1 .. -*- coding: utf-8; mode: rst -*-
5 *******************************
6 ioctl VIDIOC_QBUF, VIDIOC_DQBUF
7 *******************************
12 Exchange a buffer with the driver
18 .. c:function:: int ioctl( int fd, int request, struct v4l2_buffer *argp )
24 File descriptor returned by :ref:`open() <func-open>`.
27 VIDIOC_QBUF, VIDIOC_DQBUF
35 Applications call the ``VIDIOC_QBUF`` ioctl to enqueue an empty
36 (capturing) or filled (output) buffer in the driver's incoming queue.
37 The semantics depend on the selected I/O method.
39 To enqueue a buffer applications set the ``type`` field of a struct
40 :ref:`v4l2_buffer <v4l2-buffer>` to the same buffer type as was
41 previously used with struct :ref:`v4l2_format <v4l2-format>` ``type``
42 and struct :ref:`v4l2_requestbuffers <v4l2-requestbuffers>` ``type``.
43 Applications must also set the ``index`` field. Valid index numbers
44 range from zero to the number of buffers allocated with
45 :ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>` (struct
46 :ref:`v4l2_requestbuffers <v4l2-requestbuffers>` ``count``) minus
47 one. The contents of the struct :c:type:`struct v4l2_buffer` returned
48 by a :ref:`VIDIOC_QUERYBUF <VIDIOC_QUERYBUF>` ioctl will do as well.
49 When the buffer is intended for output (``type`` is
50 ``V4L2_BUF_TYPE_VIDEO_OUTPUT``, ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``,
51 or ``V4L2_BUF_TYPE_VBI_OUTPUT``) applications must also initialize the
52 ``bytesused``, ``field`` and ``timestamp`` fields, see :ref:`buffer`
53 for details. Applications must also set ``flags`` to 0. The
54 ``reserved2`` and ``reserved`` fields must be set to 0. When using the
55 :ref:`multi-planar API <planar-apis>`, the ``m.planes`` field must
56 contain a userspace pointer to a filled-in array of struct
57 :ref:`v4l2_plane <v4l2-plane>` and the ``length`` field must be set
58 to the number of elements in that array.
60 To enqueue a :ref:`memory mapped <mmap>` buffer applications set the
61 ``memory`` field to ``V4L2_MEMORY_MMAP``. When ``VIDIOC_QBUF`` is called
62 with a pointer to this structure the driver sets the
63 ``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_QUEUED`` flags and clears
64 the ``V4L2_BUF_FLAG_DONE`` flag in the ``flags`` field, or it returns an
67 To enqueue a :ref:`user pointer <userp>` buffer applications set the
68 ``memory`` field to ``V4L2_MEMORY_USERPTR``, the ``m.userptr`` field to
69 the address of the buffer and ``length`` to its size. When the
70 multi-planar API is used, ``m.userptr`` and ``length`` members of the
71 passed array of struct :ref:`v4l2_plane <v4l2-plane>` have to be used
72 instead. When ``VIDIOC_QBUF`` is called with a pointer to this structure
73 the driver sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the
74 ``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the
75 ``flags`` field, or it returns an error code. This ioctl locks the
76 memory pages of the buffer in physical memory, they cannot be swapped
77 out to disk. Buffers remain locked until dequeued, until the
78 :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or
79 :ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>` ioctl is called, or until the
82 To enqueue a :ref:`DMABUF <dmabuf>` buffer applications set the
83 ``memory`` field to ``V4L2_MEMORY_DMABUF`` and the ``m.fd`` field to a
84 file descriptor associated with a DMABUF buffer. When the multi-planar
85 API is used the ``m.fd`` fields of the passed array of struct
86 :ref:`v4l2_plane <v4l2-plane>` have to be used instead. When
87 ``VIDIOC_QBUF`` is called with a pointer to this structure the driver
88 sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the
89 ``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the
90 ``flags`` field, or it returns an error code. This ioctl locks the
91 buffer. Locking a buffer means passing it to a driver for a hardware
92 access (usually DMA). If an application accesses (reads/writes) a locked
93 buffer then the result is undefined. Buffers remain locked until
94 dequeued, until the :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or
95 :ref:`VIDIOC_REQBUFS <VIDIOC_REQBUFS>` ioctl is called, or until the
98 Applications call the ``VIDIOC_DQBUF`` ioctl to dequeue a filled
99 (capturing) or displayed (output) buffer from the driver's outgoing
100 queue. They just set the ``type``, ``memory`` and ``reserved`` fields of
101 a struct :ref:`v4l2_buffer <v4l2-buffer>` as above, when
102 ``VIDIOC_DQBUF`` is called with a pointer to this structure the driver
103 fills the remaining fields or returns an error code. The driver may also
104 set ``V4L2_BUF_FLAG_ERROR`` in the ``flags`` field. It indicates a
105 non-critical (recoverable) streaming error. In such case the application
106 may continue as normal, but should be aware that data in the dequeued
107 buffer might be corrupted. When using the multi-planar API, the planes
108 array must be passed in as well.
110 By default ``VIDIOC_DQBUF`` blocks when no buffer is in the outgoing
111 queue. When the ``O_NONBLOCK`` flag was given to the
112 :ref:`open() <func-open>` function, ``VIDIOC_DQBUF`` returns
113 immediately with an EAGAIN error code when no buffer is available.
115 The :c:type:`struct v4l2_buffer` structure is specified in
122 On success 0 is returned, on error -1 and the ``errno`` variable is set
123 appropriately. The generic error codes are described at the
124 :ref:`Generic Error Codes <gen-errors>` chapter.
127 Non-blocking I/O has been selected using ``O_NONBLOCK`` and no
128 buffer was in the outgoing queue.
131 The buffer ``type`` is not supported, or the ``index`` is out of
132 bounds, or no buffers have been allocated yet, or the ``userptr`` or
133 ``length`` are invalid.
136 ``VIDIOC_DQBUF`` failed due to an internal error. Can also indicate
137 temporary problems like signal loss. Note the driver might dequeue
138 an (empty) buffer despite returning an error, or even stop
139 capturing. Reusing such buffer may be unsafe though and its details
140 (e.g. ``index``) may not be returned either. It is recommended that
141 drivers indicate recoverable errors by setting the
142 ``V4L2_BUF_FLAG_ERROR`` and returning 0 instead. In that case the
143 application should be able to safely reuse the buffer and continue
147 ``VIDIOC_DQBUF`` returns this on an empty capture queue for mem2mem
148 codecs if a buffer with the ``V4L2_BUF_FLAG_LAST`` was already
149 dequeued and no new buffers are expected to become available.
152 .. ------------------------------------------------------------------------------
153 .. This file was automatically converted from DocBook-XML with the dbxml
154 .. library (https://github.com/return42/sphkerneldoc). The origin XML comes
155 .. from the linux kernel, refer to:
157 .. * https://github.com/torvalds/linux/tree/master/Documentation/DocBook
158 .. ------------------------------------------------------------------------------