problems using select() on alsa pcm.

problems using select() on alsa pcm.

[James Courtier-Dutton]

open pcm, and get a handle.

snd_pcm_poll_descriptors(handle, &pfd, err);

Get a poll file scriptor in pfd.

select(nfds, rfds, wfds, efds, tvp);

Is it possible to use this call with alsa ?
It seems that the select functions as expected with the descriptor so 
that we can do a snd_pcm_writei().

It seems that the select does not function so that we can do a 
snd_pcm_readi().

Are there any documents describing how one can get the read (rfds) working?

Cheers
James



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Re: problems using select() on alsa pcm.

[Paul Davis]

>open pcm, and get a handle.
>
>snd_pcm_poll_descriptors(handle, &pfd, err);
>
>Get a poll file scriptor in pfd.
>
>select(nfds, rfds, wfds, efds, tvp);
>
>Is it possible to use this call with alsa ?

select is generally deprecated in linux (linus says so!). but you can
use the same pfds in select as in poll (select is implemented in the
kernel using the poll code). the problem is interpreting the results
you get back (as noted recently for the dmix plugin).


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Re: problems using select() on alsa pcm.

[Jaroslav Kysela]

On Wed, 24 Mar 2004, Paul Davis wrote:

> >open pcm, and get a handle.
> >
> >snd_pcm_poll_descriptors(handle, &pfd, err);
> >
> >Get a poll file scriptor in pfd.
> >
> >select(nfds, rfds, wfds, efds, tvp);
> >
> >Is it possible to use this call with alsa ?
> 
> select is generally deprecated in linux (linus says so!). but you can
> use the same pfds in select as in poll (select is implemented in the
> kernel using the poll code). the problem is interpreting the results
> you get back (as noted recently for the dmix plugin).

Nope, the application must give results back to pfd array and call
snd_pcm_poll_descriptors_revents() function. In this way, everything
will work as expected.

						Jaroslav

-----
Jaroslav Kysela <perex@suse.cz>
Linux Kernel Sound Maintainer
ALSA Project, SuSE Labs


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Re: problems using select() on alsa pcm.

[Paul Davis]

>> select is generally deprecated in linux (linus says so!). but you can
>> use the same pfds in select as in poll (select is implemented in the
>> kernel using the poll code). the problem is interpreting the results
>> you get back (as noted recently for the dmix plugin).
>
>Nope, the application must give results back to pfd array and call
>snd_pcm_poll_descriptors_revents() function. In this way, everything
>will work as expected.

i thought that the flags set by select(2) were different than those
set by poll(2) ??


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Re: problems using select() on alsa pcm.

[Jaroslav Kysela]

On Thu, 25 Mar 2004, Paul Davis wrote:

> >> select is generally deprecated in linux (linus says so!). but you can
> >> use the same pfds in select as in poll (select is implemented in the
> >> kernel using the poll code). the problem is interpreting the results
> >> you get back (as noted recently for the dmix plugin).
> >
> >Nope, the application must give results back to pfd array and call
> >snd_pcm_poll_descriptors_revents() function. In this way, everything
> >will work as expected.
> 
> i thought that the flags set by select(2) were different than those
> set by poll(2) ??

Yes, but the results are similar. So you have to do:

snd_pcm_poll_descriptors(pfds)
pfds->events -> FD_SET(x, y)
select()
FD_ISSET(x, y) -> pfds->revents
revents = snd_pcm_poll_descriptors_revents(pfds)
if (revents & POLLOUT)
  playback_event();
if (revents & POLLIN)
  capture_event();

						Jaroslav

-----
Jaroslav Kysela <perex@suse.cz>
Linux Kernel Sound Maintainer
ALSA Project, SuSE Labs


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Re: problems using select() on alsa pcm.

