int beep(int freq, int ms) {
  static void *pcm = NULL;
  if (pcm == NULL) {
    if (snd_pcm_open(&pcm, "default", 0, 0)) {
      return -1;
    }
    snd_pcm_set_params(pcm, 1, 3, 1, 8000, 1, 20000);
  }
  unsigned char buf[2400];
  long frames;
  long phase;
  for (int i = 0; i < ms / 50; i++) {
    snd_pcm_prepare(pcm);
    for (int j = 0; j < sizeof(buf); j++) {
      buf[j] = freq > 0 ? (255 * j * freq / 8000) : 0;
    }
    int r = snd_pcm_writei(pcm, buf, sizeof(buf));
    if (r < 0) {
      snd_pcm_recover(pcm, r, 0);
    }
  }
  return 0;
}

AudioComponent output;
AudioUnit unit;
AudioComponentDescription descr;
AURenderCallbackStruct cb;
AudioStreamBasicDescription stream;
descr.componentType = kAudioUnitType_Output,
descr.componentSubType = kAudioUnitSubType_DefaultOutput,
descr.componentManufacturer = kAudioUnitManufacturer_Apple,
// Actual sound will be generated asynchronously in the callback tone_cb
cb.inputProc = tone_cb;
stream.mFormatID = kAudioFormatLinearPCM;
stream.mFormatFlags = 0;
stream.mSampleRate = 8000;
stream.mBitsPerChannel = 8;
stream.mChannelsPerFrame = 1;
stream.mFramesPerPacket = 1;
stream.mBytesPerFrame = 1;
stream.mBytesPerPacket = 1;
output = AudioComponentFindNext(NULL, &descr);
AudioComponentInstanceNew(output, &unit);
AudioUnitSetProperty(unit, kAudioUnitProperty_SetRenderCallback,
                    kAudioUnitScope_Input, 0, &cb, sizeof(cb));
AudioUnitSetProperty(unit, kAudioUnitProperty_StreamFormat,
                    kAudioUnitScope_Input, 0, &stream, sizeof(stream));
AudioUnitInitialize(unit);
AudioOutputUnitStart(unit);

static OSStatus tone_cb(void *inRefCon,
                        AudioUnitRenderActionFlags *ioActionFlags,
                        const AudioTimeStamp *inTimeStamp, UInt32 inBusNumber,
                        UInt32 inNumberFrames, AudioBufferList *ioData) {
  unsigned int frame;
  unsigned char *buf = ioData->mBuffers[0].mData;
  unsigned long i = 0;
  for (i = 0; i < inNumberFrames; i++) {
    buf[i] = beep_freq > 0 ? (255 * theta * beep_freq / 8000) : 0;
    theta++;
    counter--;
  }
  return 0;
}