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
128
129
130
|
<refentry id="func-select">
<refmeta>
<refentrytitle>V4L2 select()</refentrytitle>
&manvol;
</refmeta>
<refnamediv>
<refname>v4l2-select</refname>
<refpurpose>Synchronous I/O multiplexing</refpurpose>
</refnamediv>
<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h></funcsynopsisinfo>
<funcprototype>
<funcdef>int <function>select</function></funcdef>
<paramdef>int <parameter>nfds</parameter></paramdef>
<paramdef>fd_set *<parameter>readfds</parameter></paramdef>
<paramdef>fd_set *<parameter>writefds</parameter></paramdef>
<paramdef>fd_set *<parameter>exceptfds</parameter></paramdef>
<paramdef>struct timeval *<parameter>timeout</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>With the <function>select()</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>select()</function> returns the total
number of bits set in the <structname>fd_set</structname>s. 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>select()</function> function succeeds, setting the bit of
the file descriptor in <parameter>readfds</parameter> or
<parameter>writefds</parameter>, but subsequent &VIDIOC-DQBUF; calls
will fail.<footnote><para>The Linux kernel implements
<function>select()</function> like the &func-poll; function, but
<function>select()</function> cannot return a
<constant>POLLERR</constant>.</para>
</footnote></para>
<para>When use of the <function>read()</function> function has
been negotiated and the driver does not capture yet, the
<function>select()</function> function starts capturing. When that
fails, <function>select()</function> returns successful and a
subsequent <function>read()</function> call, which also attempts to
start capturing, will return an appropriate error code. When the
driver captures continuously (as opposed to, for example, still
images) and data is already available the
<function>select()</function> function returns immediately.</para>
<para>When use of the <function>write()</function> function has
been negotiated the <function>select()</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>select()</function> function.</para>
<para>For more details see the <function>select()</function>
manual page.</para>
</refsect1>
<refsect1>
<title>Return Value</title>
<para>On success, <function>select()</function> returns the number
of descriptors contained in the three returned descriptor sets, which
will be zero if the timeout expired. On error
<returnvalue>-1</returnvalue> is returned, and the
<varname>errno</varname> variable is set appropriately; the sets and
<parameter>timeout</parameter> are undefined. Possible error codes
are:</para>
<variablelist>
<varlistentry>
<term><errorcode>EBADF</errorcode></term>
<listitem>
<para>One or more of the file descriptor sets specified a
file descriptor that is not open.</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>The <parameter>readfds</parameter>,
<parameter>writefds</parameter>, <parameter>exceptfds</parameter> or
<parameter>timeout</parameter> pointer 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 less than
zero or greater than <constant>FD_SETSIZE</constant>.</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
</refentry>
|
