.. 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``.
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