1<refentry id="func-select"> 2 <refmeta> 3 <refentrytitle>V4L2 select()</refentrytitle> 4 &manvol; 5 </refmeta> 6 7 <refnamediv> 8 <refname>v4l2-select</refname> 9 <refpurpose>Synchronous I/O multiplexing</refpurpose> 10 </refnamediv> 11 12 <refsynopsisdiv> 13 <funcsynopsis> 14 <funcsynopsisinfo> 15#include <sys/time.h> 16#include <sys/types.h> 17#include <unistd.h></funcsynopsisinfo> 18 <funcprototype> 19 <funcdef>int <function>select</function></funcdef> 20 <paramdef>int <parameter>nfds</parameter></paramdef> 21 <paramdef>fd_set *<parameter>readfds</parameter></paramdef> 22 <paramdef>fd_set *<parameter>writefds</parameter></paramdef> 23 <paramdef>fd_set *<parameter>exceptfds</parameter></paramdef> 24 <paramdef>struct timeval *<parameter>timeout</parameter></paramdef> 25 </funcprototype> 26 </funcsynopsis> 27 </refsynopsisdiv> 28 29 <refsect1> 30 <title>Description</title> 31 32 <para>With the <function>select()</function> function applications 33can suspend execution until the driver has captured data or is ready 34to accept data for output.</para> 35 36 <para>When streaming I/O has been negotiated this function waits 37until a buffer has been filled or displayed and can be dequeued with 38the &VIDIOC-DQBUF; ioctl. When buffers are already in the outgoing 39queue of the driver the function returns immediately.</para> 40 41 <para>On success <function>select()</function> returns the total 42number of bits set in the <structname>fd_set</structname>s. When the 43function timed out it returns a value of zero. On failure it returns 44<returnvalue>-1</returnvalue> and the <varname>errno</varname> 45variable is set appropriately. When the application did not call 46&VIDIOC-QBUF; or &VIDIOC-STREAMON; yet the 47<function>select()</function> function succeeds, setting the bit of 48the file descriptor in <parameter>readfds</parameter> or 49<parameter>writefds</parameter>, but subsequent &VIDIOC-DQBUF; calls 50will fail.<footnote><para>The Linux kernel implements 51<function>select()</function> like the &func-poll; function, but 52<function>select()</function> cannot return a 53<constant>POLLERR</constant>.</para> 54 </footnote></para> 55 56 <para>When use of the <function>read()</function> function has 57been negotiated and the driver does not capture yet, the 58<function>select()</function> function starts capturing. When that 59fails, <function>select()</function> returns successful and a 60subsequent <function>read()</function> call, which also attempts to 61start capturing, will return an appropriate error code. When the 62driver captures continuously (as opposed to, for example, still 63images) and data is already available the 64<function>select()</function> function returns immediately.</para> 65 66 <para>When use of the <function>write()</function> function has 67been negotiated the <function>select()</function> function just waits 68until the driver is ready for a non-blocking 69<function>write()</function> call.</para> 70 71 <para>All drivers implementing the <function>read()</function> or 72<function>write()</function> function or streaming I/O must also 73support the <function>select()</function> function.</para> 74 75 <para>For more details see the <function>select()</function> 76manual page.</para> 77 78 </refsect1> 79 80 <refsect1> 81 <title>Return Value</title> 82 83 <para>On success, <function>select()</function> returns the number 84of descriptors contained in the three returned descriptor sets, which 85will be zero if the timeout expired. On error 86<returnvalue>-1</returnvalue> is returned, and the 87<varname>errno</varname> variable is set appropriately; the sets and 88<parameter>timeout</parameter> are undefined. Possible error codes 89are:</para> 90 91 <variablelist> 92 <varlistentry> 93 <term><errorcode>EBADF</errorcode></term> 94 <listitem> 95 <para>One or more of the file descriptor sets specified a 96file descriptor that is not open.</para> 97 </listitem> 98 </varlistentry> 99 <varlistentry> 100 <term><errorcode>EBUSY</errorcode></term> 101 <listitem> 102 <para>The driver does not support multiple read or write 103streams and the device is already in use.</para> 104 </listitem> 105 </varlistentry> 106 <varlistentry> 107 <term><errorcode>EFAULT</errorcode></term> 108 <listitem> 109 <para>The <parameter>readfds</parameter>, 110<parameter>writefds</parameter>, <parameter>exceptfds</parameter> or 111<parameter>timeout</parameter> pointer references an inaccessible memory 112area.</para> 113 </listitem> 114 </varlistentry> 115 <varlistentry> 116 <term><errorcode>EINTR</errorcode></term> 117 <listitem> 118 <para>The call was interrupted by a signal.</para> 119 </listitem> 120 </varlistentry> 121 <varlistentry> 122 <term><errorcode>EINVAL</errorcode></term> 123 <listitem> 124 <para>The <parameter>nfds</parameter> argument is less than 125zero or greater than <constant>FD_SETSIZE</constant>.</para> 126 </listitem> 127 </varlistentry> 128 </variablelist> 129 </refsect1> 130</refentry> 131 132<!-- 133Local Variables: 134mode: sgml 135sgml-parent-document: "v4l2.sgml" 136indent-tabs-mode: nil 137End: 138--> 139