1 .. -*- coding: utf-8; mode: rst -*-
5 *******************************
6 ioctl VIDIOC_QBUF, VIDIOC_DQBUF
7 *******************************
12 VIDIOC_QBUF - VIDIOC_DQBUF - Exchange a buffer with the driver
18 .. c:function:: int ioctl( int fd, VIDIOC_QBUF, struct v4l2_buffer *argp )
21 .. c:function:: int ioctl( int fd, VIDIOC_DQBUF, struct v4l2_buffer *argp )
29 File descriptor returned by :ref:`open() <func-open>`.
32 Pointer to struct :c:type:`v4l2_buffer`.
38 Applications call the ``VIDIOC_QBUF`` ioctl to enqueue an empty
39 (capturing) or filled (output) buffer in the driver's incoming queue.
40 The semantics depend on the selected I/O method.
42 To enqueue a buffer applications set the ``type`` field of a struct
43 :c:type:`v4l2_buffer` to the same buffer type as was
44 previously used with struct :c:type:`v4l2_format` ``type``
45 and struct :c:type:`v4l2_requestbuffers` ``type``.
46 Applications must also set the ``index`` field. Valid index numbers
47 range from zero to the number of buffers allocated with
48 :ref:`VIDIOC_REQBUFS` (struct
49 :c:type:`v4l2_requestbuffers` ``count``) minus
50 one. The contents of the struct :c:type:`v4l2_buffer` returned
51 by a :ref:`VIDIOC_QUERYBUF` ioctl will do as well.
52 When the buffer is intended for output (``type`` is
53 ``V4L2_BUF_TYPE_VIDEO_OUTPUT``, ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``,
54 or ``V4L2_BUF_TYPE_VBI_OUTPUT``) applications must also initialize the
55 ``bytesused``, ``field`` and ``timestamp`` fields, see :ref:`buffer`
56 for details. Applications must also set ``flags`` to 0. The
57 ``reserved2`` and ``reserved`` fields must be set to 0. When using the
58 :ref:`multi-planar API <planar-apis>`, the ``m.planes`` field must
59 contain a userspace pointer to a filled-in array of struct
60 :c:type:`v4l2_plane` and the ``length`` field must be set
61 to the number of elements in that array.
63 To enqueue a :ref:`memory mapped <mmap>` buffer applications set the
64 ``memory`` field to ``V4L2_MEMORY_MMAP``. When ``VIDIOC_QBUF`` is called
65 with a pointer to this structure the driver sets the
66 ``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_QUEUED`` flags and clears
67 the ``V4L2_BUF_FLAG_DONE`` flag in the ``flags`` field, or it returns an
68 ``EINVAL`` error code.
70 To enqueue a :ref:`user pointer <userp>` buffer applications set the
71 ``memory`` field to ``V4L2_MEMORY_USERPTR``, the ``m.userptr`` field to
72 the address of the buffer and ``length`` to its size. When the
73 multi-planar API is used, ``m.userptr`` and ``length`` members of the
74 passed array of struct :c:type:`v4l2_plane` have to be used
75 instead. When ``VIDIOC_QBUF`` is called with a pointer to this structure
76 the driver sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the
77 ``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the
78 ``flags`` field, or it returns an error code. This ioctl locks the
79 memory pages of the buffer in physical memory, they cannot be swapped
80 out to disk. Buffers remain locked until dequeued, until the
81 :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or
82 :ref:`VIDIOC_REQBUFS` ioctl is called, or until the
85 To enqueue a :ref:`DMABUF <dmabuf>` buffer applications set the
86 ``memory`` field to ``V4L2_MEMORY_DMABUF`` and the ``m.fd`` field to a
87 file descriptor associated with a DMABUF buffer. When the multi-planar
88 API is used the ``m.fd`` fields of the passed array of struct
89 :c:type:`v4l2_plane` have to be used instead. When
90 ``VIDIOC_QBUF`` is called with a pointer to this structure the driver
91 sets the ``V4L2_BUF_FLAG_QUEUED`` flag and clears the
92 ``V4L2_BUF_FLAG_MAPPED`` and ``V4L2_BUF_FLAG_DONE`` flags in the
93 ``flags`` field, or it returns an error code. This ioctl locks the
94 buffer. Locking a buffer means passing it to a driver for a hardware
95 access (usually DMA). If an application accesses (reads/writes) a locked
96 buffer then the result is undefined. Buffers remain locked until
97 dequeued, until the :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or
98 :ref:`VIDIOC_REQBUFS` ioctl is called, or until the
101 The ``request_fd`` field can be used with the ``VIDIOC_QBUF`` ioctl to specify
102 the file descriptor of a :ref:`request <media-request-api>`, if requests are
103 in use. Setting it means that the buffer will not be passed to the driver
104 until the request itself is queued. Also, the driver will apply any
105 settings associated with the request for this buffer. This field will
106 be ignored unless the ``V4L2_BUF_FLAG_REQUEST_FD`` flag is set.
