summaryrefslogtreecommitdiffstats
path: root/Documentation/DocBook/v4l/vidioc-qbuf.xml
blob: 1870817781548b409889d9ed93220cb26162ec74 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
<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 <link linkend="mmap">memory mapped</link>
buffer applications set the <structfield>type</structfield> field of a
&v4l2-buffer; to the same buffer type as previously &v4l2-format;
<structfield>type</structfield> and &v4l2-requestbuffers;
<structfield>type</structfield>, the <structfield>memory</structfield>
field to <constant>V4L2_MEMORY_MMAP</constant> 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
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. 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>type</structfield> field of a
&v4l2-buffer; to the same buffer type as previously &v4l2-format;
<structfield>type</structfield> and &v4l2-requestbuffers;
<structfield>type</structfield>, the <structfield>memory</structfield>
field to <constant>V4L2_MEMORY_USERPTR</constant> and the
<structfield>m.userptr</structfield> field to the address of the
buffer and <structfield>length</structfield> to its size. When the
buffer is intended for output additional fields must be set as above.
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 are
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> and <structfield>memory</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.</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>
      </varlistentry>
      <varlistentry>
	<term><errorcode>ENOMEM</errorcode></term>
	<listitem>
	  <para>Not enough physical or virtual memory was available to
enqueue a user pointer buffer.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<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.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>
</refentry>

<!--
Local Variables:
mode: sgml
sgml-parent-document: "v4l2.sgml"
indent-tabs-mode: nil
End:
-->
OpenPOWER on IntegriCloud