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1<refentry id="vidioc-qbuf">
2 <refmeta>
3 <refentrytitle>ioctl VIDIOC_QBUF, VIDIOC_DQBUF</refentrytitle>
4 &manvol;
5 </refmeta>
6
7 <refnamediv>
8 <refname>VIDIOC_QBUF</refname>
9 <refname>VIDIOC_DQBUF</refname>
10 <refpurpose>Exchange a buffer with the driver</refpurpose>
11 </refnamediv>
12
13 <refsynopsisdiv>
14 <funcsynopsis>
15 <funcprototype>
16 <funcdef>int <function>ioctl</function></funcdef>
17 <paramdef>int <parameter>fd</parameter></paramdef>
18 <paramdef>int <parameter>request</parameter></paramdef>
19 <paramdef>struct v4l2_buffer *<parameter>argp</parameter></paramdef>
20 </funcprototype>
21 </funcsynopsis>
22 </refsynopsisdiv>
23
24 <refsect1>
25 <title>Arguments</title>
26
27 <variablelist>
28 <varlistentry>
29 <term><parameter>fd</parameter></term>
30 <listitem>
31 <para>&fd;</para>
32 </listitem>
33 </varlistentry>
34 <varlistentry>
35 <term><parameter>request</parameter></term>
36 <listitem>
37 <para>VIDIOC_QBUF, VIDIOC_DQBUF</para>
38 </listitem>
39 </varlistentry>
40 <varlistentry>
41 <term><parameter>argp</parameter></term>
42 <listitem>
43 <para></para>
44 </listitem>
45 </varlistentry>
46 </variablelist>
47 </refsect1>
48
49 <refsect1>
50 <title>Description</title>
51
52 <para>Applications call the <constant>VIDIOC_QBUF</constant> ioctl
53to enqueue an empty (capturing) or filled (output) buffer in the
54driver's incoming queue. The semantics depend on the selected I/O
55method.</para>
56
57 <para>To enqueue a buffer applications set the <structfield>type</structfield>
58field of a &v4l2-buffer; to the same buffer type as was previously used
59with &v4l2-format; <structfield>type</structfield> and &v4l2-requestbuffers;
60<structfield>type</structfield>. Applications must also set the
61<structfield>index</structfield> field. Valid index numbers range from
62zero to the number of buffers allocated with &VIDIOC-REQBUFS;
63(&v4l2-requestbuffers; <structfield>count</structfield>) minus one. The
64contents of the struct <structname>v4l2_buffer</structname> returned
65by a &VIDIOC-QUERYBUF; ioctl will do as well. When the buffer is
66intended for output (<structfield>type</structfield> is
67<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant>,
68<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE</constant>, or
69<constant>V4L2_BUF_TYPE_VBI_OUTPUT</constant>) applications must also
70initialize the <structfield>bytesused</structfield>,
71<structfield>field</structfield> and
72<structfield>timestamp</structfield> fields, see <xref
73linkend="buffer" /> for details.
74Applications must also set <structfield>flags</structfield> to 0.
75The <structfield>reserved2</structfield> and
76<structfield>reserved</structfield> fields must be set to 0. When using
77the <link linkend="planar-apis">multi-planar API</link>, the
78<structfield>m.planes</structfield> field must contain a userspace pointer
79to a filled-in array of &v4l2-plane; and the <structfield>length</structfield>
80field must be set to the number of elements in that array.
