doc/spec/r10944.htm

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VIDIOC_TRY_FMT</TITLE
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>Video for Linux Two API Specification: Revision 0.24</TH
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><H1
><A
NAME="VIDIOC-G-FMT"
></A
>ioctl VIDIOC_G_FMT, VIDIOC_S_FMT,
VIDIOC_TRY_FMT</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN10948"
></A
><H2
>Name</H2
>VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT&nbsp;--&nbsp;Get or set the data format, try a format</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN10953"
></A
><H2
>Synopsis</H2
><DIV
CLASS="FUNCSYNOPSIS"
><P
></P
><A
NAME="AEN10954"
></A
><P
><CODE
><CODE
CLASS="FUNCDEF"
>int ioctl</CODE
>(int fd, int request, struct v4l2_format
*argp);</CODE
></P
><P
></P
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN10964"
></A
><H2
>Arguments</H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><CODE
CLASS="PARAMETER"
>fd</CODE
></DT
><DD
><P
>File descriptor returned by <A
HREF="r14090.htm"
><CODE
CLASS="FUNCTION"
>open()</CODE
></A
>.</P
></DD
><DT
><CODE
CLASS="PARAMETER"
>request</CODE
></DT
><DD
><P
>VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT</P
></DD
><DT
><CODE
CLASS="PARAMETER"
>argp</CODE
></DT
><DD
><P
></P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN10984"
></A
><H2
>Description</H2
><P
>These ioctls are used to negotiate the format of data
(typically image format) exchanged between driver and
application.</P
><P
>To query the current parameters applications set the
<CODE
CLASS="STRUCTFIELD"
>type</CODE
> field of a struct
<CODE
CLASS="STRUCTNAME"
>v4l2_format</CODE
> to the respective buffer (stream)
type. For example video capture devices use
<CODE
CLASS="CONSTANT"
>V4L2_BUF_TYPE_VIDEO_CAPTURE</CODE
>. When the application
calls the <CODE
CLASS="CONSTANT"
>VIDIOC_G_FMT</CODE
> ioctl with a pointer to
this structure the driver fills the respective member of the
<CODE
CLASS="STRUCTFIELD"
>fmt</CODE
> union. In case of video capture devices
that is the struct&nbsp;<A
HREF="c2030.htm#V4L2-PIX-FORMAT"
>v4l2_pix_format</A
> <CODE
CLASS="STRUCTFIELD"
>pix</CODE
> member.
When the requested buffer type is not supported drivers return an
<SPAN
CLASS="ERRORCODE"
>EINVAL</SPAN
> error code.</P
><P
>To change the current format parameters applications
initialize the <CODE
CLASS="STRUCTFIELD"
>type</CODE
> field and all
fields of the respective <CODE
CLASS="STRUCTFIELD"
>fmt</CODE
>
union member. For details see the documentation of the various devices
types in <A
HREF="c6488.htm"
>Chapter 4</A
>. 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 <CODE
CLASS="CONSTANT"
>VIDIOC_S_FMT</CODE
> ioctl
with a pointer to a <CODE
CLASS="STRUCTNAME"
>v4l2_format</CODE
> 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 <CODE
CLASS="CONSTANT"
>VIDIOC_S_FMT</CODE
> ioctl returns the
current format parameters as <CODE
CLASS="CONSTANT"
>VIDIOC_G_FMT</CODE
> 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
<CODE
CLASS="CONSTANT"
>VIDIOC_G_FMT</CODE
> and
<CODE
CLASS="CONSTANT"
>VIDIOC_S_FMT</CODE
> ioctl. When the requested buffer
type is not supported drivers return an <SPAN
CLASS="ERRORCODE"
>EINVAL</SPAN
> error code on a
<CODE
CLASS="CONSTANT"
>VIDIOC_S_FMT</CODE
> attempt. When I/O is already in
progress or the resource is not available for other reasons drivers
return the <SPAN
