1 .. -*- coding: utf-8; mode: rst -*-
9 A buffer contains data exchanged by application and driver using one of
10 the Streaming I/O methods. In the multi-planar API, the data is held in
11 planes, while the buffer structure acts as a container for the planes.
12 Only pointers to buffers (planes) are exchanged, the data itself is not
13 copied. These pointers, together with meta-information like timestamps
14 or field parity, are stored in a struct :ref:`struct v4l2_buffer <v4l2-buffer>`,
15 argument to the :ref:`VIDIOC_QUERYBUF`,
16 :ref:`VIDIOC_QBUF` and
17 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. In the multi-planar API,
18 some plane-specific members of struct :ref:`struct v4l2_buffer <v4l2-buffer>`,
19 such as pointers and sizes for each plane, are stored in struct
20 :ref:`struct v4l2_plane <v4l2-plane>` instead. In that case, struct
21 :ref:`struct v4l2_buffer <v4l2-buffer>` contains an array of plane structures.
23 Dequeued video buffers come with timestamps. The driver decides at which
24 part of the frame and with which clock the timestamp is taken. Please
25 see flags in the masks ``V4L2_BUF_FLAG_TIMESTAMP_MASK`` and
26 ``V4L2_BUF_FLAG_TSTAMP_SRC_MASK`` in :ref:`buffer-flags`. These flags
27 are always valid and constant across all buffers during the whole video
28 stream. Changes in these flags may take place as a side effect of
29 :ref:`VIDIOC_S_INPUT <VIDIOC_G_INPUT>` or
30 :ref:`VIDIOC_S_OUTPUT <VIDIOC_G_OUTPUT>` however. The
31 ``V4L2_BUF_FLAG_TIMESTAMP_COPY`` timestamp type which is used by e.g. on
32 mem-to-mem devices is an exception to the rule: the timestamp source
33 flags are copied from the OUTPUT video buffer to the CAPTURE video
42 .. flat-table:: struct v4l2_buffer
55 - Number of the buffer, set by the application except when calling
56 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`, then it is set by the
57 driver. This field can range from zero to the number of buffers
58 allocated with the :ref:`VIDIOC_REQBUFS` ioctl
59 (struct :ref:`v4l2_requestbuffers <v4l2-requestbuffers>`
60 ``count``), plus any buffers allocated with
61 :ref:`VIDIOC_CREATE_BUFS` minus one.
70 - Type of the buffer, same as struct
71 :ref:`v4l2_format <v4l2-format>` ``type`` or struct
72 :ref:`v4l2_requestbuffers <v4l2-requestbuffers>` ``type``, set
73 by the application. See :ref:`v4l2-buf-type`
82 - The number of bytes occupied by the data in the buffer. It depends
83 on the negotiated data format and may change with each buffer for
84 compressed variable size data like JPEG images. Drivers must set
85 this field when ``type`` refers to a capture stream, applications
86 when it refers to an output stream. If the application sets this
87 to 0 for an output stream, then ``bytesused`` will be set to the
88 size of the buffer (see the ``length`` field of this struct) by
89 the driver. For multiplanar formats this field is ignored and the
90 ``planes`` pointer is used instead.
99 - Flags set by the application or driver, see :ref:`buffer-flags`.
108 - Indicates the field order of the image in the buffer, see
109 :ref:`v4l2-field`. This field is not used when the buffer
110 contains VBI data. Drivers must set it when ``type`` refers to a
111 capture stream, applications when it refers to an output stream.
