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Proc asound documentation

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/proc/asound for fun and profit

\ Linux tends to put information about system processes, including some of the hardware configuration information, in a virtual filesystem called /proc. The items here are not actual files, they are a way of reading information from and sending information to the operating system kernel and associated kernel modules.

Reading information can be done by simply looking at the virtual file using cat, more, less, or some other pager/file viewer program. Sending commands back is done by using cat or echo to write data into the virtual file.

The /proc/asound set of files is what ALSA uses for device information and for some control purposes.


What are the files?

The /proc info, such as usb descriptor dump, is hardware (driver) dependent. Contents are up to the driver author, so a complete general description isn't really possible. That said, ALSA does have certain standards which most drivers adhere to.

The basic files, provided by ALSA itself:

/proc/asound/cardX/ (where X is the sound card number, from 0-7) : a cardX directory exists for each sound card the system knows about. : see below for information on the contents of this directory.

/proc/asound/cards (readonly) : the list of registered cards

/proc/asound/dev/ : a directory listing the specific device files used by programs for sound operations if your system uses devfs, this will be present : if your system does not use devfs (and as of 2006-06, most do not) : this file will either not exist at all, or simply be a symlink to /dev/snd

/proc/asound/devices (readonly) : the list of registered ALSA devices (major device number = 116)

/proc/asound/hwdep (readonly) : the list of hwdep (hardware dependent) controls does not appear on all systems (does this still exist??)

/proc/asound/meminfo (readonly) : memory usage information this proc file appears only when you build the alsa drivers : with memory debug (or full) option so the file shows the : currently allocated memories on kernel space.

/proc/asound/modules (readonly) : the list of registered ALSA soundcard drivers this is not all kernel modules loaded for ALSA, this is just the list of : drivers for hardware. Expect to see one line here for each soundcard that is : in use.

/proc/asound/oss/ : a directory containing info about oss emulation see below for information on the contents of this directory.

/proc/asound/pcm (readonly) : the list of allocated pcm streams note that this (probably) does not mean the list of active streams, it is a list : of devices. It's really useful for figuring out the hw:0,0 style device : names that commands like aplay want.

/proc/asound/seq/ : a directory containing info about sequencer see below for information on the contents of this directory.

/proc/asound/timers (readonly) : similar to /proc/asound/pcm, it's a list of timers ALSA knows about, and also : (seems to) describe which ones are actually being used for something at the : moment.

/proc/asound/version (readonly) : the version and date the ALSA subsystem module (or kernel) was built

Note: Devices marked "readonly" above are used only to provide information from the kernel. All other devices are read-write devices and can be used to send commands to ALSA.

Device files in /dev/snd/ (and /proc/asound/dev/)

Device files are what applications connect to in order to perform sound operations such as recording, playing, altering volume, getting timing information, and performing MIDI sequencing. They are generally found in /dev/snd, but may also be found in /proc/asound/dev on some systems.

Generally the device files are named in the form aaaCxDy

aaa is the service name

x is the card number (0-7)

y the device number (0-?)

controlC?  control devices (i.e. mixer, etc.)
hwC?D?     hwdep devices
midiC?D?   rawmidi devices
pcmC?D?p   pcm playback devices
pcmC?D?c   pcm capture devices
seq        sequencer device
timer      timer device

The /proc/asound/oss/ directory

The contents of the files under this directory are changed dynamically. When no oss emulation modules (snd-pcm-oss, snd-mixer-oss) are loaded, no pcm nor mixer devices will be listed.

/proc/asound/oss/devices (RO)
 the list of devices already registered

/proc/asound/oss/sndstat (RO)
 /dev/sndstat compatible list

The /proc/sound/cardX/ directory

id (RO)
 the id string of the card

ac97#? (RO)
 AC97 codec information

ac97#?regs (RO)
 (printable) register dump

midi? (RO)
 the current status of input/output on the
 rawmidi device

 the directory status of the given pcm playback stream
 the directory status of the given pcm capture stream

The /proc/asound/cardX/pcmXX/ directories

The files in these optional directories contain PCM stream information. Note that with Linux 2.6.17 and newer these files will only be available if CONFIG_SND_VERBOSE_PROCFS ("Verbose procfs contents") is enabled in the kernel config.

pcm??/info (RO)
 the pcm stream general info (card, device, name, etc.)

pcm??/oss (RO)
 oss emulation info (shown only when the pcm is opened
 as an oss device).

 the substream information directory

pcm??/sub?/info (RO)
 the pcm substream general info (card, device, name, etc.)

pcm??/sub?/status (RO)
 the current status of the given pcm substream
 (status, position, delay, tick time, etc.)

pcm??/sub?/hw_params (RO)
 hw_params set-up on the substream
 (buffer size, format, etc.)

pcm??/sub?/sw_params (RO)
 sw_params set-up on the substream
 (threshold, etc.)

pcm??/sub?/prealloc (RW)
 the number of pre-allocated buffer size in kb.
 you can specify the buffer size by writing to this proc file:

 # echo 128 > /proc/asound/card0/pcm0p/sub0/prealloc

 to allocate 128kbyte for playback, substream #0, stream #0
 on the card #0.

To find all the options for the alsa modules on your machine run this script...

modinfo $(modprobe -l snd-*) > ~/modinfo

The /proc/asound/seq/ directory

clients : Need info

drivers : Need info

oss : Need info

queues : Need info

timer : Need info

So what are the hardware devices?

Typical output might look like this:

prompt# cat /proc/asound/devices  
  0: [ 0]   : control 
  1:        : sequencer 
 16: [ 0- 0]: digital audio playback 
 18: [ 0- 2]: digital audio playback 
 24: [ 0- 0]: digital audio capture 
 25: [ 0- 1]: digital audio capture 
 33:        : timer 

The example above tells you that you have one control channel, two PCM playback devices (DAC's), two PCM capture devices (ADC's), a MIDI sequencer, and a timer.

On the system used for this example, with no remapping of anything to anything else, these equate to the following:


  • First PCM playback DAC

    • What it does: Plays sound
    • The device file looks like:
    • crw-rw---- 1 root audio 116, 16 Mar 4 21:30 pcmC0D0p
    • (the date on yours will probably be different)
    • What ALSA calls it: the playback half of hw:0,0, which is a duplex device
  • First PCM recording ADC

    • What it does: Plays sound
    • The device file looks like:
    • crw-rw---- 1 root audio 116, 16 Mar 4 21:30 pcmC0D0c
    • What ALSA calls it: the recording half of hw:0,0, which is a duplex device
  • Control Channel for first soundcard

    • What it Does: controls volume/recording gain (and other stuff?)
    • The device file looks like:
    • crw-rw---- 1 root audio 116, 0 Mon DD hh:mm /dev/snd/controlC0
    • (Mon DD hh:mm will be the date and time the device file was created on your system)
    • What ALSA calls it:  ??

/proc will change

In future /proc will be used for process information only, and the place to look for ALSA info will be sysfs.

In the 2.6 kernel source, there's a file called


with some info about this. As of 2006-06 (and kernel 2.6.16), /sys exists, but the /proc interface has not changed yet.

See Also

  • Procfile.txt: Takashi Iwai's Procfile.txt in the ALSA Documentation.

  • aadebug: a script you can use to provide a brief snapshot of your system suitable for emailing to someone else for help.

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