title: When to use snd_pcm_avail instead of snd_pcm_avail_update?
body: <p>ALSA functions <code>snd_pcm_avail</code> and <code>snd_pcm_avail_update</code> are very similar. Most examples seem to use <code>snd_pcm_avail_update</code>. When should I use <code>snd_pcm_avail</code> instead?</p>
  
  <p>In my test program it does not seem to make a difference which function I use. What is the difference?</p>