On Wed, Aug 17, 2022 at 08:10:37PM +0100, mick.crane wrote: > I'm just requesting some comprehensible, simple to understand, overview of > how the sound softwares are working.
Too many layers and possibilities to give a comprehensive listing. Let's pick ONE. Plain ALSA, without Pulseaudio or anything else on top of it. ALSA is implemented in the kernel, and the visible connection points to it are character device files: unicorn:~$ ls -l /dev/snd/ total 0 drwxr-xr-x 2 root root 60 Aug 16 07:35 by-id/ drwxr-xr-x 2 root root 80 Aug 16 07:35 by-path/ crw-rw----+ 1 root audio 116, 12 Aug 16 07:35 controlC0 crw-rw----+ 1 root audio 116, 14 Aug 16 07:35 controlC1 crw-rw----+ 1 root audio 116, 10 Aug 16 07:35 hwC0D0 crw-rw----+ 1 root audio 116, 11 Aug 16 07:35 hwC0D2 crw-rw----+ 1 root audio 116, 3 Aug 16 07:35 pcmC0D0c crw-rw----+ 1 root audio 116, 2 Aug 16 07:37 pcmC0D0p crw-rw----+ 1 root audio 116, 9 Aug 16 07:35 pcmC0D10p crw-rw----+ 1 root audio 116, 4 Aug 16 07:35 pcmC0D2c crw-rw----+ 1 root audio 116, 5 Aug 16 07:35 pcmC0D3p crw-rw----+ 1 root audio 116, 6 Aug 16 07:35 pcmC0D7p crw-rw----+ 1 root audio 116, 7 Aug 16 07:35 pcmC0D8p crw-rw----+ 1 root audio 116, 8 Aug 16 07:35 pcmC0D9p crw-rw----+ 1 root audio 116, 13 Aug 16 07:35 pcmC1D0c crw-rw----+ 1 root audio 116, 1 Aug 16 07:35 seq crw-rw----+ 1 root audio 116, 33 Aug 16 07:35 timer As you can see, members of group 'audio' can write to these things, and can therefore cause the computer to emit sounds, or to stop emitting them. But that's not how it normally works in modern Debian. Do you see the '+' signs after the permissions? Those indicate that there's more going on than can be shown in the standard Unix permission bits. In this case, there's an ACL. We can see it with getfacl(1) (from the 'acl' package): unicorn:~$ getfacl /dev/snd/pcmC0D0p getfacl: Removing leading '/' from absolute path names # file: dev/snd/pcmC0D0p # owner: root # group: audio user::rw- user:greg:rw- group::rw- mask::rw- other::--- The 'user:greg:rw-' part indicates that my login name has been added to the Access Control List for this file. I can do stuff with it, even if I'm not in the 'audio' group. (I happen to be in that group, but that's not important here.) So... why am I in this ACL on this file? Because when I logged in, something added me to a bunch of ACLs on my behalf. It's all part of the complex login procedure on modern Debian systems. I can't tell you off the top of my head which program or service does it, but something does. When I logout, presumably this ACL entry will be removed, and I would no longer have access to those files, if I weren't in the 'audio' group. If another user logs in, they'll get a similar ACL entry on the audio device files, and they'll be able to play sounds also -- until they logout, at which time their sound-making privileges are revoked. They don't need to be added to the 'audio' group, and in fact, they shouldn't be. That's reserved for special users, like the one who installed Debian (UID 1000). On my system, that user is 'greg'.
