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<refentry id="vidioc-qbuf">
<refentrytitle>ioctl VIDIOC_QBUF, VIDIOC_DQBUF</refentrytitle>
<refpurpose>Exchange a buffer with the driver</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>Applications call the <constant>VIDIOC_QBUF</constant> ioctl
to enqueue an empty (capturing) or filled (output) buffer in the
driver's incoming queue. The semantics depend on the selected I/O
<para>To enqueue a buffer 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>. Applications must also set 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
contents of the struct <structname>v4l2_buffer</structname> returned
by a &VIDIOC-QUERYBUF; ioctl will do as well. When the buffer is
intended for output (<structfield>type</structfield> is
<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant> or
<constant>V4L2_BUF_TYPE_VBI_OUTPUT</constant>) applications must also
initialize the <structfield>bytesused</structfield>,
<structfield>field</structfield> and
<structfield>timestamp</structfield> fields, see <xref
linkend="buffer" /> for details.
Applications must also set <structfield>flags</structfield> to 0. If a driver
supports capturing from specific video inputs and you want to specify a video
input, then <structfield>flags</structfield> should be set to
<constant>V4L2_BUF_FLAG_INPUT</constant> and the field
<structfield>input</structfield> must be initialized to the desired input.
The <structfield>reserved</structfield> field must be set to 0.
<para>To enqueue a <link linkend="mmap">memory mapped</link>
buffer applications set the <structfield>memory</structfield>
field to <constant>V4L2_MEMORY_MMAP</constant>. When
<constant>VIDIOC_QBUF</constant> is called with a pointer to this
structure the driver sets the
<constant>V4L2_BUF_FLAG_MAPPED</constant> and
<constant>V4L2_BUF_FLAG_QUEUED</constant> flags and clears the
<constant>V4L2_BUF_FLAG_DONE</constant> flag in the
<structfield>flags</structfield> field, or it returns an
<para>To enqueue a <link linkend="userp">user pointer</link>
buffer applications set the <structfield>memory</structfield>
field to <constant>V4L2_MEMORY_USERPTR</constant>, the
<structfield>m.userptr</structfield> field to the address of the
buffer and <structfield>length</structfield> to its size.
When <constant>VIDIOC_QBUF</constant> is called with a pointer to this
structure the driver sets the <constant>V4L2_BUF_FLAG_QUEUED</constant>
flag and clears the <constant>V4L2_BUF_FLAG_MAPPED</constant> and
<constant>V4L2_BUF_FLAG_DONE</constant> flags in the
<structfield>flags</structfield> field, or it returns an error code.
This ioctl locks the memory pages of the buffer in physical memory,
they cannot be swapped out to disk. Buffers remain locked until
dequeued, until the &VIDIOC-STREAMOFF; or &VIDIOC-REQBUFS; ioctl is
called, or until the device is closed.</para>
<para>Applications call the <constant>VIDIOC_DQBUF</constant>
ioctl to dequeue a filled (capturing) or displayed (output) buffer
from the driver's outgoing queue. They just set the
<structfield>type</structfield>, <structfield>memory</structfield>
and <structfield>reserved</structfield>
fields of a &v4l2-buffer; as above, when <constant>VIDIOC_DQBUF</constant>
is called with a pointer to this structure the driver fills the
remaining fields or returns an error code. The driver may also set
<constant>V4L2_BUF_FLAG_ERROR</constant> in the <structfield>flags</structfield>
field. It indicates a non-critical (recoverable) streaming error. In such case
the application may continue as normal, but should be aware that data in the
dequeued buffer might be corrupted.</para>
<para>By default <constant>VIDIOC_DQBUF</constant> blocks when no
buffer is in the outgoing queue. When the
<constant>O_NONBLOCK</constant> flag was given to the &func-open;
function, <constant>VIDIOC_DQBUF</constant> returns immediately
with an &EAGAIN; when no buffer is available.</para>
<para>The <structname>v4l2_buffer</structname> structure is
specified in <xref linkend="buffer" />.</para>
<para>Non-blocking I/O has been selected using
<constant>O_NONBLOCK</constant> and no buffer was in the outgoing
<para>The buffer <structfield>type</structfield> is not
supported, or the <structfield>index</structfield> is out of bounds,
or no buffers have been allocated yet, or the
<structfield>userptr</structfield> or
<structfield>length</structfield> are invalid.</para>
<para>Not enough physical or virtual memory was available to
enqueue a user pointer buffer.</para>
<para><constant>VIDIOC_DQBUF</constant> failed due to an
internal error. Can also indicate temporary problems like signal
loss. Note the driver might dequeue an (empty) buffer despite
returning an error, or even stop capturing. Reusing such buffer may be unsafe
though and its details (e.g. <structfield>index</structfield>) may not be
returned either. It is recommended that drivers indicate recoverable errors
by setting the <constant>V4L2_BUF_FLAG_ERROR</constant> and returning 0 instead.
In that case the application should be able to safely reuse the buffer and
continue streaming.
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