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-<refentry id="vidioc-qbuf">
- <refmeta>
- <refentrytitle>ioctl VIDIOC_QBUF, VIDIOC_DQBUF</refentrytitle>
- &manvol;
- </refmeta>
-
- <refnamediv>
- <refname>VIDIOC_QBUF</refname>
- <refname>VIDIOC_DQBUF</refname>
- <refpurpose>Exchange a buffer with the driver</refpurpose>
- </refnamediv>
-
- <refsynopsisdiv>
- <funcsynopsis>
- <funcprototype>
- <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>
- </funcprototype>
- </funcsynopsis>
- </refsynopsisdiv>
-
- <refsect1>
- <title>Arguments</title>
-
- <variablelist>
- <varlistentry>
- <term><parameter>fd</parameter></term>
- <listitem>
- <para>&fd;</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><parameter>request</parameter></term>
- <listitem>
- <para>VIDIOC_QBUF, VIDIOC_DQBUF</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><parameter>argp</parameter></term>
- <listitem>
- <para></para>
- </listitem>
- </varlistentry>
- </variablelist>
- </refsect1>
-
- <refsect1>
- <title>Description</title>
-
- <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
-method.</para>
-
- <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>,
-<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE</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. When using
-the <link linkend="planar-apis">multi-planar API</link>, the
-<structfield>m.planes</structfield> field must contain a userspace pointer
-to a filled-in array of &v4l2-plane; and the <structfield>length</structfield>
-field must be set to the number of elements in that array.
-</para>
-
- <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
-&EINVAL;.</para>
-
- <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 the multi-planar
-API is used, <structfield>m.userptr</structfield> and
-<structfield>length</structfield> members of the passed array of &v4l2-plane;
-have to be used instead. 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. When using the multi-planar API, the
-planes array does not have to be passed; the <structfield>m.planes</structfield>
-member must be set to NULL in that case.</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>
- </refsect1>
-
- <refsect1>
- &return-value;
-
- <variablelist>
- <varlistentry>
- <term><errorcode>EAGAIN</errorcode></term>
- <listitem>
- <para>Non-blocking I/O has been selected using
-<constant>O_NONBLOCK</constant> and no buffer was in the outgoing
-queue.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><errorcode>EINVAL</errorcode></term>
- <listitem>
- <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>
- </listitem>
- <term><errorcode>EIO</errorcode></term>
- <listitem>
- <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.
- </para>
- </listitem>
- </varlistentry>
- </variablelist>
- </refsect1>
-</refentry>