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<refentry id="vidioc-querybuf">
<refentrytitle>ioctl VIDIOC_QUERYBUF</refentrytitle>
<refpurpose>Query the status of a buffer</refpurpose>
<funcdef>int <function>ioctl</function></funcdef>
<paramdef>int <parameter>fd</parameter></paramdef>
<paramdef>int <parameter>request</parameter></paramdef>
<paramdef>struct v4l2_buffer *<parameter>argp</parameter></paramdef>
<para>This ioctl is part of the <link linkend="mmap">streaming
</link> I/O method. It can be used to query the status of a
buffer at any time after buffers have been allocated with the
&VIDIOC-REQBUFS; ioctl.</para>
<para>Applications set the <structfield>type</structfield> field
of a &v4l2-buffer; to the same buffer type as was previously used with
&v4l2-format; <structfield>type</structfield> and &v4l2-requestbuffers;
<structfield>type</structfield>, and the <structfield>index</structfield>
field. Valid index numbers range from zero
to the number of buffers allocated with &VIDIOC-REQBUFS;
(&v4l2-requestbuffers; <structfield>count</structfield>) minus one.
The <structfield>reserved</structfield> field should to set to 0.
When using the <link linkend="planar-apis">multi-planar API</link>, the
<structfield>m.planes</structfield> field must contain a userspace pointer to an
array of &v4l2-plane; and the <structfield>length</structfield> field has
to be set to the number of elements in that array.
After calling <constant>VIDIOC_QUERYBUF</constant> with a pointer to
this structure drivers return an error code or fill the rest of
the structure.</para>
<para>In the <structfield>flags</structfield> field the
<constant>V4L2_BUF_FLAG_QUEUED</constant> and
<constant>V4L2_BUF_FLAG_DONE</constant> flags will be valid. The
<structfield>memory</structfield> field will be set to the current
I/O method. For the single-planar API, the <structfield>m.offset</structfield>
contains the offset of the buffer from the start of the device memory,
the <structfield>length</structfield> field its size. For the multi-planar API,
fields <structfield>m.mem_offset</structfield> and
<structfield>length</structfield> in the <structfield>m.planes</structfield>
array elements will be used instead and the <structfield>length</structfield>
field of &v4l2-buffer; is set to the number of filled-in array elements.
The driver may or may not set the remaining fields and flags, they are
meaningless in this context.</para>
<para>The <structname>v4l2_buffer</structname> structure is
specified in <xref linkend="buffer" />.</para>
<para>The buffer <structfield>type</structfield> is not
supported, or the <structfield>index</structfield> is out of bounds.</para>