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+.. -*- 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.
+
+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.
+
+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.