117 lines
4 KiB
ReStructuredText
117 lines
4 KiB
ReStructuredText
.. -*- coding: utf-8; mode: rst -*-
|
|
|
|
.. _func-poll:
|
|
|
|
***********
|
|
V4L2 poll()
|
|
***********
|
|
|
|
Name
|
|
====
|
|
|
|
v4l2-poll - Wait for some event on a file descriptor
|
|
|
|
|
|
Synopsis
|
|
========
|
|
|
|
.. code-block:: c
|
|
|
|
#include <sys/poll.h>
|
|
|
|
|
|
.. c:function:: int poll( struct pollfd *ufds, unsigned int nfds, int timeout )
|
|
:name: v4l2-poll
|
|
|
|
Arguments
|
|
=========
|
|
|
|
|
|
|
|
Description
|
|
===========
|
|
|
|
With the :ref:`poll() <func-poll>` function applications can suspend execution
|
|
until the driver has captured data or is ready to accept data for
|
|
output.
|
|
|
|
When streaming I/O has been negotiated this function waits until a
|
|
buffer has been filled by the capture device and can be dequeued with
|
|
the :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. For output devices this
|
|
function waits until the device is ready to accept a new buffer to be
|
|
queued up with the :ref:`VIDIOC_QBUF <VIDIOC_QBUF>` ioctl for
|
|
display. When buffers are already in the outgoing queue of the driver
|
|
(capture) or the incoming queue isn't full (display) the function
|
|
returns immediately.
|
|
|
|
On success :ref:`poll() <func-poll>` returns the number of file descriptors
|
|
that have been selected (that is, file descriptors for which the
|
|
``revents`` field of the respective :c:func:`struct pollfd` structure
|
|
is non-zero). Capture devices set the ``POLLIN`` and ``POLLRDNORM``
|
|
flags in the ``revents`` field, output devices the ``POLLOUT`` and
|
|
``POLLWRNORM`` flags. When the function timed out it returns a value of
|
|
zero, on failure it returns -1 and the ``errno`` variable is set
|
|
appropriately. When the application did not call
|
|
:ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` the :ref:`poll() <func-poll>`
|
|
function succeeds, but sets the ``POLLERR`` flag in the ``revents``
|
|
field. When the application has called
|
|
:ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` for a capture device but
|
|
hasn't yet called :ref:`VIDIOC_QBUF <VIDIOC_QBUF>`, the
|
|
:ref:`poll() <func-poll>` function succeeds and sets the ``POLLERR`` flag in
|
|
the ``revents`` field. For output devices this same situation will cause
|
|
:ref:`poll() <func-poll>` to succeed as well, but it sets the ``POLLOUT`` and
|
|
``POLLWRNORM`` flags in the ``revents`` field.
|
|
|
|
If an event occurred (see :ref:`VIDIOC_DQEVENT`)
|
|
then ``POLLPRI`` will be set in the ``revents`` field and
|
|
:ref:`poll() <func-poll>` will return.
|
|
|
|
When use of the :ref:`read() <func-read>` function has been negotiated and the
|
|
driver does not capture yet, the :ref:`poll() <func-poll>` function starts
|
|
capturing. When that fails it returns a ``POLLERR`` as above. Otherwise
|
|
it waits until data has been captured and can be read. When the driver
|
|
captures continuously (as opposed to, for example, still images) the
|
|
function may return immediately.
|
|
|
|
When use of the :ref:`write() <func-write>` function has been negotiated and the
|
|
driver does not stream yet, the :ref:`poll() <func-poll>` function starts
|
|
streaming. When that fails it returns a ``POLLERR`` as above. Otherwise
|
|
it waits until the driver is ready for a non-blocking
|
|
:ref:`write() <func-write>` call.
|
|
|
|
If the caller is only interested in events (just ``POLLPRI`` is set in
|
|
the ``events`` field), then :ref:`poll() <func-poll>` will *not* start
|
|
streaming if the driver does not stream yet. This makes it possible to
|
|
just poll for events and not for buffers.
|
|
|
|
All drivers implementing the :ref:`read() <func-read>` or :ref:`write() <func-write>`
|
|
function or streaming I/O must also support the :ref:`poll() <func-poll>`
|
|
function.
|
|
|
|
For more details see the :ref:`poll() <func-poll>` manual page.
|
|
|
|
|
|
Return Value
|
|
============
|
|
|
|
On success, :ref:`poll() <func-poll>` returns the number structures which have
|
|
non-zero ``revents`` fields, or zero if the call timed out. On error -1
|
|
is returned, and the ``errno`` variable is set appropriately:
|
|
|
|
EBADF
|
|
One or more of the ``ufds`` members specify an invalid file
|
|
descriptor.
|
|
|
|
EBUSY
|
|
The driver does not support multiple read or write streams and the
|
|
device is already in use.
|
|
|
|
EFAULT
|
|
``ufds`` references an inaccessible memory area.
|
|
|
|
EINTR
|
|
The call was interrupted by a signal.
|
|
|
|
EINVAL
|
|
The ``nfds`` argument is greater than ``OPEN_MAX``.
|