1 What: /dev/kmsg
2 Date: Mai 2012
3 KernelVersion: 3.5
4 Contact: Kay Sievers <kay@vrfy.org>
5 Description: The /dev/kmsg character device
6 to the kernel's printk buffer.
7
8 Injecting messages:
9
10 Every write() to the opened de
11 the kernel's printk buffer.
12
13 The logged line can be prefixe
14 carries the syslog priority an
15 prefix number is composed of t
16 priority and the next 8 bits t
17
18 If no prefix is given, the pri
19 log priority and the facility
20 is not possible to inject mess
21 facility number LOG_KERN (0),
22 the messages can always be rel
23
24 Accessing the buffer:
25
26 Every read() from the opened d
27 of the kernel's printk buffer.
28
29 The first read() directly foll
30 first message in the buffer; t
31 persistent state; many readers
32 and read from it, without affe
33
34 Every read() will receive the
35 records are available read() w
36 used -EAGAIN returned.
37
38 Messages in the record ring bu
39 there are never partial messag
40
41 In case messages get overwritt
42 the device is kept open, the n
43 and the seek position be updat
44 Subsequent reads() will return
45
46 Unlike the classic syslog() in
47 sequence numbers allow to calc
48 messages, in case the buffer g
49 to reconnect to the buffer and
50 if needed, without limiting th
51
52 The device supports seek with
53
54 SEEK_SET, 0
55 seek to the first entry in t
56 SEEK_END, 0
57 seek after the last entry in
58 SEEK_DATA, 0
59 seek after the last record a
60 the last SYSLOG_ACTION_CLEAR
61
62 Other seek operations or offse
63 the special behavior this devi
64 or write only whole variable l
65 stored in a ring buffer.
66
67 Because of the non-standard be
68 non-standard. -ESPIPE is retur
69 is returned for other operatio
70 and values are historical and
71 risk of breaking userspace.
72
73 The output format consists of
74 prefix including priority and
75 sequence number and the monoto
76 and a flag field. All fields a
77
78 Future extensions might add mo
79 the terminating ';'. Unknown f
80 gracefully ignored.
81
82 The human readable text string
83 and is terminated by a '\n'. U
84 hardware or other facilities a
85 all non-printable characters a
86 are escaped by "\x00" C-style
87
88 A line starting with ' ', is a
89 key/value pairs to the log mes
90 readable context of the messag
91 userspace.
92
93 Example::
94
95 7,160,424069,-;pci_root PNP0
96 SUBSYSTEM=acpi
97 DEVICE=+acpi:PNP0A03:00
98 6,339,5140900,-;NET: Registe
99 30,340,5690716,-;udevd[80]:
100
101 The DEVICE= key uniquely ident
102
103 ============ ==============
104 b12:8 block dev_t
105 c127:3 char dev_t
106 n8 netdev ifindex
107 +sound:card0 subsystem:devn
108 ============ ==============
109
110 The flags field carries '-' by
111 fragment of a line. Note, that
112 lines are not necessarily corr
113 interleaved with unrelated mes
114 the output usually produces be
115 similar logic is used internal
116 the console, /proc/kmsg or the
117
118 By default, kernel tries to av
119 when it can and fragments are
120 console support is enabled, th
121 disabled and /dev/kmsg output
122 the log consumer performs conc
123 should be the same. In the fut
124 may be removed entirely and /d
125 implement fragment handling.
126
127 Users: dmesg(1), userspace kernel log
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