[alsa-devel] snd_pcm_poll_descriptors() error return

[Takashi Sakamoto]

Hi,

On Oct 02 2018 06:23, Alan Horstmann wrote:
> A user of the Portaudio library is reporting that from Alsa version 1.1.4 the
> function snd_pcm_poll_descriptors() can return -32 (Broken Pipe), and this
> fails an assert within Portaudio.  Earlier 1.1.3 does not seem to  show this
> issue.  For more details, see
> https://app.assembla.com/spaces/portaudio/support/tickets/268.
> 
> The Alsa docs don't mention any possible negative error code from this
> function - has the behaviour changed, or is it likely something else is
> triggering the difference?

Return values from a call of 'snd_pcm_poll_descriptors()' varies
depending on implementation of PCM plugins for used PCM node. Therefore
it's impossible to cover all of the values in documentation.

In a design of alsa-lib, 'snd_pcm_poll_descriptors()' is a thin wrapper
of '.poll_descriptors' callback of internal/external structure of PCM
plugins, therefore its behaviour is dynamically different depending on
each implementation.

For example, when using 'hw' PCM node directly, a call of this function
can returns '-ENOMEM' or '2':
http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=src/pcm/pcm_hw.c;h=59a242009e9f5f1098759305499eaa27bef6c1ab;hb=HEAD#l227

When using 'dmix' PCM node, '-EIO', '0', '-EPIPE' and '1':
http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=src/pcm/pcm_direct.c;h=2b07eff90ff6e12200e27a0d40378f29a7bf0d24;hb=HEAD#l668

When using PCM node with external PCM plugins such as 'pulse',
'snd_pcm_ioplug_create()' is called to instantiate plugin structure,
and a call of 'snd_pcm_poll_descriptors()' can return '-EIO', '0', '1',
and value depending on plugin implementation.
http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=src/pcm/pcm_ioplug.c;h=881a1a85adaf09b4732375e2161b0a1ebb53900f;hb=HEAD#l772

When having good discussion to investigate cause, discover solution for
this kind of issues, you need to identify which PCM node triggers the
issue, at least.


Regards

Takashi Sakamoto