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
101Users:		dmesg(1), userspace kernel log consumers