RE: Little question about ALSA callbacks

["Alfredo" <alfredo@vida-software.com>]

Thank you very much for your suggestions Takashi.


-----Original Message-----
From: Takashi Iwai [mailto:tiwai@suse.de]
Sent: miércoles, 27 de julio de 2005 10:48
To: Alfredo
Cc: alsa-devel@lists.sourceforge.net
Subject: Re: [Alsa-devel] Little question about ALSA callbacks

At Wed, 27 Jul 2005 11:28:26 +0200,
Alfredo wrote:
>
> Hello, I have read the article ^[$B!H^[(BIntroduction to Sound
Programming with ALSA from Linux
> Journal. This article explains:
>
> // Starts here:
>
------------------------------------------------------------------------
-------------------------------------------
>
> ^[$B!H^[(BIn the previous examples, the PCM streams were operating in
blocking mode, that is,
> the calls would not return until the data had been transferred. In an
interactive
> event-driven application, this situation could lock up the application
for
> unacceptably long periods of time. ALSA allows opening a stream in
nonblocking mode
> where the read and write functions return immediately. If data
transfers are pending
> and the calls cannot be processed, ALSA returns an error code of
EBUSY.
>
> Many graphical applications use callbacks to handle events. ALSA
supports opening a
> PCM stream in asynchronous mode. This allows registering a callback
function to be
> called when a period of sample data has been transferred.^[$B!H^[(B
>
> // Finishes here:
>
------------------------------------------------------------------------
-------
>
> I^[.F^[N"m developing an event driven Linux server that will use ALSA
library. Please, could
> you point me to some resources in order to learn how to use alsa in
asynchronous mode
> (the event driven mechanism will be based in poll()).
>
> Clemens Ladisch helped me suggesting that I must use poll to wait for
events in
> different file descriptors. I need to be notified when audio buffer
has been recorded.
> I need to learn how to use ALSA in conjunction with poll.

The async mode described above uses a signal instead of poll, so this
is not a recommended way although it seems working on most of
architectures.  Using poll is better.

If your app is a graphical one or do a heavy job (e.g. encoding)
together with a sound engine, you'll likely need a dedicated audio
thread for stability.  In that case, consdering to use JACK would be a
good idea, too...


Takashi





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