Documentation/linux_tv/media/v4l/vidioc-querybuf.rst

   1 .. -*- coding: utf-8; mode: rst -*-
   2
   3 .. _VIDIOC_QUERYBUF:
   4
   5 *********************
   6 ioctl VIDIOC_QUERYBUF
   7 *********************
   8
   9 NAME
  10 ====
  11
  12 VIDIOC_QUERYBUF - Query the status of a buffer
  13
  14 SYNOPSIS
  15 ========
  16
  17 .. cpp:function:: int ioctl( int fd, int request, struct v4l2_buffer *argp )
  18
  19
  20 ARGUMENTS
  21 =========
  22
  23 ``fd``
  24     File descriptor returned by :ref:`open() <func-open>`.
  25
  26 ``request``
  27     VIDIOC_QUERYBUF
  28
  29 ``argp``
  30
  31
  32 DESCRIPTION
  33 ===========
  34
  35 This ioctl is part of the :ref:`streaming <mmap>` I/O method. It can
  36 be used to query the status of a buffer at any time after buffers have
  37 been allocated with the :ref:`VIDIOC_REQBUFS` ioctl.
  38
  39 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 and the ``index`` field. Valid index numbers range from zero to the
  44 number of buffers allocated with
  45 :ref:`VIDIOC_REQBUFS` (struct
  46 :ref:`v4l2_requestbuffers <v4l2-requestbuffers>` ``count``) minus
  47 one. The ``reserved`` and ``reserved2`` fields must be set to 0. When
  48 using the :ref:`multi-planar API <planar-apis>`, the ``m.planes``
  49 field must contain a userspace pointer to an array of struct
  50 :ref:`v4l2_plane <v4l2-plane>` and the ``length`` field has to be set
  51 to the number of elements in that array. After calling
  52 :ref:`VIDIOC_QUERYBUF` with a pointer to this structure drivers return an
  53 error code or fill the rest of the structure.
  54
  55 In the ``flags`` field the ``V4L2_BUF_FLAG_MAPPED``,
  56 ``V4L2_BUF_FLAG_PREPARED``, ``V4L2_BUF_FLAG_QUEUED`` and
  57 ``V4L2_BUF_FLAG_DONE`` flags will be valid. The ``memory`` field will be
  58 set to the current I/O method. For the single-planar API, the
  59 ``m.offset`` contains the offset of the buffer from the start of the
  60 device memory, the ``length`` field its size. For the multi-planar API,
  61 fields ``m.mem_offset`` and ``length`` in the ``m.planes`` array
  62 elements will be used instead and the ``length`` field of struct
  63 :ref:`v4l2_buffer <v4l2-buffer>` is set to the number of filled-in
  64 array elements. The driver may or may not set the remaining fields and
  65 flags, they are meaningless in this context.
  66
  67 The :ref:`struct v4l2_buffer <v4l2-buffer>` structure is specified in
  68 :ref:`buffer`.
  69
  70
  71 RETURN VALUE
  72 ============
  73
  74 On success 0 is returned, on error -1 and the ``errno`` variable is set
  75 appropriately. The generic error codes are described at the
  76 :ref:`Generic Error Codes <gen-errors>` chapter.
  77
  78 EINVAL
  79     The buffer ``type`` is not supported, or the ``index`` is out of
  80     bounds.