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<refentry id="vidioc-g-fmt">
<refentrytitle>ioctl VIDIOC_G_FMT, VIDIOC_S_FMT,
<refpurpose>Get or set the data format, try a format</refpurpose>
<funcdef>int <function>ioctl</function></funcdef>
<paramdef>int <parameter>fd</parameter></paramdef>
<paramdef>int <parameter>request</parameter></paramdef>
<paramdef>struct v4l2_format
<para>These ioctls are used to negotiate the format of data
(typically image format) exchanged between driver and
<para>To query the current parameters applications set the
<structfield>type</structfield> field of a struct
<structname>v4l2_format</structname> to the respective buffer (stream)
type. For example video capture devices use
<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant>. When the application
calls the <constant>VIDIOC_G_FMT</constant> ioctl with a pointer to
this structure the driver fills the respective member of the
<structfield>fmt</structfield> union. In case of video capture devices
that is the &v4l2-pix-format; <structfield>pix</structfield> member.
When the requested buffer type is not supported drivers return an
<para>To change the current format parameters applications
initialize the <structfield>type</structfield> field and all
fields of the respective <structfield>fmt</structfield>
union member. For details see the documentation of the various devices
types in <xref linkend="devices" />. Good practice is to query the
current parameters first, and to
modify only those parameters not suitable for the application. When
the application calls the <constant>VIDIOC_S_FMT</constant> ioctl
with a pointer to a <structname>v4l2_format</structname> structure
the driver checks
and adjusts the parameters against hardware abilities. Drivers
should not return an error code unless the input is ambiguous, this is
a mechanism to fathom device capabilities and to approach parameters
acceptable for both the application and driver. On success the driver
may program the hardware, allocate resources and generally prepare for
data exchange.
Finally the <constant>VIDIOC_S_FMT</constant> ioctl returns the
current format parameters as <constant>VIDIOC_G_FMT</constant> does.
Very simple, inflexible devices may even ignore all input and always
return the default parameters. However all V4L2 devices exchanging
data with the application must implement the
<constant>VIDIOC_G_FMT</constant> and
<constant>VIDIOC_S_FMT</constant> ioctl. When the requested buffer
type is not supported drivers return an &EINVAL; on a
<constant>VIDIOC_S_FMT</constant> attempt. When I/O is already in
progress or the resource is not available for other reasons drivers
return the &EBUSY;.</para>
<para>The <constant>VIDIOC_TRY_FMT</constant> ioctl is equivalent
to <constant>VIDIOC_S_FMT</constant> with one exception: it does not
change driver state. It can also be called at any time, never
returning <errorcode>EBUSY</errorcode>. This function is provided to
negotiate parameters, to learn about hardware limitations, without
disabling I/O or possibly time consuming hardware preparations.
Although strongly recommended drivers are not required to implement
this ioctl.</para>
<table pgwide="1" frame="none" id="v4l2-format">
<title>struct <structname>v4l2_format</structname></title>
<tgroup cols="4">
<colspec colname="c1" />
<colspec colname="c2" />
<colspec colname="c3" />
<colspec colname="c4" />
<tbody valign="top">
<entry>Type of the data stream, see <xref
linkend="v4l2-buf-type" />.</entry>
<entry>Definition of an image format, see <xref
linkend="pixfmt" />, used by video capture and output
<entry>Definition of an overlaid image, see <xref
linkend="overlay" />, used by video overlay devices.</entry>
<entry>Raw VBI capture or output parameters. This is
discussed in more detail in <xref linkend="raw-vbi" />. Used by raw VBI
capture and output devices.</entry>
<entry>Sliced VBI capture or output parameters. See
<xref linkend="sliced" /> for details. Used by sliced VBI
capture and output devices.</entry>
<entry>Place holder for future extensions and custom
(driver defined) formats with <structfield>type</structfield>
<constant>V4L2_BUF_TYPE_PRIVATE</constant> and higher.</entry>
<para>The data format cannot be changed at this
time, for example because I/O is already in progress.</para>
<para>The &v4l2-format; <structfield>type</structfield>
field is invalid, the requested buffer type not supported, or
<constant>VIDIOC_TRY_FMT</constant> was called and is not
supported with this buffer type.</para>
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