1 .. SPDX-License-Identifier: GPL-2.0
2
3 .. _GPIO_LINEINFO_CHANGED_READ:
4
5 **************************
6 GPIO_LINEINFO_CHANGED_READ
7 **************************
8
9 .. warning::
10 This ioctl is part of chardev_v1.rst and i
11 gpio-v2-lineinfo-changed-read.rst.
12
13 Name
14 ====
15
16 GPIO_LINEINFO_CHANGED_READ - Read line info ch
17 from the chip.
18
19 Synopsis
20 ========
21
22 ``int read(int chip_fd, void *buf, size_t coun
23
24 Arguments
25 =========
26
27 ``chip_fd``
28 The file descriptor of the GPIO character
29
30 ``buf``
31 The buffer to contain the :c:type:`events<
32
33 ``count``
34 The number of bytes available in ``buf``,
35 of a :c:type:`gpioline_info_changed` event
36
37 Description
38 ===========
39
40 Read line info change events for watched lines
41
42 .. note::
43 Monitoring line info changes is not genera
44 only be performed by a system monitoring c
45
46 These events relate to changes in a line's
47 not its value. Use gpio-lineevent-data-rea
48 line changes value.
49
50 A line must be watched using gpio-get-lineinfo
51 info changed events. Subsequently, a request,
52 of the line will generate an info changed even
53
54 The kernel timestamps events when they occur a
55 from where they can be read by userspace at it
56
57 The size of the kernel event buffer is fixed a
58
59 The buffer may overflow if bursts of events oc
60 by userspace. If an overflow occurs then the m
61 Overflow cannot be detected from userspace.
62
63 Events read from the buffer are always in the
64 detected by the kernel, including when multipl
65 the one ``chip_fd``.
66
67 To minimize the number of calls required to co
68 userspace, `read()` supports copying multiple
69 copied is the lower of the number available in
70 number that will fit in the userspace buffer (
71
72 A `read()` will block if no event is available
73 been set **O_NONBLOCK**.
74
75 The presence of an event can be tested for by
76 readable using `poll()` or an equivalent.
77
78 First added in 5.7.
79
80 Return Value
81 ============
82
83 On success the number of bytes read, which wil
84 a :c:type:`gpioline_info_changed` event.
85
86 On error -1 and the ``errno`` variable is set
87 Common error codes are described in error-code
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