xref: /kernel/linux/linux-5.10/Documentation/userspace-api/media/v4l/func-select.rst

.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
.. c:namespace:: V4L

.. _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 )

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 :c: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 :c:func:`select()` returns the total number of bits set in
``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 :c: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 :c:func:`read()` function has been negotiated and the
driver does not capture yet, the :c:func:`select()` function starts
capturing. When that fails, :c:func:`select()` returns successful and
a subsequent :c: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 :c:func:`select()` function returns
immediately.

When use of the :c:func:`write()` function has been negotiated the
:c:func:`select()` function just waits until the driver is ready for a
non-blocking :c:func:`write()` call.

All drivers implementing the :c:func:`read()` or :c:func:`write()`
function or streaming I/O must also support the :c:func:`select()`
function.

For more details see the :c:func:`select()` manual page.

Return Value
============

On success, :c: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 :c:func:`select()` like the
   :c:func:`poll()` function, but :c:func:`select()` cannot
   return a ``POLLERR``.