CLASS="ERRORCODE"
>EBUSY</SPAN
> error code.</P
><P
>The <CODE
CLASS="CONSTANT"
>VIDIOC_TRY_FMT</CODE
> ioctl is equivalent
to <CODE
CLASS="CONSTANT"
>VIDIOC_S_FMT</CODE
> with one exception: it does not
change driver state. It can also be called at any time, never
returning <SPAN
CLASS="ERRORCODE"
>EBUSY</SPAN
>. 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.</P
><DIV
CLASS="TABLE"
><A
NAME="V4L2-FORMAT"
></A
><P
><B
>Table 1. struct <CODE
CLASS="STRUCTNAME"
>v4l2_format</CODE
></B
></P
><TABLE
BORDER="0"
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><COL
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><TR
><TD
>enum&nbsp;<A
HREF="x5953.htm#V4L2-BUF-TYPE"
>v4l2_buf_type</A
></TD
><TD
><CODE
CLASS="STRUCTFIELD"
>type</CODE
></TD
><TD
>&nbsp;</TD
><TD
>Type of the data stream, see <A
HREF="x5953.htm#V4L2-BUF-TYPE"
>Table 3-2</A
>.</TD
></TR
><TR
><TD
>union</TD
><TD
><CODE
CLASS="STRUCTFIELD"
>fmt</CODE
></TD
><TD
>&nbsp;</TD
><TD
>&nbsp;</TD
></TR
><TR
><TD
>&nbsp;</TD
><TD
>struct&nbsp;<A
HREF="c2030.htm#V4L2-PIX-FORMAT"
>v4l2_pix_format</A
></TD
><TD
><CODE
CLASS="STRUCTFIELD"
>pix</CODE
></TD
><TD
>Definition of an image format, see <A
HREF="c2030.htm"
>Chapter 2</A
>, used by video capture and output
devices.</TD
></TR
><TR
><TD
>&nbsp;</TD
><TD
>struct&nbsp;<A
HREF="x6570.htm#V4L2-WINDOW"
>v4l2_window</A
></TD
><TD
><CODE
CLASS="STRUCTFIELD"
>win</CODE
></TD
><TD
>Definition of an overlaid image, see <A
HREF="x6570.htm"
>Section 4.2</A
>, used by video overlay devices.</TD
></TR
><TR
><TD
>&nbsp;</TD
><TD
>struct&nbsp;<A
HREF="x7013.htm#V4L2-VBI-FORMAT"
>v4l2_vbi_format</A
></TD
><TD
><CODE
CLASS="STRUCTFIELD"
>vbi</CODE
></TD
><TD
>Raw VBI capture or output parameters. This is
discussed in more detail in <A
HREF="x7013.htm"
>Section 4.7</A
>. Used by raw VBI
capture and output devices.</TD
></TR
><TR
><TD
>&nbsp;</TD
><TD
>struct&nbsp;<A
HREF="x7236.htm#V4L2-SLICED-VBI-FORMAT"
>v4l2_sliced_vbi_format</A
></TD
><TD
><CODE
CLASS="STRUCTFIELD"
>sliced</CODE
></TD
><TD
>Sliced VBI capture or output parameters. See
<A
HREF="x7236.htm"
>Section 4.8</A
> for details. Used by sliced VBI
capture and output devices.</TD
></TR
><TR
><TD
>&nbsp;</TD
><TD
>__u8</TD
><TD
><CODE
CLASS="STRUCTFIELD"
>raw_data</CODE
>[200]</TD
><TD
>Place holder for future extensions and custom
(driver defined) formats with <CODE
CLASS="STRUCTFIELD"
>type</CODE
>
<CODE
CLASS="CONSTANT"
>V4L2_BUF_TYPE_PRIVATE</CODE
> and higher.</TD
></TR
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><DIV
CLASS="REFSECT1"
><A
NAME="AEN11074"
></A
><H2
>Return Value</H2
><P
>On success <SPAN
CLASS="RETURNVALUE"
>0</SPAN
> is returned, on error <SPAN
CLASS="RETURNVALUE"
>-1</SPAN
> and the <CODE
CLASS="VARNAME"
>errno</CODE
> variable is set appropriately:</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><SPAN
CLASS="ERRORCODE"
>EBUSY</SPAN
></DT
><DD
><P
>The data format cannot be changed at this
time, for example because I/O is already in progress.</P
></DD
><DT
><SPAN
CLASS="ERRORCODE"
>EINVAL</SPAN
></DT
><DD
><P
>The struct&nbsp;<A
HREF="r10944.htm#V4L2-FORMAT"
>v4l2_format</A
> <CODE
CLASS="STRUCTFIELD"
>type</CODE
>
field is invalid, the requested buffer type not supported, or
<CODE
CLASS="CONSTANT"
>VIDIOC_TRY_FMT</CODE
> was called and is not
supported with this buffer type.</P
></DD
></DL
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