On 13.03.2013 17:47, Paul D. DeRocco wrote:
From: Clemens Ladisch [mailto:cladisch@googlemail.com]
#include <stdio.h> #include <stdlib.h> #include <alsa/asoundlib.h>
static void check(int err, const char *f) { if (err < 0) { fprintf(stderr, "%s failed: %s\n", f, snd_strerror(err)); exit(EXIT_FAILURE); } } #define CHECK(f) check(f, #f)
int main() { snd_ctl_t *ctl; snd_ctl_elem_value_t *value;
CHECK(snd_ctl_open(&ctl, "default", 0)); snd_ctl_elem_value_alloca(&value); snd_ctl_elem_value_set_interface(value,SND_CTL_ELEM_IFACE_MIXER); snd_ctl_elem_value_set_name(value, "Master Playback Volume"); snd_ctl_elem_value_set_integer(value, 0, 42); CHECK(snd_ctl_elem_write(ctl, value));
snd_ctl_close(ctl); return 0;}
Well, I finally got a chance to try this. It "works", but the snd_ctl_elem_write call takes a couple of milliseconds (that's around a million instructions on my Atom), and it apparently spends this time in kernel state, because it causes my sound thread to underrun, so it's not usable.
So why are you doing this from your sound thread then?
Daniel
If I use alsamixer, I don't have this problem; I can manipulate the master volume smoothly while I'm playing my synth app. So I went through the source, and found that it's using snd_mixer_selem_set_playback_volume. This is apparently part of something called the "Simple Mixer Interface", yet another section of ALSA which is completely undocumented, other than the Doxygen stuff (which is only meaningful as a reference for someone who already knows it inside out, but can't remember e.g. whether playback_pcm comes before or after capture_pcm in the snd_mixer_selem_regopt structure).
So what's the minimum I need to do in order to get at the Master Playback Volume, via the Simple Mixer Interface?