120 - For capture streams this is time when the first data byte was
121 captured, as returned by the :c:func:`clock_gettime()` function
122 for the relevant clock id; see ``V4L2_BUF_FLAG_TIMESTAMP_*`` in
123 :ref:`buffer-flags`. For output streams the driver stores the
124 time at which the last data byte was actually sent out in the
125 ``timestamp`` field. This permits applications to monitor the
126 drift between the video and system clock. For output streams that
127 use ``V4L2_BUF_FLAG_TIMESTAMP_COPY`` the application has to fill
128 in the timestamp which will be copied by the driver to the capture
133 - struct :ref:`v4l2_timecode <v4l2-timecode>`
138 - When ``type`` is ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` and the
139 ``V4L2_BUF_FLAG_TIMECODE`` flag is set in ``flags``, this
140 structure contains a frame timecode. In
141 :ref:`V4L2_FIELD_ALTERNATE <v4l2-field>` mode the top and
142 bottom field contain the same timecode. Timecodes are intended to
143 help video editing and are typically recorded on video tapes, but
144 also embedded in compressed formats like MPEG. This field is
145 independent of the ``timestamp`` and ``sequence`` fields.
154 - Set by the driver, counting the frames (not fields!) in sequence.
155 This field is set for both input and output devices.
161 In :ref:`V4L2_FIELD_ALTERNATE <v4l2-field>` mode the top and
162 bottom field have the same sequence number. The count starts at
163 zero and includes dropped or repeated frames. A dropped frame was
164 received by an input device but could not be stored due to lack of
165 free buffer space. A repeated frame was displayed again by an
166 output device because the application did not pass new data in
169 Note this may count the frames received e.g. over USB, without
170 taking into account the frames dropped by the remote hardware due
171 to limited compression throughput or bus bandwidth. These devices
172 identify by not enumerating any video standards, see
182 - This field must be set by applications and/or drivers in
183 accordance with the selected I/O method. See :ref:`v4l2-memory`
198 - For the single-planar API and when ``memory`` is
199 ``V4L2_MEMORY_MMAP`` this is the offset of the buffer from the
200 start of the device memory. The value is returned by the driver
201 and apart of serving as parameter to the
202 :ref:`mmap() <func-mmap>` function not useful for applications.
203 See :ref:`mmap` for details
212 - For the single-planar API and when ``memory`` is
213 ``V4L2_MEMORY_USERPTR`` this is a pointer to the buffer (casted to
214 unsigned long type) in virtual memory, set by the application. See
215 :ref:`userp` for details.
224 - When using the multi-planar API, contains a userspace pointer to
225 an array of struct :ref:`v4l2_plane <v4l2-plane>`. The size of
226 the array should be put in the ``length`` field of this
227 :ref:`struct v4l2_buffer <v4l2-buffer>` structure.
236 - For the single-plane API and when ``memory`` is
237 ``V4L2_MEMORY_DMABUF`` this is the file descriptor associated with
247 - Size of the buffer (not the payload) in bytes for the
248 single-planar API. This is set by the driver based on the calls to
249 :ref:`VIDIOC_REQBUFS` and/or
250 :ref:`VIDIOC_CREATE_BUFS`. For the
251 multi-planar API the application sets this to the number of
252 elements in the ``planes`` array. The driver will fill in the
253 actual number of valid elements in that array.
262 - A place holder for future extensions. Drivers and applications
272 - A place holder for future extensions. Drivers and applications
295 - The number of bytes occupied by data in the plane (its payload).
296 Drivers must set this field when ``type`` refers to a capture
297 stream, applications when it refers to an output stream. If the
298 application sets this to 0 for an output stream, then
299 ``bytesused`` will be set to the size of the plane (see the
300 ``length`` field of this struct) by the driver. Note that the
301 actual image data starts at ``data_offset`` which may not be 0.
310 - Size in bytes of the plane (not its payload). This is set by the
311 driver based on the calls to
312 :ref:`VIDIOC_REQBUFS` and/or
313 :ref:`VIDIOC_CREATE_BUFS`.
331 - When the memory type in the containing struct
332 :ref:`v4l2_buffer <v4l2-buffer>` is ``V4L2_MEMORY_MMAP``, this
333 is the value that should be passed to :ref:`mmap() <func-mmap>`,
334 similar to the ``offset`` field in struct
335 :ref:`v4l2_buffer <v4l2-buffer>`.
