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pcm: clarify documentation of poll descriptor usage

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This is based on my understanding of the intended behavior, the
test/pcm.c example code, as well as the github pull request
discussion (#370).

There needs to be more clarifiaction regarding the exact semantics
of the value of the revents output parameter of
snd_pcm_poll_descriptors_revents, since there are events that do
not necessarily correspond to POLLIN or POLLOUT (such as period
events), but I believe this is a lot less obvious and needs
confirmation first.

Closes: https://github.com/alsa-project/alsa-lib/pull/370
Signed-off-by: Zeno Endemann <zeno.endemann@mailbox.org>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
This commit is contained in:
Zeno Endemann 2023-11-24 14:33:47 +01:00 • committed by Jaroslav Kysela
parent 454638ef30
commit cd04da2bcf
1 changed files with 12 additions and 0 deletions
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12
src/pcm/pcm.c
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@ -1811,6 +1811,11 @@ static int __snd_pcm_poll_descriptors(snd_pcm_t *pcm, struct pollfd *pfds,
* corresponding FD_SET arrays and demangle events using
* \link ::snd_pcm_poll_descriptors_revents() \endlink .
*
* It is guaranteed that for the given PCM handle, the output poll
* descriptor structs (and their count) will never change, thus it is
* valid to call the function once and reuse its output for the
* lifetime of the PCM device.
*
* The function is thread-safe when built with the proper option.
*/
int snd_pcm_poll_descriptors(snd_pcm_t *pcm, struct pollfd *pfds, unsigned int space)
@ -1846,6 +1851,13 @@ static int __snd_pcm_poll_revents(snd_pcm_t *pcm, struct pollfd *pfds,
* Note: Even if multiple poll descriptors are used (i.e. pfds > 1),
* this function returns only a single event.
*
* The passed in count of poll descriptors must be equal to
* \link ::snd_pcm_poll_descriptors_count() \endlink and the passed in array
* must match the array returned by \link ::snd_pcm_poll_descriptors() \endlink
* (in its full length and original order) with the revent fields updated
* according to the poll() result. This function will not modify the file
* descriptor or event field of any element of the given poll descriptor array.
*
* The function is thread-safe when built with the proper option.
*/
int snd_pcm_poll_descriptors_revents(snd_pcm_t *pcm, struct pollfd *pfds, unsigned int nfds, unsigned short *revents)