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