[alsa-devel] handling of underrunning snd_pcm_writei

Mads Kiilerich mads at kiilerich.com
Wed Nov 11 17:50:12 CET 2009


I AM trying to convert an internal app from oss to alsa and is trying to 
grok the alsa API and the examples I can find.

I would like to ask some questions to avoid guessing and making wrong 
assumptions. Feel free to redirect me to where I should have found the 
answer myself.

says about snd_pcm_writei:

> Returns: a positive number of frames actually written otherwise a negative error code
> ...
> Return values:
> ...
> -EPIPE 	an underrun occurred
> ...
> The count of bytes can be less only if a signal or underrun occurred.

So what is returned in case of underrun? Error code or count of bytes?

Should snd_pcm_recover always be used to get the byte count after 
-EPIPE? snd_pcm_recover is neither mentioned in the link above nor in 
http://www.alsa-project.org/alsa-doc/alsa-lib/pcm.html , so I don't feel 
confident about that.

And is it measured in "positive number of frames actually written" or 
"count of bytes"?

It surprises me that snd_pcm_writei shows non-blocking behaviour in case 
of underruns and don't use all the offered data. I would guess that it 
either blocked and enjoyed the data it finally got, or that it gave an 
error and didn't take any data at all. But if this is how it is, then 
... that is how it is ;-)



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