1.. SPDX-License-Identifier: GPL-2.0
2
3.. _GPIO_LINEINFO_CHANGED_READ:
4
5**************************
6GPIO_LINEINFO_CHANGED_READ
7**************************
8
9.. warning::
10    This ioctl is part of chardev_v1.rst and is obsoleted by
11    gpio-v2-lineinfo-changed-read.rst.
12
13Name
14====
15
16GPIO_LINEINFO_CHANGED_READ - Read line info change events for watched lines
17from the chip.
18
19Synopsis
20========
21
22``int read(int chip_fd, void *buf, size_t count)``
23
24Arguments
25=========
26
27``chip_fd``
28    The file descriptor of the GPIO character device returned by `open()`.
29
30``buf``
31    The buffer to contain the :c:type:`events<gpioline_info_changed>`.
32
33``count``
34    The number of bytes available in ``buf``, which must be at least the size
35    of a :c:type:`gpioline_info_changed` event.
36
37Description
38===========
39
40Read line info change events for watched lines from the chip.
41
42.. note::
43    Monitoring line info changes is not generally required, and would typically
44    only be performed by a system monitoring component.
45
46    These events relate to changes in a line's request state or configuration,
47    not its value. Use gpio-lineevent-data-read.rst to receive events when a
48    line changes value.
49
50A line must be watched using gpio-get-lineinfo-watch-ioctl.rst to generate
51info changed events.  Subsequently, a request, release, or reconfiguration
52of the line will generate an info changed event.
53
54The kernel timestamps events when they occur and stores them in a buffer
55from where they can be read by userspace at its convenience using `read()`.
56
57The size of the kernel event buffer is fixed at 32 events per ``chip_fd``.
58
59The buffer may overflow if bursts of events occur quicker than they are read
60by userspace. If an overflow occurs then the most recent event is discarded.
61Overflow cannot be detected from userspace.
62
63Events read from the buffer are always in the same order that they were
64detected by the kernel, including when multiple lines are being monitored by
65the one ``chip_fd``.
66
67To minimize the number of calls required to copy events from the kernel to
68userspace, `read()` supports copying multiple events. The number of events
69copied is the lower of the number available in the kernel buffer and the
70number that will fit in the userspace buffer (``buf``).
71
72A `read()` will block if no event is available and the ``chip_fd`` has not
73been set **O_NONBLOCK**.
74
75The presence of an event can be tested for by checking that the ``chip_fd`` is
76readable using `poll()` or an equivalent.
77
78First added in 5.7.
79
80Return Value
81============
82
83On success the number of bytes read, which will be a multiple of the size of
84a :c:type:`gpioline_info_changed` event.
85
86On error -1 and the ``errno`` variable is set appropriately.
87Common error codes are described in error-codes.rst.
88