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/* Compile with gcc -lasound -pthread threadaudio.c */

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

unsigned char audiobuffer[0x400];
pthread_mutex_t audiomutex = PTHREAD_MUTEX_INITIALIZER;

void changeaudio (int volume) {
  int i;
  pthread_mutex_lock(&audiomutex);
  for (i = 0; i < sizeof(audiobuffer); i++)
    audiobuffer[i] = (random() & 0xff) * volume / 10;
  pthread_mutex_unlock(&audiomutex);
}

void *startaudio (void *param)
{
  static char *device = "default";
  snd_output_t *output = NULL;
  int *audiostop = (int*)param;
  int err;
  snd_pcm_t *handle;
  snd_pcm_sframes_t frames;

  changeaudio(5);

  if ((err = snd_pcm_open(&handle, device, SND_PCM_STREAM_PLAYBACK, 0)) < 0) {
    printf("Playback open error: %s\n", snd_strerror(err));
    exit(EXIT_FAILURE);
  }

  if ((err = snd_pcm_set_params(handle,
                SND_PCM_FORMAT_U8,
                SND_PCM_ACCESS_RW_INTERLEAVED,
                1,
                48000,
                1,
                100000)) < 0) {   /* 0.1sec */
    printf("Playback open error: %s\n", snd_strerror(err));
    exit(EXIT_FAILURE);
  }

  while (!*audiostop) {
    err = snd_pcm_wait(handle, 1000);
    if (err < 0) {
      fprintf (stderr, "poll failed (%d)\n", err);
      break;
    }

    pthread_mutex_lock(&audiomutex);
    frames = snd_pcm_writei(handle, audiobuffer, sizeof(audiobuffer));
    pthread_mutex_unlock(&audiomutex);

    if (frames < 0)
      err = snd_pcm_recover(handle, frames, 0);
    if (err < 0) {
      printf("snd_pcm_writei failed: %s\n", snd_strerror(err));
      break;
    }

    if (frames > 0 && frames < (long)sizeof(audiobuffer))
      printf("Short write (expected %li, wrote %li)\n", (long)sizeof(audiobuffer), frames);
  }
  snd_pcm_close(handle);  
}

int main(void)
{
  pthread_t audiothread;
  int audiostop = 0;
  int volume;

  pthread_create(&audiothread, NULL, startaudio, &audiostop);

  while (1) {
    printf("Enter volume 1 through 10. [0 to quit.]: ");
    scanf("%d", &volume);
    if (volume == 0) break;
    changeaudio(volume);
  }

  audiostop = 1;
  pthread_join(audiothread, NULL);

  return 0;
}