81</para>
82
83 <para>To enqueue a <link linkend="mmap">memory mapped</link>
84buffer applications set the <structfield>memory</structfield>
85field to <constant>V4L2_MEMORY_MMAP</constant>. When
86<constant>VIDIOC_QBUF</constant> is called with a pointer to this
87structure the driver sets the
88<constant>V4L2_BUF_FLAG_MAPPED</constant> and
89<constant>V4L2_BUF_FLAG_QUEUED</constant> flags and clears the
90<constant>V4L2_BUF_FLAG_DONE</constant> flag in the
91<structfield>flags</structfield> field, or it returns an
92&EINVAL;.</para>
93
94 <para>To enqueue a <link linkend="userp">user pointer</link>
95buffer applications set the <structfield>memory</structfield>
96field to <constant>V4L2_MEMORY_USERPTR</constant>, the
97<structfield>m.userptr</structfield> field to the address of the
98buffer and <structfield>length</structfield> to its size. When the multi-planar
99API is used, <structfield>m.userptr</structfield> and
100<structfield>length</structfield> members of the passed array of &v4l2-plane;
101have to be used instead. When <constant>VIDIOC_QBUF</constant> is called with
102a pointer to this structure the driver sets the
103<constant>V4L2_BUF_FLAG_QUEUED</constant> flag and clears the
104<constant>V4L2_BUF_FLAG_MAPPED</constant> and
105<constant>V4L2_BUF_FLAG_DONE</constant> flags in the
106<structfield>flags</structfield> field, or it returns an error code.
107This ioctl locks the memory pages of the buffer in physical memory,
108they cannot be swapped out to disk. Buffers remain locked until
109dequeued, until the &VIDIOC-STREAMOFF; or &VIDIOC-REQBUFS; ioctl is
110called, or until the device is closed.</para>
111
112 <para>Applications call the <constant>VIDIOC_DQBUF</constant>
113ioctl to dequeue a filled (capturing) or displayed (output) buffer
114from the driver's outgoing queue. They just set the
115<structfield>type</structfield>, <structfield>memory</structfield>
116and <structfield>reserved</structfield>
117fields of a &v4l2-buffer; as above, when <constant>VIDIOC_DQBUF</constant>
118is called with a pointer to this structure the driver fills the
119remaining fields or returns an error code. The driver may also set
120<constant>V4L2_BUF_FLAG_ERROR</constant> in the <structfield>flags</structfield>
121field. It indicates a non-critical (recoverable) streaming error. In such case
122the application may continue as normal, but should be aware that data in the
123dequeued buffer might be corrupted. When using the multi-planar API, the
124planes array does not have to be passed; the <structfield>m.planes</structfield>
125member must be set to NULL in that case.</para>
126
127 <para>By default <constant>VIDIOC_DQBUF</constant> blocks when no
128buffer is in the outgoing queue. When the
129<constant>O_NONBLOCK</constant> flag was given to the &func-open;
130function, <constant>VIDIOC_DQBUF</constant> returns immediately
131with an &EAGAIN; when no buffer is available.</para>
132
133 <para>The <structname>v4l2_buffer</structname> structure is
134specified in <xref linkend="buffer" />.</para>
135 </refsect1>
136
137 <refsect1>
138 &return-value;
139
140 <variablelist>
141 <varlistentry>
142 <term><errorcode>EAGAIN</errorcode></term>
143 <listitem>
144 <para>Non-blocking I/O has been selected using
145<constant>O_NONBLOCK</constant> and no buffer was in the outgoing
146queue.</para>
147 </listitem>
148 </varlistentry>
149 <varlistentry>
150 <term><errorcode>EINVAL</errorcode></term>
151 <listitem>
152 <para>The buffer <structfield>type</structfield> is not
153supported, or the <structfield>index</structfield> is out of bounds,
154or no buffers have been allocated yet, or the
155<structfield>userptr</structfield> or
156<structfield>length</structfield> are invalid.</para>
157 </listitem>
158 <term><errorcode>EIO</errorcode></term>
159 <listitem>
160 <para><constant>VIDIOC_DQBUF</constant> failed due to an
161internal error. Can also indicate temporary problems like signal
162loss. Note the driver might dequeue an (empty) buffer despite
163returning an error, or even stop capturing. Reusing such buffer may be unsafe
164though and its details (e.g. <structfield>index</structfield>) may not be
165returned either. It is recommended that drivers indicate recoverable errors
166by setting the <constant>V4L2_BUF_FLAG_ERROR</constant> and returning 0 instead.
167In that case the application should be able to safely reuse the buffer and
168continue streaming.
169 </para>
170 </listitem>
171 </varlistentry>
172 </variablelist>
173 </refsect1>
174</refentry>