How to use alsa sound and/or snd_pcm_open in docker? - linux

Question

I am running an Ubuntu 12.04 Docker container on an Ubuntu 16.04 host. Some test code I have exercises 'snd_pcm_open'/'snd_pcm_close' operations with the SND_PCM_STREAM_PLAYBACK and SND_PCM_STREAM_CAPTURE stream types.
I do not need any actual sound/audio capabilities but just getting the 'snd_pcm_open' return 0 with a valid handle, then 'snd_pcm_close' to return 0 on the same handle would be good enough for my purposes. I do not want to modify the code as it's already got some not-so-nice platform dependent switches and I am not the maintainer.
I am using the simple code and compiling it as 'g++ alsa_test.cpp -lasound'
#include <stdio.h>
#include <alsa/asoundlib.h>
int main() {
snd_pcm_t* handle;
snd_pcm_stream_t stream_type[]= {SND_PCM_STREAM_PLAYBACK, SND_PCM_STREAM_CAPTURE};
printf("\nstarting\n");
for (unsigned char i = 0; i < sizeof(stream_type) / sizeof(stream_type[0]); ++i) {
printf(">>>>>>>>\n\n");
int deviceResult = snd_pcm_open(&handle, "default" , stream_type[i], 0);
printf("\n%d open: %d\n", stream_type[i], deviceResult);
if (deviceResult >= 0) {
printf("attempting to close %d\n", stream_type[i]);
snd_pcm_drain(handle);
deviceResult = snd_pcm_close(handle);
printf("%d close: %d\n\n", stream_type[i], deviceResult);
}
printf("<<<<<<<<\n\n");
}
return 0;
}
It works just fine on the host but despite all the different things I tried, 'snd_pcm_open' returns '-2' for both stream types in the container.
I tried installing the 'libasound2.dev' but 'modinfo soundcore' is empty and '/dev/snd' does not exist.
Also tried running the container with the options below, even though it feels like a massive over kill for such a simple purpose
--privileged --cap-add=ALL -v /dev:/dev -v /lib/modules:/lib/modules
After these extra parameters to the container, following commands generate the same output both in the host and the container.
root#31142791f82d:/export# modinfo soundcore
filename: /lib/modules/4.4.0-59-generic/kernel/sound/soundcore.ko
alias: char-major-14-*
license: GPL
author: Alan Cox
description: Core sound module
srcversion: C941364F5CD0B525693B243
depends:
intree: Y
vermagic: 4.4.0-59-generic SMP mod_unload modversions
parm: preclaim_oss:int
root#31142791f82d:/export# ls -l /dev/snd/
total 0
drwxr-xr-x 2 root root 100 Feb 2 21:10 by-path
crw-rw----+ 1 root audio 116, 2 Feb 2 07:42 controlC0
crw-rw----+ 1 root audio 116, 7 Feb 2 07:42 controlC1
crw-rw----+ 1 root audio 116, 12 Feb 2 21:10 controlC2
crw-rw----+ 1 root audio 116, 6 Feb 2 07:42 hwC0D0
crw-rw----+ 1 root audio 116, 11 Feb 2 07:42 hwC1D0
crw-rw----+ 1 root audio 116, 3 Feb 2 07:42 pcmC0D3p
crw-rw----+ 1 root audio 116, 4 Feb 2 07:42 pcmC0D7p
crw-rw----+ 1 root audio 116, 5 Feb 2 07:42 pcmC0D8p
crw-rw----+ 1 root audio 116, 9 Feb 2 10:44 pcmC1D0c
crw-rw----+ 1 root audio 116, 8 Feb 2 07:42 pcmC1D0p
crw-rw----+ 1 root audio 116, 10 Feb 2 21:30 pcmC1D1p
crw-rw----+ 1 root audio 116, 14 Feb 2 21:10 pcmC2D0c
crw-rw----+ 1 root audio 116, 13 Feb 2 21:10 pcmC2D0p
crw-rw----+ 1 root audio 116, 1 Feb 2 07:42 seq
crw-rw----+ 1 root audio 116, 33 Feb 2 07:42 timer
The container only has the 'root' user by the way, so, access rights shouldn't be an issue either.
What would be the easiest and least hacky way to get this working? I'd rather get rid off the privileged mode and dev/modules mapping to the container however, these containers are not accessed from the outside world and are only created/destroyed for some short lived tasks. So, safety isn't exactly a massive concern.
Thanks in advance.

Answer 1

If you don't actually need the device to work correctly, use the null device instead of default.
To make the null plugin the default one, put this into the container's /etc/asound.conf, or into the user's ~/.asoundrc:
pcm.!default = null;