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Diffstat (limited to 'Documentation/DocBook/media/v4l/func-select.xml')
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diff --git a/Documentation/DocBook/media/v4l/func-select.xml b/Documentation/DocBook/media/v4l/func-select.xml new file mode 100644 index 00000000..e12a60d9 --- /dev/null +++ b/Documentation/DocBook/media/v4l/func-select.xml @@ -0,0 +1,130 @@ +<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> |