aboutsummaryrefslogtreecommitdiff
path: root/Documentation/DocBook/media/v4l/func-poll.xml
blob: ec3c718f59631fc1afe0acd412e1ebe0b9c869a6 (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
<refentry id="func-poll">
  <refmeta>
    <refentrytitle>V4L2 poll()</refentrytitle>
    &manvol;
  </refmeta>

  <refnamediv>
    <refname>v4l2-poll</refname>
    <refpurpose>Wait for some event on a file descriptor</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;sys/poll.h&gt;</funcsynopsisinfo>
      <funcprototype>
	<funcdef>int <function>poll</function></funcdef>
	<paramdef>struct pollfd *<parameter>ufds</parameter></paramdef>
	<paramdef>unsigned int <parameter>nfds</parameter></paramdef>
	<paramdef>int <parameter>timeout</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>With the <function>poll()</function> function applications
can suspend execution until the driver has captured data or is ready
to accept data for output.</para>

    <para>When streaming I/O has been negotiated this function waits
until a buffer has been filled or displayed and can be dequeued with
the &VIDIOC-DQBUF; ioctl. When buffers are already in the outgoing
queue of the driver the function returns immediately.</para>

    <para>On success <function>poll()</function> returns the number of
file descriptors that have been selected (that is, file descriptors
for which the <structfield>revents</structfield> field of the
respective <structname>pollfd</structname> structure is non-zero).
Capture devices set the <constant>POLLIN</constant> and
<constant>POLLRDNORM</constant> flags in the
<structfield>revents</structfield> field, output devices the
<constant>POLLOUT</constant> and <constant>POLLWRNORM</constant>
flags. When the function timed out it returns a value of zero, on
failure it returns <returnvalue>-1</returnvalue> and the
<varname>errno</varname> variable is set appropriately. When the
application did not call &VIDIOC-QBUF; or &VIDIOC-STREAMON; yet the
<function>poll()</function> function succeeds, but sets the
<constant>POLLERR</constant> flag in the
<structfield>revents</structfield> field.</para>

    <para>When use of the <function>read()</function> function has
been negotiated and the driver does not capture yet, the
<function>poll</function> function starts capturing. When that fails
it returns a <constant>POLLERR</constant> as above. Otherwise it waits
until data has been captured and can be read. When the driver captures
continuously (as opposed to, for example, still images) the function
may return immediately.</para>

    <para>When use of the <function>write()</function> function has
been negotiated the <function>poll</function> function just waits
until the driver is ready for a non-blocking
<function>write()</function> call.</para>

    <para>All drivers implementing the <function>read()</function> or
<function>write()</function> function or streaming I/O must also
support the <function>poll()</function> function.</para>

    <para>For more details see the
<function>poll()</function> manual page.</para>
  </refsect1>

  <refsect1>
    <title>Return Value</title>

    <para>On success, <function>poll()</function> returns the number
structures which have non-zero <structfield>revents</structfield>
fields, or zero if the call timed out. On error
<returnvalue>-1</returnvalue> is returned, and the
<varname>errno</varname> variable is set appropriately:</para>

    <variablelist>
      <varlistentry>
	<term><errorcode>EBADF</errorcode></term>
	<listitem>
	  <para>One or more of the <parameter>ufds</parameter> members
specify an invalid file descriptor.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><errorcode>EBUSY</errorcode></term>
	<listitem>
	  <para>The driver does not support multiple read or write
streams and the device is already in use.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><errorcode>EFAULT</errorcode></term>
	<listitem>
	  <para><parameter>ufds</parameter> references an inaccessible
memory area.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><errorcode>EINTR</errorcode></term>
	<listitem>
	  <para>The call was interrupted by a signal.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><errorcode>EINVAL</errorcode></term>
	<listitem>
	  <para>The <parameter>nfds</parameter> argument is greater
than <constant>OPEN_MAX</constant>.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>
</refentry>

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