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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 <sys/poll.h></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 | |
28 | can suspend execution until the driver has captured data or is ready | |
29 | to accept data for output.</para> | |
30 | ||
31 | <para>When streaming I/O has been negotiated this function waits | |
32 | until a buffer has been filled or displayed and can be dequeued with | |
33 | the &VIDIOC-DQBUF; ioctl. When buffers are already in the outgoing | |
34 | queue of the driver the function returns immediately.</para> | |
35 | ||
36 | <para>On success <function>poll()</function> returns the number of | |
37 | file descriptors that have been selected (that is, file descriptors | |
38 | for which the <structfield>revents</structfield> field of the | |
39 | respective <structname>pollfd</structname> structure is non-zero). | |
40 | Capture 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> | |
44 | flags. When the function timed out it returns a value of zero, on | |
45 | failure it returns <returnvalue>-1</returnvalue> and the | |
46 | <varname>errno</varname> variable is set appropriately. When the | |
47 | application 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 | |
53 | been negotiated and the driver does not capture yet, the | |
54 | <function>poll</function> function starts capturing. When that fails | |
55 | it returns a <constant>POLLERR</constant> as above. Otherwise it waits | |
56 | until data has been captured and can be read. When the driver captures | |
57 | continuously (as opposed to, for example, still images) the function | |
58 | may return immediately.</para> | |
59 | ||
60 | <para>When use of the <function>write()</function> function has | |
61 | been negotiated the <function>poll</function> function just waits | |
62 | until 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 | |
67 | support 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 | |
77 | structures which have non-zero <structfield>revents</structfield> | |
78 | fields, 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 | |
87 | specify 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 | |
94 | streams 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 | |
101 | memory 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 | |
114 | than <constant>OPEN_MAX</constant>.</para> | |
115 | </listitem> | |
116 | </varlistentry> | |
117 | </variablelist> | |
118 | </refsect1> | |
119 | </refentry> |