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		The values are separated by a ','. Future extensions might
63		add more comma separated values before the terminating ';'.
64		Unknown values should be gracefully ignored.
65
66		The human readable text string starts directly after the ';'
67		and is terminated by a '\n'. Untrusted values derived from
68		hardware or other facilities are printed, therefore
69		all non-printable characters in the log message are escaped
70		by "\x00" C-style hex encoding.
71
72		A line starting with ' ', is a continuation line, adding
73		key/value pairs to the log message, which provide the machine
74		readable context of the message, for reliable processing in
75		userspace.
76
77		Example:
78		7,160,424069;pci_root PNP0A03:00: host bridge window [io  0x0000-0x0cf7] (ignored)
79		 SUBSYSTEM=acpi
80		 DEVICE=+acpi:PNP0A03:00
81		6,339,5140900;NET: Registered protocol family 10
82		30,340,5690716;udevd[80]: starting version 181
83
84		The DEVICE= key uniquely identifies devices the following way:
85		  b12:8        - block dev_t
86		  c127:3       - char dev_t
87		  n8           - netdev ifindex
88		  +sound:card0 - subsystem:devname
89
90Users:		dmesg(1), userspace kernel log consumers
91