A program attempting to open a sound device for exclusive access may fail, if that audio device is already in use by another program. Also, depending on the capabilities of the audio device, such as whether it is full-duplex or not, errors may occur if capture is requested during playback. The actual error messages produced are program and operating system dependent. However, some typical error messages are listed below.

Audio device unavailable.
Cannot open audio device.
Could not gain access to /dev/dsp for writing.
Audio capabilities are not available on this machine.
Can't open output file '/dev/dsp': Device or resource busy. # ©2007 dsplabs.com.au
Error Initializing Audio: There was an error initializing the audio i/o layer.

The reminder of this article briefly discusses the Linux sound architecture, followed by an explanation on how to locate and close applications that are using an audio device.

Linux audio
There are two main audio APIs used under Linux, namely the Open Sound System (OSS) and the Advanced Linux Sound Architecture (ALSA). There are also some alternatives, however OSS and ALSA are by far most popular audio interfaces. If you would like to read a bit more on how OSS and ALSA APIs compare then have a look at the ALSA vs. OSS article. In our article, we will mostly limit our discussion to the ALSA API. Note however that ALSA does provide a compatibility mode for OSS. Some very useful information about ALSA can be found in Alsa-sound-mini-HOWTO. Well, let me squeeze in another useful reference before proceeding. The beautiful thing about Linux is how simple things are under it. Under Linux, the audio devices are simply character devices. This makes it easy to access and use them. If you are planning on writing some code that requires audio device manipulation, then A Tutorial on Using the ALSA Audio API is well worth a read.

To start, lets have a look at audio cards installed on your system, by running the following command.

cat /proc/asound/cards

If, for instance, your sound card is based on NVidia NFORCE chip-set the output might look something like this:

 0 [CK804          ]: NFORCE - NVidia CK804
                      NVidia CK804 with ALC850 at 0xd0103000, irq 233

On my system, the sound card is part of the Intel chip-set and is listed below as card 0. There is also another card with audio capabilities (card 1) which is an analog modem (in this case also a part of the Intel chip-set).

 0 [I82801DBICH4   ]: ICH4 - Intel 82801DB-ICH4
                      Intel 82801DB-ICH4 with unknown codec at 0xe0100c00, irq 10
 1 [Modem          ]: ICH-MODEM - Intel 82801DB-ICH4 Modem
                      Intel 82801DB-ICH4 Modem at 0x2400, irq 10

Strictly speaking, an audio card could have a number of on-board devices. These devices are available under the /dev tree. The OSS device nodes are located in /dev/dsp* while ALSA uses /dev/snd/*. Lets have a closer look at ALSA’s device nodes.

ls -la /dev/snd

The above command should produce an output similar to the one shown below, provided that you do have a sound card and that ALSA is present and loaded on your system.

total 0
drwxr-xr-x  2 root  root     280 Oct 31 13:48 .
drwxr-xr-x 12 root  root    3840 Oct 31 13:49 ..
crw-------  1 kamil root 116, 10 Oct 31 13:48 controlC0
crw-------  1 kamil root 116, 13 Oct 31 13:48 controlC1
crw-------  1 kamil root 116,  9 Oct 31 13:48 pcmC0D0c
crw-------  1 kamil root 116,  8 Oct 31 13:48 pcmC0D0p
crw-------  1 kamil root 116,  7 Oct 31 13:48 pcmC0D1c
crw-------  1 kamil root 116,  6 Oct 31 13:48 pcmC0D2c
crw-------  1 kamil root 116,  5 Oct 31 13:48 pcmC0D3c
crw-------  1 kamil root 116,  4 Oct 31 13:48 pcmC0D4p
crw-------  1 kamil root 116, 12 Oct 31 13:48 pcmC1D0c
crw-------  1 kamil root 116, 11 Oct 31 13:48 pcmC1D0p
crw-------  1 kamil root 116,  3 Oct 31 13:48 seq
crw-------  1 kamil root 116,  2 Oct 31 13:48 timer

The above output shows a number of devices. Amongst others, there is the /dev/snd/controlC0 which is the control device for card 0, as well as /dev/snd/pcmC0D0p which is the raw audio playback device 0 on card 0. Note that the current naming convention is such that the card number follows the letter C and device number follows the letter D. The last letter on the pcm devices, c or p, indicates if the device is used for capture or playback, respectively.

Reproducing the error
Let us try to reproduce a conflict described in the introduction. First, we’ll run mplayer to play an mp3 file song.mp3, as follows.

mplayer song.mp3

Then we’ll try to play an audio file from Matlab.

>> [x, fs, nbits] = wavread('audio.wav');
>> sound(x, fs);
>> Unable to open the audio device for playback:
   Audio Device Unavailable

The Java sound interface requires an exclusive access to the audio device, and hence the above error is produced. This is just one, somewhat artificial, example.

Freeing the audio device
So, the question is: how to free a Linux sound device? Well, the simplest way is to close, or end, the program that has opened it in the first place. A program will open an audio device with some read/write permissions and will hold a handle to that device. A well behaved application should release any open file handles (in-fact any resources) on exit back to the operating system. If an application fails to do that the operating system may collect these resources after the application terminates.

One may ask how to find out which program, or programs, have gained access to the audio device? That is where a little knowledge of Linux tools, and especially the lsof utility, comes in handy. Since we know where in the /dev tree the devices are mapped under ALSA we can use lsof, which stands for `list open files’, to locate open sound device files. lsof will also tell us which programs have opened them.

So, lets run lsof and grep snd from its output to find out which sound devices are open.

lsof |grep snd

The output below shows that the KDE sound mixer, kmix, has opened the control devices for both cards (the sound card and the modem). More importantly, the mplayer is using the pcm device (i.e. the raw audio device) for playback. It is for this reason that the playback failed in Matlab.

kmix       5444     kamil   10u      CHR     116,10      4807 /dev/snd/controlC0
kmix       5444     kamil   11u      CHR     116,13      4944 /dev/snd/controlC1
mplayer   14004     kamil  mem       CHR      116,8      4684 /dev/snd/pcmC0D0p
mplayer   14004     kamil    4r      CHR      116,2      3914 /dev/snd/timer
mplayer   14004     kamil    5u      CHR      116,8      4684 /dev/snd/pcmC0D0p

Once the program that holds the audio device hostage has been identified, one can terminate it to free the device. If it is a shell program then ending it can be achieved with Ctrl+c. If it is a GUI based program then Alt+F4 may accomplish it, or f.e. File->Quit menu will also do it. Note however, that there are times when programs become unresponsive, or even crash, without properly freeing the open file handles. In such a case, the use of lsof becomes even more useful. Once the program is identified, a killall or kill commands can be used to terminate such a program. You could ask the application to close gracefully, i.e. give it time to perform any exit tasks (including properly freeing resources) by using

killall -TERM mplayer

or you could could force its termination by using

killall -SIGKILL mplayer

If you want to be more specific (and not kill every running instance of mplayer) then you could terminate a specific program using its process id (PID). Note that PID information is part of the lsof output. In our example the mplayer’s PID is 14004. The command below was used to terminate it.

kill -TERM 14004

After terminating the program we can have another look at open audio devices.

lsof |grep snd

Notice, in the output below, that the raw audio device is no longer in use by mplayer.

kmix       5444     kamil   10u      CHR     116,10      4807 /dev/snd/controlC0
kmix       5444     kamil   11u      CHR     116,13      4944 /dev/snd/controlC1

Note that lsof is an extremely usesful utility. It finds many applications beyond the simple example described here. Another very useful utility is lslk — list local locks (i.e. list lock files). I will describe the use of lslk in a future article.

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