v4.19.13 snapshot.
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+.. -*- coding: utf-8; mode: rst -*-
+
+.. _func-select:
+
+*************
+V4L2 select()
+*************
+
+Name
+====
+
+v4l2-select - Synchronous I/O multiplexing
+
+
+Synopsis
+========
+
+.. code-block:: c
+
+ #include <sys/time.h>
+ #include <sys/types.h>
+ #include <unistd.h>
+
+
+.. c:function:: int select( int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout )
+ :name: v4l2-select
+
+Arguments
+=========
+
+``nfds``
+ The highest-numbered file descriptor in any of the three sets, plus 1.
+
+``readfds``
+ File descriptions to be watched if a read() call won't block.
+
+``writefds``
+ File descriptions to be watched if a write() won't block.
+
+``exceptfds``
+ File descriptions to be watched for V4L2 events.
+
+``timeout``
+ Maximum time to wait.
+
+
+Description
+===========
+
+With the :ref:`select() <func-select>` function applications can suspend
+execution until the driver has captured data or is ready to accept data
+for output.
+
+When streaming I/O has been negotiated this function waits until a
+buffer has been filled or displayed and can be dequeued with the
+:ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. When buffers are already in
+the outgoing queue of the driver the function returns immediately.
+
+On success :ref:`select() <func-select>` returns the total number of bits set in
+:c:func:`struct fd_set`. When the function timed out it returns
+a value of zero. On failure it returns -1 and the ``errno`` variable is
+set appropriately. When the application did not call
+:ref:`VIDIOC_QBUF` or
+:ref:`VIDIOC_STREAMON` yet the :ref:`select() <func-select>`
+function succeeds, setting the bit of the file descriptor in ``readfds``
+or ``writefds``, but subsequent :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`
+calls will fail. [#f1]_
+
+When use of the :ref:`read() <func-read>` function has been negotiated and the
+driver does not capture yet, the :ref:`select() <func-select>` function starts
+capturing. When that fails, :ref:`select() <func-select>` returns successful and
+a subsequent :ref:`read() <func-read>` 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 :ref:`select() <func-select>` function returns
+immediately.
+
+When use of the :ref:`write() <func-write>` function has been negotiated the
+:ref:`select() <func-select>` function just waits until the driver is ready for a
+non-blocking :ref:`write() <func-write>` call.
+
+All drivers implementing the :ref:`read() <func-read>` or :ref:`write() <func-write>`
+function or streaming I/O must also support the :ref:`select() <func-select>`
+function.
+
+For more details see the :ref:`select() <func-select>` manual page.
+
+
+Return Value
+============
+
+On success, :ref:`select() <func-select>` returns the number of descriptors
+contained in the three returned descriptor sets, which will be zero if
+the timeout expired. On error -1 is returned, and the ``errno`` variable
+is set appropriately; the sets and ``timeout`` are undefined. Possible
+error codes are:
+
+EBADF
+ One or more of the file descriptor sets specified a file descriptor
+ that is not open.
+
+EBUSY
+ The driver does not support multiple read or write streams and the
+ device is already in use.
+
+EFAULT
+ The ``readfds``, ``writefds``, ``exceptfds`` or ``timeout`` pointer
+ references an inaccessible memory area.
+
+EINTR
+ The call was interrupted by a signal.
+
+EINVAL
+ The ``nfds`` argument is less than zero or greater than
+ ``FD_SETSIZE``.
+
+.. [#f1]
+ The Linux kernel implements :ref:`select() <func-select>` like the
+ :ref:`poll() <func-poll>` function, but :ref:`select() <func-select>` cannot
+ return a ``POLLERR``.