vidioc-qbuf.rst

.. -*- coding: utf-8; mode: rst -*-

.. _VIDIOC_QBUF:

*******************************
ioctl VIDIOC_QBUF, VIDIOC_DQBUF
*******************************

Name
====

VIDIOC_QBUF - VIDIOC_DQBUF - Exchange a buffer with the driver


Synopsis
========

.. c:function:: int ioctl( int fd, VIDIOC_QBUF, struct v4l2_buffer *argp )
    :name: VIDIOC_QBUF

.. c:function:: int ioctl( int fd, VIDIOC_DQBUF, struct v4l2_buffer *argp )
    :name: VIDIOC_DQBUF


Arguments
=========

``fd``
    File descriptor returned by :ref:`open() <func-open>`.

``argp``
    Pointer to struct :c:type:`v4l2_buffer`.


Description
===========

Applications call the ``VIDIOC_QBUF`` ioctl to enqueue an empty
(capturing) or filled (output) buffer in the driver's incoming queue.
The semantics depend on the selected I/O method.

To enqueue a buffer applications set the ``type`` field of a struct
:c:type:`v4l2_buffer` to the same buffer type as was
previously used with struct :c:type:`v4l2_format` ``type``
and struct :c:type:`v4l2_requestbuffers` ``type``.
Applications must also set the ``index`` field. Valid index numbers
range from zero to the number of buffers allocated with
:ref:`VIDIOC_REQBUFS` (struct
:c:type:`v4l2_requestbuffers` ``count``) minus
one. The contents of the struct :c:type:`v4l2_buffer` returned
by a :ref:`VIDIOC_QUERYBUF` ioctl will do as well.
When the buffer is intended for output (``type`` is
``V4L2_BUF_TYPE_VIDEO_OUTPUT``, ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``,
or ``V4L2_BUF_TYPE_VBI_OUTPUT``) applications must also initialize the
``bytesused``, ``field`` and ``timestamp`` fields, see :ref:`buffer`
for details. Applications must also set ``flags`` to 0. The
``reserved2`` and ``reserved`` fields must be set to 0. When using the
:ref:`multi-planar API <planar-apis>`, the ``m.planes`` field must
contain a userspace pointer to a filled-in array of struct
:c:type:`v4l2_plane` and the ``length`` field must be set
to the number of elements in that array.

To enqueue a :ref:`memory mapped <mmap>` buffer applications set the
``memory`` field to ``V4L2_MEMORY_MMAP``. When ``VIDIOC_QBUF`` is called
with a pointer to this structure the driver sets the
``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_QUEUED`` flags and clears
the ``V4L2_BUF_FLAG_DONE`` flag in the ``flags`` field, or it returns an
``EINVAL`` error code.

To enqueue a :ref:`user pointer <userp>` buffer applications set the
``memory`` field to ``V4L2_MEMORY_USERPTR``, the ``m.userptr`` field to
the address of the buffer and ``length`` to its size. When the
multi-planar API is used, ``m.userptr`` and ``length`` members of the
passed array of struct :c:type:`v4l2_plane` have to be used
instead. When ``VIDIOC_QBUF`` is called with a pointer to this structure
the driver sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the
``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the
``flags`` field, or it returns an error code. This ioctl locks the
memory pages of the buffer in physical memory, they cannot be swapped
out to disk. Buffers remain locked until dequeued, until the
:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or
:ref:`VIDIOC_REQBUFS` ioctl is called, or until the
device is closed.

To enqueue a :ref:`DMABUF <dmabuf>` buffer applications set the
``memory`` field to ``V4L2_MEMORY_DMABUF`` and the ``m.fd`` field to a
file descriptor associated with a DMABUF buffer. When the multi-planar
API is used the ``m.fd`` fields of the passed array of struct
:c:type:`v4l2_plane` have to be used instead. When
``VIDIOC_QBUF`` is called with a pointer to this structure the driver
sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the
``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the
``flags`` field, or it returns an error code. This ioctl locks the
buffer. Locking a buffer means passing it to a driver for a hardware
access (usually DMA). If an application accesses (reads/writes) a locked
buffer then the result is undefined. Buffers remain locked until
dequeued, until the :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or
:ref:`VIDIOC_REQBUFS` ioctl is called, or until the
device is closed.

