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1<refentry id="func-read">
2 <refmeta>
3 <refentrytitle>V4L2 read()</refentrytitle>
4 &manvol;
5 </refmeta>
6
7 <refnamediv>
8 <refname>v4l2-read</refname>
9 <refpurpose>Read from a V4L2 device</refpurpose>
10 </refnamediv>
11
12 <refsynopsisdiv>
13 <funcsynopsis>
14 <funcsynopsisinfo>#include &lt;unistd.h&gt;</funcsynopsisinfo>
15 <funcprototype>
16 <funcdef>ssize_t <function>read</function></funcdef>
17 <paramdef>int <parameter>fd</parameter></paramdef>
18 <paramdef>void *<parameter>buf</parameter></paramdef>
19 <paramdef>size_t <parameter>count</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>buf</parameter></term>
36 <listitem>
37 <para></para>
38 </listitem>
39 </varlistentry>
40 <varlistentry>
41 <term><parameter>count</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><function>read()</function> attempts to read up to
53<parameter>count</parameter> bytes from file descriptor
54<parameter>fd</parameter> into the buffer starting at
55<parameter>buf</parameter>. The layout of the data in the buffer is
56discussed in the respective device interface section, see ##. If <parameter>count</parameter> is zero,
57<function>read()</function> returns zero and has no other results. If
58<parameter>count</parameter> is greater than
59<constant>SSIZE_MAX</constant>, the result is unspecified. Regardless
60of the <parameter>count</parameter> value each
61<function>read()</function> call will provide at most one frame (two
62fields) worth of data.</para>
63
64 <para>By default <function>read()</function> blocks until data
65becomes available. When the <constant>O_NONBLOCK</constant> flag was
66given to the &func-open; function it
67returns immediately with an &EAGAIN; when no data is available. The
68&func-select; or &func-poll; functions
69can always be used to suspend execution until data becomes available. All
70drivers supporting the <function>read()</function> function must also
71support <function>select()</function> and
72<function>poll()</function>.</para>
73
74 <para>Drivers can implement read functionality in different
75ways, using a single or multiple buffers and discarding the oldest or
76newest frames once the internal buffers are filled.</para>
77
78 <para><function>read()</function> never returns a "snapshot" of a
79buffer being filled. Using a single buffer the driver will stop
80capturing when the application starts reading the buffer until the
81read is finished. Thus only the period of the vertical blanking
82interval is available for reading, or the capture rate must fall below
83the nominal frame rate of the video standard.</para>
84
85<para>The behavior of
86<function>read()</function> when called during the active picture
87period or the vertical blanking separating the top and bottom field
88depends on the discarding policy. A driver discarding the oldest
89frames keeps capturing into an internal buffer, continuously
90overwriting the previously, not read frame, and returns the frame
91being received at the time of the <function>read()</function> call as
92soon as it is complete.</para>
93
94 <para>A driver discarding the newest frames stops capturing until
95the next <function>read()</function> call. The frame being received at
96<function>read()</function> time is discarded, returning the following
97frame instead. Again this implies a reduction of the capture rate to
98one half or less of the nominal frame rate. An example of this model
99is the video read mode of the bttv driver, initiating a DMA to user
100memory when <function>read()</function> is called and returning when
101the DMA finished.</para>
102
103 <para>In the multiple buffer model drivers maintain a ring of
104internal buffers, automatically advancing to the next free buffer.
105This allows continuous capturing when the application can empty the
106buffers fast enough. Again, the behavior when the driver runs out of
107free buffers depends on the discarding policy.</para>
108
109 <para>Applications can get and set the number of buffers used
110internally by the driver with the &VIDIOC-G-PARM; and &VIDIOC-S-PARM;
111ioctls. They are optional, however. The discarding policy is not
112reported and cannot be changed. For minimum requirements see <xref
113 linkend="devices" />.</para>
114 </refsect1>
115
116 <refsect1>
117 <title>Return Value</title>
118
119 <para>On success, the number of bytes read is returned. It is not
120an error if this number is smaller than the number of bytes requested,
121or the amount of data required for one frame. This may happen for
122example because <function>read()</function> was interrupted by a
123signal. On error, -1 is returned, and the <varname>errno</varname>
124variable is set appropriately. In this case the next read will start
125at the beginning of a new frame. Possible error codes are:</para>
126
127 <variablelist>
128 <varlistentry>
129 <term><errorcode>EAGAIN</errorcode></term>
130 <listitem>
131 <para>Non-blocking I/O has been selected using
132O_NONBLOCK and no data was immediately available for reading.</para>
133 </listitem>
134 </varlistentry>
135 <varlistentry>
136 <term><errorcode>EBADF</errorcode></term>
137 <listitem>
138 <para><parameter>fd</parameter> is not a valid file
139descriptor or is not open for reading, or the process already has the
140maximum number of files open.</para>
141 </listitem>
142 </varlistentry>
143 <varlistentry>
144 <term><errorcode>EBUSY</errorcode></term>
145 <listitem>
146 <para>The driver does not support multiple read streams and the
147device is already in use.</para>
148 </listitem>
149 </varlistentry>
150 <varlistentry>
151 <term><errorcode>EFAULT</errorcode></term>
152 <listitem>
153 <para><parameter>buf</parameter> references an inaccessible
154memory area.</para>
155 </listitem>
156 </varlistentry>
157 <varlistentry>
158 <term><errorcode>EINTR</errorcode></term>
159 <listitem>
160 <para>The call was interrupted by a signal before any
161data was read.</para>
162 </listitem>
163 </varlistentry>
164 <varlistentry>
165 <term><errorcode>EIO</errorcode></term>
166 <listitem>
167 <para>I/O error. This indicates some hardware problem or a
168failure to communicate with a remote device (USB camera etc.).</para>
169 </listitem>
170 </varlistentry>
171 <varlistentry>
172 <term><errorcode>EINVAL</errorcode></term>
173 <listitem>
174 <para>The <function>read()</function> function is not
175supported by this driver, not on this device, or generally not on this
176type of device.</para>
177 </listitem>
178 </varlistentry>
179 </variablelist>
180 </refsect1>
181</refentry>
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