344 - When the memory type in the containing struct
345 :ref:`v4l2_buffer <v4l2-buffer>` is ``V4L2_MEMORY_USERPTR``,
346 this is a userspace pointer to the memory allocated for this plane
356 - When the memory type in the containing struct
357 :ref:`v4l2_buffer <v4l2-buffer>` is ``V4L2_MEMORY_DMABUF``,
358 this is a file descriptor associated with a DMABUF buffer, similar
359 to the ``fd`` field in struct :ref:`v4l2_buffer <v4l2-buffer>`.
368 - Offset in bytes to video data in the plane. Drivers must set this
369 field when ``type`` refers to a capture stream, applications when
370 it refers to an output stream. Note that data_offset is included
371 in ``bytesused``. So the size of the image in the plane is
372 ``bytesused``-``data_offset`` at offset ``data_offset`` from the
382 - Reserved for future use. Should be zeroed by drivers and
400 - ``V4L2_BUF_TYPE_VIDEO_CAPTURE``
404 - Buffer of a single-planar video capture stream, see
409 - ``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``
413 - Buffer of a multi-planar video capture stream, see
418 - ``V4L2_BUF_TYPE_VIDEO_OUTPUT``
422 - Buffer of a single-planar video output stream, see
427 - ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``
431 - Buffer of a multi-planar video output stream, see :ref:`output`.
435 - ``V4L2_BUF_TYPE_VIDEO_OVERLAY``
439 - Buffer for video overlay, see :ref:`overlay`.
443 - ``V4L2_BUF_TYPE_VBI_CAPTURE``
447 - Buffer of a raw VBI capture stream, see :ref:`raw-vbi`.
451 - ``V4L2_BUF_TYPE_VBI_OUTPUT``
455 - Buffer of a raw VBI output stream, see :ref:`raw-vbi`.
459 - ``V4L2_BUF_TYPE_SLICED_VBI_CAPTURE``
463 - Buffer of a sliced VBI capture stream, see :ref:`sliced`.
467 - ``V4L2_BUF_TYPE_SLICED_VBI_OUTPUT``
471 - Buffer of a sliced VBI output stream, see :ref:`sliced`.
475 - ``V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY``
479 - Buffer for video output overlay (OSD), see :ref:`osd`.
483 - ``V4L2_BUF_TYPE_SDR_CAPTURE``
487 - Buffer for Software Defined Radio (SDR) capture stream, see
492 - ``V4L2_BUF_TYPE_SDR_OUTPUT``
496 - Buffer for Software Defined Radio (SDR) output stream, see
514 - ``V4L2_BUF_FLAG_MAPPED``
518 - The buffer resides in device memory and has been mapped into the
519 application's address space, see :ref:`mmap` for details.
520 Drivers set or clear this flag when the
521 :ref:`VIDIOC_QUERYBUF`,
522 :ref:`VIDIOC_QBUF` or
523 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl is called. Set by the
528 - ``V4L2_BUF_FLAG_QUEUED``
532 - Internally drivers maintain two buffer queues, an incoming and
533 outgoing queue. When this flag is set, the buffer is currently on
534 the incoming queue. It automatically moves to the outgoing queue
535 after the buffer has been filled (capture devices) or displayed
536 (output devices). Drivers set or clear this flag when the
537 ``VIDIOC_QUERYBUF`` ioctl is called. After (successful) calling
538 the ``VIDIOC_QBUF``\ ioctl it is always set and after
539 ``VIDIOC_DQBUF`` always cleared.
543 - ``V4L2_BUF_FLAG_DONE``
547 - When this flag is set, the buffer is currently on the outgoing
548 queue, ready to be dequeued from the driver. Drivers set or clear
549 this flag when the ``VIDIOC_QUERYBUF`` ioctl is called. After
550 calling the ``VIDIOC_QBUF`` or ``VIDIOC_DQBUF`` it is always
551 cleared. Of course a buffer cannot be on both queues at the same
552 time, the ``V4L2_BUF_FLAG_QUEUED`` and ``V4L2_BUF_FLAG_DONE`` flag
553 are mutually exclusive. They can be both cleared however, then the
554 buffer is in "dequeued" state, in the application domain so to
559 - ``V4L2_BUF_FLAG_ERROR``
563 - When this flag is set, the buffer has been dequeued successfully,
564 although the data might have been corrupted. This is recoverable,
565 streaming may continue as normal and the buffer may be reused
566 normally. Drivers set this flag when the ``VIDIOC_DQBUF`` ioctl is
571 - ``V4L2_BUF_FLAG_KEYFRAME``
575 - Drivers set or clear this flag when calling the ``VIDIOC_DQBUF``
576 ioctl. It may be set by video capture devices when the buffer
577 contains a compressed image which is a key frame (or field), i. e.