[James Courtier-Dutton]

Jaroslav Kysela wrote:
> On Wed, 24 Mar 2004, Paul Davis wrote:
> 
> 
>>>open pcm, and get a handle.
>>>
>>>snd_pcm_poll_descriptors(handle, &pfd, err);
>>>
>>>Get a poll file scriptor in pfd.
>>>
>>>select(nfds, rfds, wfds, efds, tvp);
>>>
>>>Is it possible to use this call with alsa ?
>>
>>select is generally deprecated in linux (linus says so!). but you can
>>use the same pfds in select as in poll (select is implemented in the
>>kernel using the poll code). the problem is interpreting the results
>>you get back (as noted recently for the dmix plugin).
> 
> 
> Nope, the application must give results back to pfd array and call
> snd_pcm_poll_descriptors_revents() function. In this way, everything
> will work as expected.
> 
> 						Jaroslav
So which of the following is needed: -

snd_pcm_poll_descriptors(handle, &pfd, err);
select(nfds, rfds, wfds, efds, tvp);
snd_pcm_poll_descriptors_revents()  <- decode the results of the select.

or
snd_pcm_poll_descriptors(handle, &pfd, err);
snd_pcm_poll_descriptors_revents()  <- rearrange the pfd so that select 
works as expected.
select(nfds, rfds, wfds, efds, tvp);

Cheers
James


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Re: problems using select() on alsa pcm.

[Jaroslav Kysela]

On Thu, 25 Mar 2004, James Courtier-Dutton wrote:

> Jaroslav Kysela wrote:
> > On Wed, 24 Mar 2004, Paul Davis wrote:
> > 
> > 
> >>>open pcm, and get a handle.
> >>>
> >>>snd_pcm_poll_descriptors(handle, &pfd, err);
> >>>
> >>>Get a poll file scriptor in pfd.
> >>>
> >>>select(nfds, rfds, wfds, efds, tvp);
> >>>
> >>>Is it possible to use this call with alsa ?
> >>
> >>select is generally deprecated in linux (linus says so!). but you can
> >>use the same pfds in select as in poll (select is implemented in the
> >>kernel using the poll code). the problem is interpreting the results
> >>you get back (as noted recently for the dmix plugin).
> > 
> > 
> > Nope, the application must give results back to pfd array and call
> > snd_pcm_poll_descriptors_revents() function. In this way, everything
> > will work as expected.
> > 
> > 						Jaroslav
> So which of the following is needed: -
> 
> snd_pcm_poll_descriptors(handle, &pfd, err);
> select(nfds, rfds, wfds, efds, tvp);
> snd_pcm_poll_descriptors_revents()  <- decode the results of the select.

This one. The *revents() function mangles the returned ->revents back to 
the useful meaning for applications.

> or
> snd_pcm_poll_descriptors(handle, &pfd, err);
> snd_pcm_poll_descriptors_revents()  <- rearrange the pfd so that select 
> works as expected.
> select(nfds, rfds, wfds, efds, tvp);

That's wrong.

						Jaroslav

-----
Jaroslav Kysela <perex@suse.cz>
Linux Kernel Sound Maintainer
ALSA Project, SuSE Labs


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Re: problems using select() on alsa pcm.

[Martijn Sipkema]

> >open pcm, and get a handle.
> >
> >snd_pcm_poll_descriptors(handle, &pfd, err);
> >
> >Get a poll file scriptor in pfd.
> >
> >select(nfds, rfds, wfds, efds, tvp);
> >
> >Is it possible to use this call with alsa ?
> 
> select is generally deprecated in linux (linus says so!). but you can
> use the same pfds in select as in poll (select is implemented in the
> kernel using the poll code). the problem is interpreting the results
> you get back (as noted recently for the dmix plugin).

select and pselect do allow for a more accurate timeout specification.
Why is select deprecated?

--ms




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Re: problems using select() on alsa pcm.

[Paul Davis]

>> >open pcm, and get a handle.
>> >
>> >snd_pcm_poll_descriptors(handle, &pfd, err);
>> >
>> >Get a poll file scriptor in pfd.
>> >
>> >select(nfds, rfds, wfds, efds, tvp);
>> >
>> >Is it possible to use this call with alsa ?
>> 
>> select is generally deprecated in linux (linus says so!). but you can
>> use the same pfds in select as in poll (select is implemented in the
>> kernel using the poll code). the problem is interpreting the results
>> you get back (as noted recently for the dmix plugin).
>
>select and pselect do allow for a more accurate timeout specification.
>Why is select deprecated?

well, one reason might be gleaned from the man page for select_tut:

      [ ... ]

       mode  at all (see ioctl(2)). It is easy to introduce subtle errors that
       will remove the advantage of using select, hence I will present a  list
       of essentials to watch for when using the select call.
 
