[alsa-devel] Detect when sound card is in use

Clemens Ladisch clemens at ladisch.de
Wed Apr 29 09:16:37 CEST 2009

Takashi Iwai wrote:
> Claus Ilginnis wrote:
> > I have a speaker set connected to an USB-Power-Plug (by which I can turn
> > the speakers on and off )
> > 
> > Now I want to detect, when the soundcard is in use and when not.
> > (to turn the speakers on when needed...)
> > 
> > Which way is best to detect wheather my sound card is in use or not ?
> > Is there a general overview/documentation of the sound kernel modules
> > which describes the general relationship between them ?
> > What would you do ?
> The easiest way would be to check the status of device files, such as
> /dev/snd/*, whether any process opens it, e.g. via lsof command.

This would require polling.

> But, this doesn't cover the case like an app such as a mixer applet
> which keeps opening the device but access only occasionally.

I guess the mixer doesn't matter as long as nothing is playing, so it
would be enough to monitor /dev/snd/pcm*.

The easiest way would be to wrap your playback application in a script,
but this works only if that is the only application that plays sounds.

If all your applications use the ALSA API (and not OSS), you could
write a filter plugin that just passes through all requests to the
actual device and switches the power plug in the open/close callbacks.

Best regards,

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