alsa_user_playback_interrupt程序

/* ALSA用户空间播放程序(基于"中断") */

#include
#include
#include
#include
#include

snd_pcm_t *playback_handle;
short buf[4096];

int playback_callback(snd_pcm_sframes_t nframes)
{
 int err;
 printf("playback callback called with %u frames\n", nframes);

 /* 填充缓冲区 */
 if((err = snd_pcm_writei(playback_handle, buf, nframes)) < 0)
 {
  fprintf(stderr, "write failed (%s)\n", snd_strerror(err));
 }

 return err;
}

int main(int argc, char *argv[])
{
 snd_pcm_hw_params_t *hw_params;
 snd_pcm_sw_params_t *sw_params;
 snd_pcm_sframes_t frames_to_deliver;
 int nfds;
 int err;
 struct pollfd *pfds;

 if((err = snd_pcm_open(&playback_handle, argv[1], SND_PCM_STREAM_PLAYBACK, 0)) < 0)
 {
  fprintf(stderr, "cannot open audio device %s (%s)\n", argv[1], snd_strerror(err));
  exit(1);
 }

 if((err = snd_pcm_hw_params_malloc(&hw_params)) < 0)
 {
  fprintf(stderr, "cannot allocate hardware parameter structure (%s)\n", snd_strerror(err));
  exit(1);
 }

 if((err = snd_pcm_hw_params_any(playback_handle, hw_params)) < 0)
 {
  fprintf(stderr, "cannot initialize hardware parameter structure (%s)\n", snd_strerror(err));
  exit(1);
 }

 if((err = snd_pcm_hw_params_set_access(playback_handle, hw_params, SND_PCM_ACCESS_RW_INTERLEAVED)) < 0)
 {
  fprintf(stderr, "cannot set access type (%s)\n", snd_strerror(err));
  exit(1);
 }

 if((err = snd_pcm_hw_params_set_format(playback_handle, hw_params, SND_PCM_FORMAT_S16_LE)) < 0)
 {
  fprintf(stderr, "cannot set sample format (%s)\n", snd_strerror(err));
  exit(1);
 }

 if((err = snd_pcm_hw_params_set_rate_near(playback_handle, hw_params, 44100, 0)) < 0)
 {
  fprintf(stderr, "cannot set sample rate (%s)\n", snd_strerror(err));
  exit(1);
 }

 if((err = snd_set_params(playback_handle, hw_params)) < 0)
 {
  fprintf(stderr, "cannot set parameters (%s)\n", snd_strerror(err));
  exit(1);
 }

 snd_pcm_hw_params_free(hw_params);

 /* 告诉ALSA当4096个以上帧可以传递时唤醒我们 */
 if((err = snd_pcm_sw_params_malloc(&sw_params)) < 0)
 {
  fprintf(stderr, "cannot allocate software parameters structure (%s)\n", snd_strerror(err));
  exit(1);
 } 

 if((err = snd_pcm_sw_params_current(playback_handle, sw_params)) < 0)
 {
  fprintf(stderr, "cannot initialize software parameters structure (%s)\n", snd_strerror(err));
  exit(1);
 }

 /* 设置4096帧传递一次数据 */
 if((err = snd_pcm_sw_params_set_avail_min(playback_handle, sw_params, 4096)) < 0)
 {
  fprintf(stderr, "cannot set minimun available count (%s)\n", snd_strerror(err));
  exit(1);
 }

 /*一旦有数据就开始播放 */
 if((err = snd_pcm_sw_params_set_start_threshold(playback_handle, sw_params, 0U)) < 0)
 {
  fprintf(stderr, "cannot set start mode (%s)\n", snd_strerror(err));
  exit(1);
 }

 if((err = snd_pcm_sw_params(playback_handle, sw_params)) < 0)
 {
  fprintf(stderr, "cannot set software parameters (%s)\n", snd_strerror(err));
  exit(1);
 }

 /* 每4096帧接口将中断内核,ALSA将很快唤醒本程序 */
 if((err = snd_pcm_prepare(playback_handle)) < 0)
 {
  fprintf(stderr, "cannot prepare audio interface for use (%s)\n", snd_strerror(err));
  exit(1);
 }

 while(1)
 {
  /* 等待,直到接口准备好传递数据,或者1s超时发生 */
  if((err = snd_pcm_wait(playback_handle, 1000)) < 0)
  {
   fprintf(stderr, "poll failed (%s)\n", snd_strerror(err));
   break;
  }

  /* 查出有多少空间可放置playback数据 */
  if((frames_to_deliver = snd_pcm_avail_update(playback_handle)) < 0)
  {
   if(frames_to_deliver == -EPIPE)
   {
    fprintf(stderr, "an xrun occured\n");
    break;
   }
   else
   {
    fprintf(stderr, "unknown ALSA avail update return value (%d)\n", frames_to_deliver);
    break;
   }
  }

  frames_to_deliver = frames_to_deliver > 4096 ? 4096 : frames_to_deliver;

  /* 传递数据 */
  if(playback_callback(frames_to_deliver) != frames_to_deliver)
  {
    fprintf(stderr, "playback callback failed\n");
    break;
  }
 }

 snd_pcm_close(playback_handle);
 exit(0);
  
 return 0;
}