       1.     You should always try use select without a timeout. Your program
              should have nothing to do if there is no  data  available.  Code
              that  depends  on timeouts is not usually portable and difficult
              to debug.

of course, for main event loops in things like glib, this can't ever
really be true, but it is a rather salient point for most callers of
select(). 

i think the main reason for its "deprecation" (which is very mild,
given that its an official POSIX call) is that its semantics are not
quite as clear as poll(2). its even possible that i've missed a
retraction from linus about this.

--p


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Re: problems using select() on alsa pcm.

[Takashi Iwai]

At Thu, 25 Mar 2004 11:25:56 +0100,
Martijn Sipkema wrote:
> 
> > >open pcm, and get a handle.
> > >
> > >snd_pcm_poll_descriptors(handle, &pfd, err);
> > >
> > >Get a poll file scriptor in pfd.
> > >
> > >select(nfds, rfds, wfds, efds, tvp);
> > >
> > >Is it possible to use this call with alsa ?
> > 
> > select is generally deprecated in linux (linus says so!). but you can
> > use the same pfds in select as in poll (select is implemented in the
> > kernel using the poll code). the problem is interpreting the results
> > you get back (as noted recently for the dmix plugin).
> 
> select and pselect do allow for a more accurate timeout specification.

from the spec, yes.  but nsec resolution would be never implemented :)

> Why is select deprecated?

because select is just a wrapper of poll in fact (on linux)?


Takashi


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Re: problems using select() on alsa pcm.

[Martijn Sipkema]

> > > >open pcm, and get a handle.
> > > >
> > > >snd_pcm_poll_descriptors(handle, &pfd, err);
> > > >
> > > >Get a poll file scriptor in pfd.
> > > >
> > > >select(nfds, rfds, wfds, efds, tvp);
> > > >
> > > >Is it possible to use this call with alsa ?
> > > 
> > > select is generally deprecated in linux (linus says so!). but you can
> > > use the same pfds in select as in poll (select is implemented in the
> > > kernel using the poll code). the problem is interpreting the results
> > > you get back (as noted recently for the dmix plugin).
> > 
> > select and pselect do allow for a more accurate timeout specification.
> 
> from the spec, yes.  but nsec resolution would be never implemented :)

Not nsec resolution perhaps, but a higher than msec resolution might
be useful in some cases. The fact that select was updated in the POSIX
specs to support a timespec timeout (pselect) and poll only supports
msec timeout might indicate that pselect is better suited for realtime
use. Not to mention that pselect supports at least a 31 day timeout and
that it can set the signal mask...


--ms




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Re: problems using select() on alsa pcm.

[Glenn Maynard]

On Thu, Mar 25, 2004 at 03:00:58PM +0100, Takashi Iwai wrote:
> > Why is select deprecated?
> 
> because select is just a wrapper of poll in fact (on linux)?

So?  There are a few other operating systems out there, and select() is
very portable.

(select isn't deprecated; it's simply not Linus's API to deprecate. :)

-- 
Glenn Maynard


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Re: problems using select() on alsa pcm.

[Jaroslav Kysela]

On Thu, 25 Mar 2004, James Courtier-Dutton wrote:

> open pcm, and get a handle.
> 
> snd_pcm_poll_descriptors(handle, &pfd, err);
> 
> Get a poll file scriptor in pfd.
> 
> select(nfds, rfds, wfds, efds, tvp);
> 
> Is it possible to use this call with alsa ?

Yes, snd_pcm_poll_descriptors() gives you all hints.
See to alsa-oss/alsa/alsa-oss.c - select() wrapper, but
man select and man poll will help you also.

						Jaroslav

-----
Jaroslav Kysela <perex@suse.cz>
Linux Kernel Sound Maintainer
ALSA Project, SuSE Labs


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