The ``request_fd`` field can be used with the ``VIDIOC_QBUF`` ioctl to specify
the file descriptor of a :ref:`request <media-request-api>`, if requests are
in use. Setting it means that the buffer will not be passed to the driver
until the request itself is queued. Also, the driver will apply any
settings associated with the request for this buffer. This field will
be ignored unless the ``V4L2_BUF_FLAG_REQUEST_FD`` flag is set.
If the device does not support requests, then ``EACCES`` will be returned.
If requests are supported but an invalid request file descriptor is given,
then ``EINVAL`` will be returned.

.. caution::
   It is not allowed to mix queuing requests with queuing buffers directly.
   ``EBUSY`` will be returned if the first buffer was queued directly and
   then the application tries to queue a request, or vice versa.

   For :ref:`memory-to-memory devices <codec>` you can specify the
   ``request_fd`` only for output buffers, not for capture buffers. Attempting
   to specify this for a capture buffer will result in an ``EACCES`` error.

Applications call the ``VIDIOC_DQBUF`` ioctl to dequeue a filled
(capturing) or displayed (output) buffer from the driver's outgoing
queue. They just set the ``type``, ``memory`` and ``reserved`` fields of
a struct :c:type:`v4l2_buffer` as above, when
``VIDIOC_DQBUF`` is called with a pointer to this structure the driver
fills the remaining fields or returns an error code. The driver may also
set ``V4L2_BUF_FLAG_ERROR`` in the ``flags`` field. It indicates a
non-critical (recoverable) streaming error. In such case the application
may continue as normal, but should be aware that data in the dequeued
buffer might be corrupted. When using the multi-planar API, the planes
array must be passed in as well.

By default ``VIDIOC_DQBUF`` blocks when no buffer is in the outgoing
queue. When the ``O_NONBLOCK`` flag was given to the
:ref:`open() <func-open>` function, ``VIDIOC_DQBUF`` returns
immediately with an ``EAGAIN`` error code when no buffer is available.

The struct :c:type:`v4l2_buffer` structure is specified in
:ref:`buffer`.


Return Value
============

On success 0 is returned, on error -1 and the ``errno`` variable is set
appropriately. The generic error codes are described at the
:ref:`Generic Error Codes <gen-errors>` chapter.

EAGAIN
    Non-blocking I/O has been selected using ``O_NONBLOCK`` and no
    buffer was in the outgoing queue.

EINVAL
    The buffer ``type`` is not supported, or the ``index`` is out of
    bounds, or no buffers have been allocated yet, or the ``userptr`` or
    ``length`` are invalid, or the ``V4L2_BUF_FLAG_REQUEST_FD`` flag was
    set but the the given ``request_fd`` was invalid, or ``m.fd`` was
    an invalid DMABUF file descriptor.

EIO
    ``VIDIOC_DQBUF`` failed due to an internal error. Can also indicate
    temporary problems like signal loss.

    .. note::

       The driver might dequeue an (empty) buffer despite returning
       an error, or even stop capturing. Reusing such buffer may be unsafe
       though and its details (e.g. ``index``) may not be returned either.
       It is recommended that drivers indicate recoverable errors by setting
       the ``V4L2_BUF_FLAG_ERROR`` and returning 0 instead. In that case the
       application should be able to safely reuse the buffer and continue
       streaming.

EPIPE
    ``VIDIOC_DQBUF`` returns this on an empty capture queue for mem2mem
    codecs if a buffer with the ``V4L2_BUF_FLAG_LAST`` was already
    dequeued and no new buffers are expected to become available.

EACCES
    The ``V4L2_BUF_FLAG_REQUEST_FD`` flag was set but the device does not
    support requests for the given buffer type.

EBUSY
    The first buffer was queued via a request, but the application now tries
    to queue it directly, or vice versa (it is not permitted to mix the two
    APIs).