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Mauro Carvalho Chehab8e080c22009-09-13 22:16:04 -03001<refentry id="func-poll">
2 <refmeta>
3 <refentrytitle>V4L2 poll()</refentrytitle>
4 &manvol;
5 </refmeta>
6
7 <refnamediv>
8 <refname>v4l2-poll</refname>
9 <refpurpose>Wait for some event on a file descriptor</refpurpose>
10 </refnamediv>
11
12 <refsynopsisdiv>
13 <funcsynopsis>
14 <funcsynopsisinfo>#include &lt;sys/poll.h&gt;</funcsynopsisinfo>
15 <funcprototype>
16 <funcdef>int <function>poll</function></funcdef>
17 <paramdef>struct pollfd *<parameter>ufds</parameter></paramdef>
18 <paramdef>unsigned int <parameter>nfds</parameter></paramdef>
19 <paramdef>int <parameter>timeout</parameter></paramdef>
20 </funcprototype>
21 </funcsynopsis>
22 </refsynopsisdiv>
23
24 <refsect1>
25 <title>Description</title>
26
27 <para>With the <function>poll()</function> function applications
28can suspend execution until the driver has captured data or is ready
29to accept data for output.</para>
30
31 <para>When streaming I/O has been negotiated this function waits
32until a buffer has been filled or displayed and can be dequeued with
33the &VIDIOC-DQBUF; ioctl. When buffers are already in the outgoing
34queue of the driver the function returns immediately.</para>
35
36 <para>On success <function>poll()</function> returns the number of
37file descriptors that have been selected (that is, file descriptors
38for which the <structfield>revents</structfield> field of the
39respective <structname>pollfd</structname> structure is non-zero).
40Capture devices set the <constant>POLLIN</constant> and
41<constant>POLLRDNORM</constant> flags in the
42<structfield>revents</structfield> field, output devices the
43<constant>POLLOUT</constant> and <constant>POLLWRNORM</constant>
44flags. When the function timed out it returns a value of zero, on
45failure it returns <returnvalue>-1</returnvalue> and the
46<varname>errno</varname> variable is set appropriately. When the
47application did not call &VIDIOC-QBUF; or &VIDIOC-STREAMON; yet the
48<function>poll()</function> function succeeds, but sets the
49<constant>POLLERR</constant> flag in the
50<structfield>revents</structfield> field.</para>
51
52 <para>When use of the <function>read()</function> function has
53been negotiated and the driver does not capture yet, the
54<function>poll</function> function starts capturing. When that fails
55it returns a <constant>POLLERR</constant> as above. Otherwise it waits
56until data has been captured and can be read. When the driver captures
57continuously (as opposed to, for example, still images) the function
58may return immediately.</para>
59
60 <para>When use of the <function>write()</function> function has
61been negotiated the <function>poll</function> function just waits
62until the driver is ready for a non-blocking
63<function>write()</function> call.</para>
64
65 <para>All drivers implementing the <function>read()</function> or
66<function>write()</function> function or streaming I/O must also
67support the <function>poll()</function> function.</para>
68
69 <para>For more details see the
70<function>poll()</function> manual page.</para>
71 </refsect1>
72
73 <refsect1>
74 <title>Return Value</title>
75
76 <para>On success, <function>poll()</function> returns the number
77structures which have non-zero <structfield>revents</structfield>
78fields, or zero if the call timed out. On error
79<returnvalue>-1</returnvalue> is returned, and the
80<varname>errno</varname> variable is set appropriately:</para>
81
82 <variablelist>
83 <varlistentry>
84 <term><errorcode>EBADF</errorcode></term>
85 <listitem>
86 <para>One or more of the <parameter>ufds</parameter> members
87specify an invalid file descriptor.</para>
88 </listitem>
89 </varlistentry>
90 <varlistentry>
91 <term><errorcode>EBUSY</errorcode></term>
92 <listitem>
93 <para>The driver does not support multiple read or write
94streams and the device is already in use.</para>
95 </listitem>
96 </varlistentry>
97 <varlistentry>
98 <term><errorcode>EFAULT</errorcode></term>
99 <listitem>
100 <para><parameter>ufds</parameter> references an inaccessible
101memory area.</para>
102 </listitem>
103 </varlistentry>
104 <varlistentry>
105 <term><errorcode>EINTR</errorcode></term>
106 <listitem>
107 <para>The call was interrupted by a signal.</para>
108 </listitem>
109 </varlistentry>
110 <varlistentry>
111 <term><errorcode>EINVAL</errorcode></term>
112 <listitem>
113 <para>The <parameter>nfds</parameter> argument is greater
114than <constant>OPEN_MAX</constant>.</para>
115 </listitem>
116 </varlistentry>
117 </variablelist>
118 </refsect1>
119</refentry>
120
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