[alsa-devel] snd_pcm_wait function

Daniel Sanz daniellsanz2 at gmail.com
Fri Nov 16 11:41:54 CET 2012

Thanks, Trent, I've been playing a bit with snd_pcm_drain,
snd_pcm_writei, blocking mode, non-blocking mode, etc. but even if I
fill a full last period, audio does not play from the beginning to end
unless I call snd_pcm_drain just before closing the stream. I think
there is some concept that I'm missing about how ALSA works, or maybe
my driver/ALSA configuration is broken (I'm using Arch Linux).

Do you have a brief example of playing a short WAV file using ALSA?

On Tue, Nov 13, 2012 at 5:45 AM, Trent Piepho <tpiepho at gmail.com> wrote:
> snd_pcm_drain() will drain all the pending periods.  In effect, it
> rounds up however many frames are left in the buffer to the next whole
> period.  There is no way to just drain a partially full period.
> If you always wrote full periods, and didn't use non-blocking mode so
> that each writei call always wrote everything before returning, then
> you shouldn't have a partially filled period at the end of the buffer.
> ALSA has the exact buffer pointer value of the app pointer and also
> when a period should elapse, and could easily tell you the buffer fill
> value mod period size, but I don't think there is any API for that.
> On Mon, Nov 12, 2012 at 7:15 PM, Daniel Sanz <daniellsanz2 at gmail.com> wrote:
>> Can I expect that there won't be any pending frames (so no need to
>> call snd_pcm_drain) if I always use snd_pcm_writei to write multiples
>> of the period size?
>> And what if the sound is very short or the period size is big? the
>> latency of hardware is negligible?
>> Thanks,
>> On Mon, Nov 12, 2012 at 9:57 PM, Trent Piepho <tpiepho at gmail.com> wrote:
>>> You do have to fill to a full period before calling drain, or it won't
>>> work in a very useful way.  It only stops when periods elapse.  So if
>>> you have a half full period of audio at the end of the buffer you'll
>>> get garbage when it plays to the end of that period before stopping.
>>> On Mon, Nov 12, 2012 at 2:36 AM, Clemens Ladisch <clemens at ladisch.de> wrote:
>>>> Enno Fennema wrote:
>>>>> I want to close the pcm AFTER the sound finished.
>>>> Then close it after snd_pcm_drain() has returned.
>>>>> The docs say calling drain() puts the pcm in the DRAINING state.
>>>> It also waits until the device has drained (unless the device is in
>>>> non-blocking mode).
>>>>> When I obtain the state immediately after drain() it is already SETUP.
>>>>> I hoped a state change from DRAINING to SETUP  would be the right time
>>>>> to close the pcm but there appears to be no such state change.
>>>> That change happens immediately before snd_pcm_drain() returns.
>>>> Regards,
>>>> Clemens
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