1 .. SPDX-License-Identifier: GFDL-1.1-no-invari
2 .. c:namespace:: V4L
3
4 .. _func-poll:
5
6 ***********
7 V4L2 poll()
8 ***********
9
10 Name
11 ====
12
13 v4l2-poll - Wait for some event on a file desc
14
15 Synopsis
16 ========
17
18 .. code-block:: c
19
20 #include <sys/poll.h>
21
22 .. c:function:: int poll( struct pollfd *ufds,
23
24 Arguments
25 =========
26
27
28 Description
29 ===========
30
31 With the :c:func:`poll()` function application
32 until the driver has captured data or is ready
33 output.
34
35 When streaming I/O has been negotiated this fu
36 buffer has been filled by the capture device a
37 the :ref:`VIDIOC_DQBUF <VIDIOC_QBUF>` ioctl. F
38 function waits until the device is ready to ac
39 queued up with the :ref:`VIDIOC_QBUF <VIDIOC_Q
40 display. When buffers are already in the outgo
41 (capture) or the incoming queue isn't full (di
42 returns immediately.
43
44 On success :c:func:`poll()` returns the number
45 that have been selected (that is, file descrip
46 ``revents`` field of the respective ``struct p
47 is non-zero). Capture devices set the ``POLLIN
48 flags in the ``revents`` field, output devices
49 ``POLLWRNORM`` flags. When the function timed
50 zero, on failure it returns -1 and the ``errno
51 appropriately. When the application did not ca
52 :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` the :
53 function succeeds, but sets the ``POLLERR`` fl
54 field. When the application has called
55 :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` for a
56 hasn't yet called :ref:`VIDIOC_QBUF <VIDIOC_QB
57 :c:func:`poll()` function succeeds and sets th
58 the ``revents`` field. For output devices this
59 :c:func:`poll()` to succeed as well, but it se
60 ``POLLWRNORM`` flags in the ``revents`` field.
61
62 If an event occurred (see :ref:`VIDIOC_DQEVENT
63 then ``POLLPRI`` will be set in the ``revents`
64 :c:func:`poll()` will return.
65
66 When use of the :c:func:`read()` function has
67 driver does not capture yet, the :c:func:`poll
68 capturing. When that fails it returns a ``POLL
69 it waits until data has been captured and can
70 captures continuously (as opposed to, for exam
71 function may return immediately.
72
73 When use of the :c:func:`write()` function has
74 driver does not stream yet, the :c:func:`poll(
75 streaming. When that fails it returns a ``POLL
76 it waits until the driver is ready for a non-b
77 :c:func:`write()` call.
78
79 If the caller is only interested in events (ju
80 the ``events`` field), then :c:func:`poll()` w
81 streaming if the driver does not stream yet. T
82 just poll for events and not for buffers.
83
84 All drivers implementing the :c:func:`read()`
85 function or streaming I/O must also support th
86 function.
87
88 For more details see the :c:func:`poll()` manu
89
90 Return Value
91 ============
92
93 On success, :c:func:`poll()` returns the numbe
94 non-zero ``revents`` fields, or zero if the ca
95 is returned, and the ``errno`` variable is set
96
97 EBADF
98 One or more of the ``ufds`` members specif
99 descriptor.
100
101 EBUSY
102 The driver does not support multiple read
103 device is already in use.
104
105 EFAULT
106 ``ufds`` references an inaccessible memory
107
108 EINTR
109 The call was interrupted by a signal.
110
111 EINVAL
112 The ``nfds`` value exceeds the ``RLIMIT_NO
113 ``getrlimit()`` to obtain this value.
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