On Mon, 1 Jul 2002, Joshua Haberman wrote:
> * Paul Davis ([EMAIL PROTECTED]) wrote:
> > >Also, how should programs interpret multiple poll descriptors: if the
> > >poll succeeds on some descriptors but not others, does this mean the
> > >stream is only "partially" ready?
> >
> > yes. see the internals of JACK's jack/drivers/alsa/alsa_driver.c
>
> Honestly, I'm afraid to look at that file too carefully since it is GPL
> and PortAudio is BSD. This puts me in kind of a weird situation.
>
> > >Also, is snd_pcm_poll_descriptors_revents() understood to return the
> > >events that have triggered on just *one* poll descriptor or on *all*? In
> > >other words, are you ANDing bitmasks or ORing them? Right now the
> > >semantics are impossible to deduce because the function just returns the
> > >revents verbatim since there is only one poll descriptor.
> >
> > The semantics are those of POSIX poll(2): it returns the status for
> > each file descriptor. ALSA's low level and mid level drivers don't
> > modify this in anyway. If there 3 fd's for playback and 2 for capture,
> > then the return data would indicate status for each of the 5 fd's
> > (assuming you asked for it for all 5).
>
> The prototype for this function is:
>
> int snd_pcm_poll_descriptors_revents ( snd_pcm_t *pcm,
> struct pollfd *pfds,
> unsigned int nfds,
> unsigned short * revents
> )
>
> Are you saying that the revents paramter is treated as an array? I was
> under the impression that it is treated as a pointer to a single short.
> This code fragment from test/pcm.c seems to confirm this interpretation:
>
> static int wait_for_poll(snd_pcm_t *handle, struct pollfd *ufds, unsigned
> int count)
> {
> unsigned short revents;
>
> while (1) {
> poll(ufds, count, -1);
> snd_pcm_poll_descriptors_revents(handle, ufds, count, &revents);
> if (revents & POLLERR)
> return -EIO;
> if (revents & POLLOUT)
> return 0;
> }
> }
>
> This gives me the impression that snd_pcm_poll_descriptors_revents() is a
> function that somehow "summarizes" all of the pfds into a single revents
> that speaks to the status of the entire PCM handle.
Yes, it translates multiple revents into one for application. Please, use
this function for the library flexibility.
Jaroslav
-----
Jaroslav Kysela <[EMAIL PROTECTED]>
Linux Kernel Sound Maintainer
ALSA Project http://www.alsa-project.org
SuSE Linux http://www.suse.com
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