/controlC1
$ fuser /dev/snd/*
/dev/snd/controlC0:   4155
/dev/snd/controlC1:   4155
$ ps -c 4155
  PID CLS PRI TTY      STAT   TIME COMMAND
 4155 TS   19 ?        S      0:00 kmix [kdeinit] -autostart                   

kmix, the KDE sound mixer program, is pretty good about things, and thus we no longer have a conflict:

$ ./dspinfo
Information on /dev/dsp:

Defaults:
  sampling rate: 8000 Hz
  channels: 1
  sample size: 8 bits
  block size: 1023 bytes

Supported Formats:
  mu-law
  unsigned 8-bit (default)
  signed 16-bit little-endian
  signed 16-bit big-endian
  signed 8-bit
  unsigned 16-bit little-endian
  unsigned 16-bit big-endian

Capabilities:
  revision: 1
  full duplex: yes
  real-time: yes
  batch: no
  coprocessor: no
  trigger: yes
  mmap: yes

Modes and Limits:
Device    Sample    Minimum   Maximum
Channels  Size      Rate      Rate
--------  --------  --------  --------
       1         8      1000    100000
       1        16      1000    100000
       2         8      1000    100000
       2        16      1000    100000

dspinfo is a small program I adapted from the exerpt for the old O'Reilly book Linux Multimedia Guide, which is Chapter 14. Programming Sound Devices. As I said, OSS (Open Sound System) is an old Linux standard, pretty much passed by in these days of 2.6 kernels.

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