1 .. -*- coding: utf-8; mode: rst -*-
5 ************************************************
6 ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT
7 ************************************************
12 VIDIOC_G_FMT - VIDIOC_S_FMT - VIDIOC_TRY_FMT - Get or set the data format, try a format
18 .. cpp:function:: int ioctl( int fd, int request, struct v4l2_format *argp )
25 File descriptor returned by :ref:`open() <func-open>`.
28 VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT
36 These ioctls are used to negotiate the format of data (typically image
37 format) exchanged between driver and application.
39 To query the current parameters applications set the ``type`` field of a
40 struct :ref:`struct v4l2_format <v4l2-format>` to the respective buffer (stream)
41 type. For example video capture devices use
42 ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or
43 ``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``. When the application calls the
44 :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer to this structure the driver fills
45 the respective member of the ``fmt`` union. In case of video capture
46 devices that is either the struct
47 :ref:`v4l2_pix_format <v4l2-pix-format>` ``pix`` or the struct
48 :ref:`v4l2_pix_format_mplane <v4l2-pix-format-mplane>` ``pix_mp``
49 member. When the requested buffer type is not supported drivers return
50 an ``EINVAL`` error code.
52 To change the current format parameters applications initialize the
53 ``type`` field and all fields of the respective ``fmt`` union member.
54 For details see the documentation of the various devices types in
55 :ref:`devices`. Good practice is to query the current parameters
56 first, and to modify only those parameters not suitable for the
57 application. When the application calls the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl with
58 a pointer to a :ref:`struct v4l2_format <v4l2-format>` structure the driver
59 checks and adjusts the parameters against hardware abilities. Drivers
60 should not return an error code unless the ``type`` field is invalid,
61 this is a mechanism to fathom device capabilities and to approach
62 parameters acceptable for both the application and driver. On success
63 the driver may program the hardware, allocate resources and generally
64 prepare for data exchange. Finally the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl returns
65 the current format parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. Very simple,
66 inflexible devices may even ignore all input and always return the
67 default parameters. However all V4L2 devices exchanging data with the
68 application must implement the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
69 ioctl. When the requested buffer type is not supported drivers return an
70 EINVAL error code on a :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` attempt. When I/O is already in
71 progress or the resource is not available for other reasons drivers
72 return the ``EBUSY`` error code.
74 The :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl is equivalent to :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` with one
75 exception: it does not change driver state. It can also be called at any
76 time, never returning ``EBUSY``. This function is provided to negotiate
77 parameters, to learn about hardware limitations, without disabling I/O
78 or possibly time consuming hardware preparations. Although strongly
79 recommended drivers are not required to implement this ioctl.
81 The format as returned by :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` must be identical to what
82 :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` returns for the same input or output.
87 .. flat-table:: struct v4l2_format
99 - Type of the data stream, see :ref:`v4l2-buf-type`.
110 - struct :ref:`v4l2_pix_format <v4l2-pix-format>`
114 - Definition of an image format, see :ref:`pixfmt`, used by video
115 capture and output devices.
120 - struct :ref:`v4l2_pix_format_mplane <v4l2-pix-format-mplane>`
124 - Definition of an image format, see :ref:`pixfmt`, used by video
125 capture and output devices that support the
126 :ref:`multi-planar version of the API <planar-apis>`.
131 - struct :ref:`v4l2_window <v4l2-window>`
135 - Definition of an overlaid image, see :ref:`overlay`, used by
136 video overlay devices.
141 - struct :ref:`v4l2_vbi_format <v4l2-vbi-format>`
145 - Raw VBI capture or output parameters. This is discussed in more
146 detail in :ref:`raw-vbi`. Used by raw VBI capture and output
152 - struct :ref:`v4l2_sliced_vbi_format <v4l2-sliced-vbi-format>`
156 - Sliced VBI capture or output parameters. See :ref:`sliced` for
157 details. Used by sliced VBI capture and output devices.
162 - struct :ref:`v4l2_sdr_format <v4l2-sdr-format>`
166 - Definition of a data format, see :ref:`pixfmt`, used by SDR
167 capture and output devices.
174 - ``raw_data``\ \[200\]
176 - Place holder for future extensions.
182 On success 0 is returned, on error -1 and the ``errno`` variable is set
183 appropriately. The generic error codes are described at the
184 :ref:`Generic Error Codes <gen-errors>` chapter.
187 The struct :ref:`v4l2_format <v4l2-format>` ``type`` field is
188 invalid or the requested buffer type not supported.