578 can be decompressed on its own. Also known as an I-frame.
579 Applications can set this bit when ``type`` refers to an output
584 - ``V4L2_BUF_FLAG_PFRAME``
588 - Similar to ``V4L2_BUF_FLAG_KEYFRAME`` this flags predicted frames
589 or fields which contain only differences to a previous key frame.
590 Applications can set this bit when ``type`` refers to an output
595 - ``V4L2_BUF_FLAG_BFRAME``
599 - Similar to ``V4L2_BUF_FLAG_KEYFRAME`` this flags a bi-directional
600 predicted frame or field which contains only the differences
601 between the current frame and both the preceding and following key
602 frames to specify its content. Applications can set this bit when
603 ``type`` refers to an output stream.
607 - ``V4L2_BUF_FLAG_TIMECODE``
611 - The ``timecode`` field is valid. Drivers set or clear this flag
612 when the ``VIDIOC_DQBUF`` ioctl is called. Applications can set
613 this bit and the corresponding ``timecode`` structure when
614 ``type`` refers to an output stream.
618 - ``V4L2_BUF_FLAG_PREPARED``
622 - The buffer has been prepared for I/O and can be queued by the
623 application. Drivers set or clear this flag when the
624 :ref:`VIDIOC_QUERYBUF`,
625 :ref:`VIDIOC_PREPARE_BUF <VIDIOC_QBUF>`,
626 :ref:`VIDIOC_QBUF` or
627 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl is called.
631 - ``V4L2_BUF_FLAG_NO_CACHE_INVALIDATE``
635 - Caches do not have to be invalidated for this buffer. Typically
636 applications shall use this flag if the data captured in the
637 buffer is not going to be touched by the CPU, instead the buffer
638 will, probably, be passed on to a DMA-capable hardware unit for
639 further processing or output.
643 - ``V4L2_BUF_FLAG_NO_CACHE_CLEAN``
647 - Caches do not have to be cleaned for this buffer. Typically
648 applications shall use this flag for output buffers if the data in
649 this buffer has not been created by the CPU but by some
650 DMA-capable unit, in which case caches have not been used.
654 - ``V4L2_BUF_FLAG_LAST``
658 - Last buffer produced by the hardware. mem2mem codec drivers set
659 this flag on the capture queue for the last buffer when the
660 :ref:`VIDIOC_QUERYBUF` or
661 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl is called. Due to
662 hardware limitations, the last buffer may be empty. In this case
663 the driver will set the ``bytesused`` field to 0, regardless of
664 the format. Any Any subsequent call to the
665 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl will not block anymore,
666 but return an ``EPIPE`` error code.
670 - ``V4L2_BUF_FLAG_TIMESTAMP_MASK``
674 - Mask for timestamp types below. To test the timestamp type, mask
675 out bits not belonging to timestamp type by performing a logical
676 and operation with buffer flags and timestamp mask.
680 - ``V4L2_BUF_FLAG_TIMESTAMP_UNKNOWN``
684 - Unknown timestamp type. This type is used by drivers before Linux
685 3.9 and may be either monotonic (see below) or realtime (wall
686 clock). Monotonic clock has been favoured in embedded systems
687 whereas most of the drivers use the realtime clock. Either kinds
688 of timestamps are available in user space via
689 :c:func:`clock_gettime(2)` using clock IDs ``CLOCK_MONOTONIC``
690 and ``CLOCK_REALTIME``, respectively.
