alsa snd_pcm_writei underrun error

Question

I am using alsa in arm linux 3.10. There are 3 wireless devices: two capture and send pcm using snd_pcm_readi and sendmsg and one as receiver, playback what was received using snd_pcm_writei.

Here is procedure: programs works fine while 1 sender and 1 receiver. But when I stop first sender and start the second sender, receiver side has underrun error. For each sender, receiver has a unique pcm handle to deal with data. And all sides are using the same configuration except sender handle open with name default[snd_pcm_open(pcm_handle, "default", stream,0)] and receiver handle are using plug:dmix.[snd_pcm_open(pcm_handle, "plug:dmix", stream,0)]

while(1)
{
  recv_from();
  if sender one or two's first packet
     alsa init handle1 or handle2
  int err = snd_pcm_writei(handle1 or 2, buffer, 224);
  if(err == -EPIPE)
  {
    fprintf(stderr, "underrun occurred\n");
    snd_pcm_prepare(handle);
    continue;
  }
}

The purpose is to make the receiver be able to play mixed sound for both senders. Can anyone help me with this, I am new to alsa programming. Thanks in advance!

Jasper Jasper · Accepted Answer · 2016-05-15T08:08:33

I might have found the issue. The default start threshold in configuration is 1. And when I set this to n times of my writing frames (n = 4 is what I used), the underrun issue drops its occurrences. Only once in several minutes underrun may occur. So I assume the problem is that sound card will trying to get frames while the buffer size larger than 1 (threshold).

alsa snd_pcm_writei underrun error

1 Answers