1What:		/dev/kmsg
2Date:		Mai 2012
3KernelVersion:	3.5
4Contact:	Kay Sievers <kay@vrfy.org>
5Description:	The /dev/kmsg character device node provides userspace access
6		to the kernel's printk buffer.
7
8		Injecting messages:
9		Every write() to the opened device node places a log entry in
10		the kernel's printk buffer.
11
12		The logged line can be prefixed with a <N> syslog prefix, which
13		carries the syslog priority and facility. The single decimal
14		prefix number is composed of the 3 lowest bits being the syslog
15		priority and the higher bits the syslog facility number.
16
17		If no prefix is given, the priority number is the default kernel
18		log priority and the facility number is set to LOG_USER (1). It
19		is not possible to inject messages from userspace with the
20		facility number LOG_KERN (0), to make sure that the origin of
21		the messages can always be reliably determined.
22
23		Accessing the buffer:
24		Every read() from the opened device node receives one record
25		of the kernel's printk buffer.
26
27		The first read() directly following an open() always returns
28		first message in the buffer; there is no kernel-internal
29		persistent state; many readers can concurrently open the device
30		and read from it, without affecting other readers.
31
32		Every read() will receive the next available record. If no more
33		records are available read() will block, or if O_NONBLOCK is
34		used -EAGAIN returned.
35
36		Messages in the record ring buffer get overwritten as whole,
37		there are never partial messages received by read().
38
39		In case messages get overwritten in the circular buffer while
40		the device is kept open, the next read() will return -EPIPE,
41		and the seek position be updated to the next available record.
42		Subsequent reads() will return available records again.
43
44		Unlike the classic syslog() interface, the 64 bit record
45		sequence numbers allow to calculate the amount of lost
46		messages, in case the buffer gets overwritten. And they allow
47		to reconnect to the buffer and reconstruct the read position
48		if needed, without limiting the interface to a single reader.
49
50		The device supports seek with the following parameters:
51		SEEK_SET, 0
52		  seek to the first entry in the buffer
53		SEEK_END, 0
54		  seek after the last entry in the buffer
55		SEEK_DATA, 0
56		  seek after the last record available at the time
57		  the last SYSLOG_ACTION_CLEAR was issued.
58
59		The output format consists of a prefix carrying the syslog
60		prefix including priority and facility, the 64 bit message
61		sequence number and the monotonic timestamp in microseconds,
62		and a flag field. All fields are separated by a ','.
63
64		Future extensions might add more comma separated values before
65		the terminating ';'. Unknown fields and values should be
66		gracefully ignored.
67
68		The human readable text string starts directly after the ';'
69		and is terminated by a '\n'. Untrusted values derived from
70		hardware or other facilities are printed, therefore
71		all non-printable characters and '\' itself in the log message
72		are escaped by "\x00" C-style hex encoding.
73
74		A line starting with ' ', is a continuation line, adding
75		key/value pairs to the log message, which provide the machine
76		readable context of the message, for reliable processing in
77		userspace.
78
79		Example:
80		7,160,424069,-;pci_root PNP0A03:00: host bridge window [io  0x0000-0x0cf7] (ignored)
81		 SUBSYSTEM=acpi
82		 DEVICE=+acpi:PNP0A03:00
83		6,339,5140900,-;NET: Registered protocol family 10
84		30,340,5690716,-;udevd[80]: starting version 181
85
86		The DEVICE= key uniquely identifies devices the following way:
87		  b12:8        - block dev_t
88		  c127:3       - char dev_t
89		  n8           - netdev ifindex
90		  +sound:card0 - subsystem:devname
91
92		The flags field carries '-' by default. A 'c' indicates a
93		fragment of a line. All following fragments are flagged with
94		'+'. Note, that these hints about continuation lines are not
95		necessarily correct, and the stream could be interleaved with
96		unrelated messages, but merging the lines in the output
97		usually produces better human readable results. A similar
98		logic is used internally when messages are printed to the
99		console, /proc/kmsg or the syslog() syscall.
100
101		By default, kernel tries to avoid fragments by concatenating
102		when it can and fragments are rare; however, when extended
103		console support is enabled, the in-kernel concatenation is
104		disabled and /dev/kmsg output will contain more fragments. If
105		the log consumer performs concatenation, the end result
106		should be the same. In the future, the in-kernel concatenation
107		may be removed entirely and /dev/kmsg users are recommended to
108		implement fragment handling.
109
110Users:		dmesg(1), userspace kernel log consumers
111