694 - ``V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC``
698 - The buffer timestamp has been taken from the ``CLOCK_MONOTONIC``
699 clock. To access the same clock outside V4L2, use
700 :c:func:`clock_gettime(2)`.
704 - ``V4L2_BUF_FLAG_TIMESTAMP_COPY``
708 - The CAPTURE buffer timestamp has been taken from the corresponding
709 OUTPUT buffer. This flag applies only to mem2mem devices.
713 - ``V4L2_BUF_FLAG_TSTAMP_SRC_MASK``
717 - Mask for timestamp sources below. The timestamp source defines the
718 point of time the timestamp is taken in relation to the frame.
719 Logical 'and' operation between the ``flags`` field and
720 ``V4L2_BUF_FLAG_TSTAMP_SRC_MASK`` produces the value of the
721 timestamp source. Applications must set the timestamp source when
722 ``type`` refers to an output stream and
723 ``V4L2_BUF_FLAG_TIMESTAMP_COPY`` is set.
727 - ``V4L2_BUF_FLAG_TSTAMP_SRC_EOF``
731 - End Of Frame. The buffer timestamp has been taken when the last
732 pixel of the frame has been received or the last pixel of the
733 frame has been transmitted. In practice, software generated
734 timestamps will typically be read from the clock a small amount of
735 time after the last pixel has been received or transmitten,
736 depending on the system and other activity in it.
740 - ``V4L2_BUF_FLAG_TSTAMP_SRC_SOE``
744 - Start Of Exposure. The buffer timestamp has been taken when the
745 exposure of the frame has begun. This is only valid for the
746 ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` buffer type.
763 - ``V4L2_MEMORY_MMAP``
767 - The buffer is used for :ref:`memory mapping <mmap>` I/O.
771 - ``V4L2_MEMORY_USERPTR``
775 - The buffer is used for :ref:`user pointer <userp>` I/O.
779 - ``V4L2_MEMORY_OVERLAY``
787 - ``V4L2_MEMORY_DMABUF``
791 - The buffer is used for :ref:`DMA shared buffer <dmabuf>` I/O.
798 The :ref:`struct v4l2_timecode <v4l2-timecode>` structure is designed to hold a
799 :ref:`smpte12m` or similar timecode. (struct
800 :c:type:`struct timeval` timestamps are stored in struct
801 :ref:`v4l2_buffer <v4l2-buffer>` field ``timestamp``.)
821 - Frame rate the timecodes are based on, see :ref:`timecode-type`.
829 - Timecode flags, see :ref:`timecode-flags`.
837 - Frame count, 0 ... 23/24/29/49/59, depending on the type of
846 - Seconds count, 0 ... 59. This is a binary, not BCD number.
854 - Minutes count, 0 ... 59. This is a binary, not BCD number.
862 - Hours count, 0 ... 29. This is a binary, not BCD number.
870 - The "user group" bits from the timecode.
887 - ``V4L2_TC_TYPE_24FPS``
891 - 24 frames per second, i. e. film.
895 - ``V4L2_TC_TYPE_25FPS``
899 - 25 frames per second, i. e. PAL or SECAM video.
903 - ``V4L2_TC_TYPE_30FPS``
907 - 30 frames per second, i. e. NTSC video.
911 - ``V4L2_TC_TYPE_50FPS``
919 - ``V4L2_TC_TYPE_60FPS``
940 - ``V4L2_TC_FLAG_DROPFRAME``
944 - Indicates "drop frame" semantics for counting frames in 29.97 fps
945 material. When set, frame numbers 0 and 1 at the start of each
946 minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from the
951 - ``V4L2_TC_FLAG_COLORFRAME``
955 - The "color frame" flag.
959 - ``V4L2_TC_USERBITS_field``
963 - Field mask for the "binary group flags".
967 - ``V4L2_TC_USERBITS_USERDEFINED``
971 - Unspecified format.
975 - ``V4L2_TC_USERBITS_8BITCHARS``
979 - 8-bit ISO characters.
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