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138 lines
4.8 KiB
138 lines
4.8 KiB
<refentry id="func-select">
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<refmeta>
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<refentrytitle>V4L2 select()</refentrytitle>
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&manvol;
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</refmeta>
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<refnamediv>
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<refname>v4l2-select</refname>
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<refpurpose>Synchronous I/O multiplexing</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>
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#include <sys/time.h>
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#include <sys/types.h>
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#include <unistd.h></funcsynopsisinfo>
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<funcprototype>
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<funcdef>int <function>select</function></funcdef>
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<paramdef>int <parameter>nfds</parameter></paramdef>
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<paramdef>fd_set *<parameter>readfds</parameter></paramdef>
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<paramdef>fd_set *<parameter>writefds</parameter></paramdef>
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<paramdef>fd_set *<parameter>exceptfds</parameter></paramdef>
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<paramdef>struct timeval *<parameter>timeout</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>With the <function>select()</function> function applications
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can suspend execution until the driver has captured data or is ready
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to accept data for output.</para>
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<para>When streaming I/O has been negotiated this function waits
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until a buffer has been filled or displayed and can be dequeued with
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the &VIDIOC-DQBUF; ioctl. When buffers are already in the outgoing
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queue of the driver the function returns immediately.</para>
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<para>On success <function>select()</function> returns the total
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number of bits set in the <structname>fd_set</structname>s. When the
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function timed out it returns a value of zero. On failure it returns
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<returnvalue>-1</returnvalue> and the <varname>errno</varname>
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variable is set appropriately. When the application did not call
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&VIDIOC-QBUF; or &VIDIOC-STREAMON; yet the
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<function>select()</function> function succeeds, setting the bit of
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the file descriptor in <parameter>readfds</parameter> or
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<parameter>writefds</parameter>, but subsequent &VIDIOC-DQBUF; calls
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will fail.<footnote><para>The Linux kernel implements
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<function>select()</function> like the &func-poll; function, but
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<function>select()</function> cannot return a
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<constant>POLLERR</constant>.</para>
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</footnote></para>
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<para>When use of the <function>read()</function> function has
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been negotiated and the driver does not capture yet, the
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<function>select()</function> function starts capturing. When that
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fails, <function>select()</function> returns successful and a
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subsequent <function>read()</function> call, which also attempts to
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start capturing, will return an appropriate error code. When the
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driver captures continuously (as opposed to, for example, still
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images) and data is already available the
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<function>select()</function> function returns immediately.</para>
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<para>When use of the <function>write()</function> function has
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been negotiated the <function>select()</function> function just waits
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until the driver is ready for a non-blocking
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<function>write()</function> call.</para>
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<para>All drivers implementing the <function>read()</function> or
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<function>write()</function> function or streaming I/O must also
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support the <function>select()</function> function.</para>
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<para>For more details see the <function>select()</function>
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manual page.</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para>On success, <function>select()</function> returns the number
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of descriptors contained in the three returned descriptor sets, which
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will be zero if the timeout expired. On error
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<returnvalue>-1</returnvalue> is returned, and the
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<varname>errno</varname> variable is set appropriately; the sets and
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<parameter>timeout</parameter> are undefined. Possible error codes
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are:</para>
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<variablelist>
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<varlistentry>
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<term><errorcode>EBADF</errorcode></term>
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<listitem>
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<para>One or more of the file descriptor sets specified a
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file descriptor that is not open.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><errorcode>EBUSY</errorcode></term>
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<listitem>
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<para>The driver does not support multiple read or write
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streams and the device is already in use.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><errorcode>EFAULT</errorcode></term>
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<listitem>
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<para>The <parameter>readfds</parameter>,
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<parameter>writefds</parameter>, <parameter>exceptfds</parameter> or
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<parameter>timeout</parameter> pointer references an inaccessible memory
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area.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><errorcode>EINTR</errorcode></term>
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<listitem>
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<para>The call was interrupted by a signal.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><errorcode>EINVAL</errorcode></term>
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<listitem>
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<para>The <parameter>nfds</parameter> argument is less than
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zero or greater than <constant>FD_SETSIZE</constant>.</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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</refentry>
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