1<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