amixer-volume-loop.c

#include <stdio.h>
#include <alsa/asoundlib.h>

static char card[64] = "default";


static void error(const char *fmt,...)
{
  va_list va;

  va_start(va, fmt);
  fprintf(stderr, "amixer: ");
  vfprintf(stderr, fmt, va);
  fprintf(stderr, "\n");
  va_end(va);
  exit(-1);
}


int main() {
  int err;
  snd_mixer_t *handle = NULL;
  snd_mixer_elem_t *elem;
  snd_mixer_selem_id_t *sid;
  long pvol, pmin, pmax;
  snd_mixer_selem_id_alloca(&sid);

  fprintf(stderr, "welcome\n");


  if ( (err = snd_mixer_open(&handle, 1)) < 0)
    error("Control %s open error: %s\n", card, snd_strerror(err));

  fprintf(stderr, "attach\n");

  if ((err = snd_mixer_attach(handle, card)) < 0) {
    snd_mixer_close(handle);
    error("Mixer attach %s error: %s", card, snd_strerror(err));
  }
  fprintf(stderr, "register\n");

  if ((err = snd_mixer_selem_register(handle, NULL, NULL)) < 0) {
    snd_mixer_close(handle);
    error("Mixer register error: %s", snd_strerror(err));
  }
  fprintf(stderr, "load\n");

  err = snd_mixer_load(handle);
  if (err < 0) {
    snd_mixer_close(handle);
    error("Mixer %s load error: %s", card, snd_strerror(err));
  }

  fprintf(stderr, "set_index\n");

  snd_mixer_selem_id_set_index(sid, 0); // spotify does this
  snd_mixer_selem_id_set_name(sid, "Master");

  fprintf(stderr, "find_selem\n");

  elem = snd_mixer_find_selem(handle, sid);
  if (!elem) {
    snd_mixer_close(handle);
    error("Unable to find simple control '%s',%i\n",
          snd_mixer_selem_id_get_name(sid),
          snd_mixer_selem_id_get_index(sid));
  }

  fprintf(stderr, "get_index\n");

  printf("Simple mixer control '%s',%i\n",
         snd_mixer_selem_id_get_name(sid),
         snd_mixer_selem_id_get_index(sid));
  //show_selem(handle, sid, "  ", 1);

  //  err = snd_mixer_selem_set_playback_volume(elem, SND_MIXER_SCHN_MONO, 33);
  //if(err != 0) fprintf(stderr, "error: could not get playback range\n");

  // keep printing mixer values for a while. they don't seem to update
  // when updated externally!
  while(1) {
    err = snd_mixer_selem_get_playback_volume_range(elem, &pmin, &pmax);
    if(err != 0) fprintf(stderr, "error: could not get playback range\n");

    err = snd_mixer_selem_get_playback_volume(elem, SND_MIXER_SCHN_MONO, &pvol);
    if(err != 0) fprintf(stderr, "error: could not get playback range\n");

    fprintf(stderr, "vol: %ld [%ld, %ld]\n", pvol, pmin, pmax);

    pvol = 0;
    usleep(1000 * 1000);
  }

  err = snd_mixer_close(handle);
  if(err != 0)
    fprintf(stderr, "could not close alsa handle!\n");

  return 0;
}

this program is missing handle_events. have a look here, if you're interested