107 If the device does not support requests, then ``EACCES`` will be returned.
108 If requests are supported but an invalid request file descriptor is given,
109 then ``EINVAL`` will be returned.
112 It is not allowed to mix queuing requests with queuing buffers directly.
113 ``EBUSY`` will be returned if the first buffer was queued directly and
114 then the application tries to queue a request, or vice versa. After
115 closing the file descriptor, calling
116 :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>` or calling :ref:`VIDIOC_REQBUFS`
117 the check for this will be reset.
119 For :ref:`memory-to-memory devices <codec>` you can specify the
120 ``request_fd`` only for output buffers, not for capture buffers. Attempting
121 to specify this for a capture buffer will result in an ``EACCES`` error.
123 Applications call the ``VIDIOC_DQBUF`` ioctl to dequeue a filled
124 (capturing) or displayed (output) buffer from the driver's outgoing
125 queue. They just set the ``type``, ``memory`` and ``reserved`` fields of
126 a struct :c:type:`v4l2_buffer` as above, when
127 ``VIDIOC_DQBUF`` is called with a pointer to this structure the driver
128 fills the remaining fields or returns an error code. The driver may also
129 set ``V4L2_BUF_FLAG_ERROR`` in the ``flags`` field. It indicates a
130 non-critical (recoverable) streaming error. In such case the application
131 may continue as normal, but should be aware that data in the dequeued
132 buffer might be corrupted. When using the multi-planar API, the planes
133 array must be passed in as well.
135 By default ``VIDIOC_DQBUF`` blocks when no buffer is in the outgoing
136 queue. When the ``O_NONBLOCK`` flag was given to the
137 :ref:`open() <func-open>` function, ``VIDIOC_DQBUF`` returns
138 immediately with an ``EAGAIN`` error code when no buffer is available.
140 The struct :c:type:`v4l2_buffer` structure is specified in
147 On success 0 is returned, on error -1 and the ``errno`` variable is set
148 appropriately. The generic error codes are described at the
149 :ref:`Generic Error Codes <gen-errors>` chapter.
152 Non-blocking I/O has been selected using ``O_NONBLOCK`` and no
153 buffer was in the outgoing queue.
156 The buffer ``type`` is not supported, or the ``index`` is out of
157 bounds, or no buffers have been allocated yet, or the ``userptr`` or
158 ``length`` are invalid, or the ``V4L2_BUF_FLAG_REQUEST_FD`` flag was
159 set but the the given ``request_fd`` was invalid, or ``m.fd`` was
160 an invalid DMABUF file descriptor.
163 ``VIDIOC_DQBUF`` failed due to an internal error. Can also indicate
164 temporary problems like signal loss.
168 The driver might dequeue an (empty) buffer despite returning
169 an error, or even stop capturing. Reusing such buffer may be unsafe
170 though and its details (e.g. ``index``) may not be returned either.
171 It is recommended that drivers indicate recoverable errors by setting
172 the ``V4L2_BUF_FLAG_ERROR`` and returning 0 instead. In that case the
173 application should be able to safely reuse the buffer and continue
177 ``VIDIOC_DQBUF`` returns this on an empty capture queue for mem2mem
178 codecs if a buffer with the ``V4L2_BUF_FLAG_LAST`` was already
179 dequeued and no new buffers are expected to become available.
182 The ``V4L2_BUF_FLAG_REQUEST_FD`` flag was set but the device does not
183 support requests for the given buffer type.
186 The first buffer was queued via a request, but the application now tries
187 to queue it directly, or vice versa (it is not permitted to mix the two