117 lines
3.4 KiB
ReStructuredText
117 lines
3.4 KiB
ReStructuredText
|
.. 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``.
|