[alsa-devel] "Resource temporarily unavailable" while reading although poll returns POLLIN event

[Stefan Schoenleitner]

Hi,

I wrote a small application that opens both streams (i.e. capture and playback)
of a PCM in non blocking (i.e. SND_PCM_NONBLOCK) mode.

During device configuration I also set up the minimum number of
frames to consider a PCM ready:

---------------------------------------------------------------------------------
// when to consider the device to be ready for the next data transfer operation
if ((ret=snd_pcm_sw_params_set_avail_min(handle, swparams, pcm_period_size))<0)
{
	fprintf(stderr, "could not set avail_min: %s\n", snd_strerror(ret));
	return NULL;
}
---------------------------------------------------------------------------------

As you can see it is set to the number of frames one period contains.

In other words, each time there are at least that many frames available for reading
on the capture device, we should get a POLLIN event when using poll().
On the other side we should get a POLLOUT event if that many frames can be written
to the playback device.


Then, later in my application, I use poll() on the fds of the PCM devices.
As, according to the ALSA reference, poll events can be "mangeled", I use
snd_pcm_poll_descriptors_revents() to "demangle" the events:

---------------------------------------------------------------------------------
if ((ret=snd_pcm_poll_descriptors_revents(device_handle, &poll_fds[i], 1, &revents))<0)
{
	fprintf(stderr, "could not snd_pcm_poll_descriptors_revents: %s\n", snd_strerror(ret));
	exit(EXIT_FAILURE);
}
---------------------------------------------------------------------------------



After that, each time there was a POLLIN event, I am reading exactly one period from the
capture PCM.
On the other side, if there was a POLLOUT event, I am writing exactly one period to the
playback PCM.


While this works for a small number of periods (which is not always the same), I always end up
with a "Resource temporarily unavailable" error when trying to read one period from the capture PCM:

---------------------------------------------------------------------------------
demangled poll: POLLIN  on capture device
could not read from capture device: Resource temporarily unavailable
---------------------------------------------------------------------------------


Since a POLLIN event only occurs after at least a full period is available for
reading (as set up by snd_pcm_sw_params_set_avail_min() above)
and I only read after a POLLIN event occured on the capture device fd,
I really do not understand why I get the above error.

So why does the above happen ?
What am I doing wrong ?

If it helps, I pasted the source code here so that you can view it nicely: http://pastebin.com/fCicqctq

cheers,
stefan