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 by the capture device and can be dequeued
33with the &VIDIOC-DQBUF; ioctl. For output devices this function waits
34until the device is ready to accept a new buffer to be queued up with
35the &VIDIOC-QBUF; ioctl for display. When buffers are already in the outgoing
36queue of the driver (capture) or the incoming queue isn't full (display)
37the function returns immediately.</para>
38
39    <para>On success <function>poll()</function> returns the number of
40file descriptors that have been selected (that is, file descriptors
41for which the <structfield>revents</structfield> field of the
42respective <structname>pollfd</structname> structure is non-zero).
43Capture devices set the <constant>POLLIN</constant> and
44<constant>POLLRDNORM</constant> flags in the
45<structfield>revents</structfield> field, output devices the
46<constant>POLLOUT</constant> and <constant>POLLWRNORM</constant>
47flags. When the function timed out it returns a value of zero, on
48failure it returns <returnvalue>-1</returnvalue> and the
49<varname>errno</varname> variable is set appropriately. When the
50application did not call &VIDIOC-STREAMON; the
51<function>poll()</function> function succeeds, but sets the
52<constant>POLLERR</constant> flag in the
53<structfield>revents</structfield> field. When the
54application has called &VIDIOC-STREAMON; for a capture device but hasn't
55yet called &VIDIOC-QBUF;, the <function>poll()</function> function
56succeeds and sets the <constant>POLLERR</constant> flag in the
57<structfield>revents</structfield> field. For output devices this
58same situation will cause <function>poll()</function> to succeed
59as well, but it sets the <constant>POLLOUT</constant> and
60<constant>POLLWRNORM</constant> flags in the <structfield>revents</structfield>
61field.</para>
62
63    <para>If an event occurred (see &VIDIOC-DQEVENT;) then
64<constant>POLLPRI</constant> will be set in the <structfield>revents</structfield>
65field and <function>poll()</function> will return.</para>
66
67    <para>When use of the <function>read()</function> function has
68been negotiated and the driver does not capture yet, the
69<function>poll</function> function starts capturing. When that fails
70it returns a <constant>POLLERR</constant> as above. Otherwise it waits
71until data has been captured and can be read. When the driver captures
72continuously (as opposed to, for example, still images) the function
73may return immediately.</para>
74
75    <para>When use of the <function>write()</function> function has
76been negotiated and the driver does not stream yet, the
77<function>poll</function> function starts streaming. When that fails
78it returns a <constant>POLLERR</constant> as above. Otherwise it waits
79until the driver is ready for a non-blocking
80<function>write()</function> call.</para>
81
82    <para>If the caller is only interested in events (just
83<constant>POLLPRI</constant> is set in the <structfield>events</structfield>
84field), then <function>poll()</function> will <emphasis>not</emphasis>
85start streaming if the driver does not stream yet. This makes it
86possible to just poll for events and not for buffers.</para>
87
88    <para>All drivers implementing the <function>read()</function> or
89<function>write()</function> function or streaming I/O must also
90support the <function>poll()</function> function.</para>
91
92    <para>For more details see the
93<function>poll()</function> manual page.</para>
94  </refsect1>
95
96  <refsect1>
97    <title>Return Value</title>
98
99    <para>On success, <function>poll()</function> returns the number
100structures which have non-zero <structfield>revents</structfield>
101fields, or zero if the call timed out. On error
102<returnvalue>-1</returnvalue> is returned, and the
103<varname>errno</varname> variable is set appropriately:</para>
104
105    <variablelist>
106      <varlistentry>
107	<term><errorcode>EBADF</errorcode></term>
108	<listitem>
109	  <para>One or more of the <parameter>ufds</parameter> members
110specify an invalid file descriptor.</para>
111	</listitem>
112      </varlistentry>
113      <varlistentry>
114	<term><errorcode>EBUSY</errorcode></term>
115	<listitem>
116	  <para>The driver does not support multiple read or write
117streams and the device is already in use.</para>
118	</listitem>
119      </varlistentry>
120      <varlistentry>
121	<term><errorcode>EFAULT</errorcode></term>
122	<listitem>
123	  <para><parameter>ufds</parameter> references an inaccessible
124memory area.</para>
125	</listitem>
126      </varlistentry>
127      <varlistentry>
128	<term><errorcode>EINTR</errorcode></term>
129	<listitem>
130	  <para>The call was interrupted by a signal.</para>
131	</listitem>
132      </varlistentry>
133      <varlistentry>
134	<term><errorcode>EINVAL</errorcode></term>
135	<listitem>
136	  <para>The <parameter>nfds</parameter> argument is greater
137than <constant>OPEN_MAX</constant>.</para>
138	</listitem>
139      </varlistentry>
140    </variablelist>
141  </refsect1>
142</refentry>
143