[alsa-devel] snd_pcm_poll_descriptors() error return

[Takashi Iwai]

On Tue, 02 Oct 2018 01:32:10 +0200,
Takashi Sakamoto wrote:
> 
> 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.

Yes, this function may return an error code, and it's the defined
behavior from the beginning.  So, please fix the error handling in the
application side.


thanks,

Takashi