"Default sound unit" functionality is needed.

[Vasily Postnicov]

Hello!

I need functionality similar to FreeBSD hw.snd.default_unit sysctl. It
defines which audio devices (/dev/dspN, /dev/mixerN) will be opened if
someone opens /dev/dsp or /dev/mixer.

I don't have a good idea how this can be done. I've made a quick patch for
dsp devices, it is attached.
The problem is that now the "default" node is opened all the time, no
matter if I open /dev/dsp or a specific /dev/dspN device.

My first thought was to compare ap->a_name to "dsp" to know if we just open
/dev/dsp or more specific /dev/dspN, but ap->a_name always has a form
"dspN", even if /dev/dsp is opened.

Any ideas how to implement this correctly?

                Vasily
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