1 .. -*- coding: utf-8; mode: rst -*-
12 v4l2-select - Synchronous I/O multiplexing
21 #include <sys/types.h>
25 .. cpp:function:: int select( int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout )
36 With the :ref:`select() <func-select>` function applications can suspend
37 execution until the driver has captured data or is ready to accept data
40 When streaming I/O has been negotiated this function waits until a
41 buffer has been filled or displayed and can be dequeued with the
42 :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. When buffers are already in
43 the outgoing queue of the driver the function returns immediately.
45 On success :ref:`select() <func-select>` returns the total number of bits set in
46 :c:func:`struct fd_set`. When the function timed out it returns
47 a value of zero. On failure it returns -1 and the ``errno`` variable is
48 set appropriately. When the application did not call
50 :ref:`VIDIOC_STREAMON` yet the :ref:`select() <func-select>`
51 function succeeds, setting the bit of the file descriptor in ``readfds``
52 or ``writefds``, but subsequent :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>`
53 calls will fail. [#f1]_
55 When use of the :ref:`read() <func-read>` function has been negotiated and the
56 driver does not capture yet, the :ref:`select() <func-select>` function starts
57 capturing. When that fails, :ref:`select() <func-select>` returns successful and
58 a subsequent :ref:`read() <func-read>` call, which also attempts to start
59 capturing, will return an appropriate error code. When the driver
60 captures continuously (as opposed to, for example, still images) and
61 data is already available the :ref:`select() <func-select>` function returns
64 When use of the :ref:`write() <func-write>` function has been negotiated the
65 :ref:`select() <func-select>` function just waits until the driver is ready for a
66 non-blocking :ref:`write() <func-write>` call.
68 All drivers implementing the :ref:`read() <func-read>` or :ref:`write() <func-write>`
69 function or streaming I/O must also support the :ref:`select() <func-select>`
72 For more details see the :ref:`select() <func-select>` manual page.
78 On success, :ref:`select() <func-select>` returns the number of descriptors
79 contained in the three returned descriptor sets, which will be zero if
80 the timeout expired. On error -1 is returned, and the ``errno`` variable
81 is set appropriately; the sets and ``timeout`` are undefined. Possible
85 One or more of the file descriptor sets specified a file descriptor
89 The driver does not support multiple read or write streams and the
90 device is already in use.
93 The ``readfds``, ``writefds``, ``exceptfds`` or ``timeout`` pointer
94 references an inaccessible memory area.
97 The call was interrupted by a signal.
100 The ``nfds`` argument is less than zero or greater than
104 The Linux kernel implements :ref:`select() <func-select>` like the
105 :ref:`poll() <func-poll>` function, but :ref:`select() <func-select>` cannot
106 return a ``POLLERR``.