What:		/dev/kmsgDate:		Mai 2012KernelVersion:	3.5Contact:	Kay Sievers <kay@vrfy.org>Description:	The /dev/kmsg character device node provides userspace access		to the kernel's printk buffer. 		Injecting messages: 		Every write() to the opened device node places a log entry in		the kernel's printk buffer. 		The logged line can be prefixed with a <N> syslog prefix, which		carries the syslog priority and facility. The single decimal		prefix number is composed of the 3 lowest bits being the syslog		priority and the next 8 bits the syslog facility number. 		If no prefix is given, the priority number is the default kernel		log priority and the facility number is set to LOG_USER (1). It		is not possible to inject messages from userspace with the		facility number LOG_KERN (0), to make sure that the origin of		the messages can always be reliably determined. 		Accessing the buffer: 		Every read() from the opened device node receives one record		of the kernel's printk buffer. 		The first read() directly following an open() always returns		first message in the buffer; there is no kernel-internal		persistent state; many readers can concurrently open the device		and read from it, without affecting other readers. 		Every read() will receive the next available record. If no more		records are available read() will block, or if O_NONBLOCK is		used -EAGAIN returned. 		Messages in the record ring buffer get overwritten as whole,		there are never partial messages received by read(). 		In case messages get overwritten in the circular buffer while		the device is kept open, the next read() will return -EPIPE,		and the seek position be updated to the next available record.		Subsequent reads() will return available records again. 		Unlike the classic syslog() interface, the 64 bit record		sequence numbers allow to calculate the amount of lost		messages, in case the buffer gets overwritten. And they allow		to reconnect to the buffer and reconstruct the read position		if needed, without limiting the interface to a single reader. 		The device supports seek with the following parameters: 		SEEK_SET, 0		  seek to the first entry in the buffer		SEEK_END, 0		  seek after the last entry in the buffer		SEEK_DATA, 0		  seek after the last record available at the time		  the last SYSLOG_ACTION_CLEAR was issued. 		Other seek operations or offsets are not supported because of		the special behavior this device has. The device allows to read		or write only whole variable length messages (records) that are		stored in a ring buffer. 		Because of the non-standard behavior also the error values are		non-standard. -ESPIPE is returned for non-zero offset. -EINVAL		is returned for other operations, e.g. SEEK_CUR. This behavior		and values are historical and could not be modified without the		risk of breaking userspace. 		The output format consists of a prefix carrying the syslog		prefix including priority and facility, the 64 bit message		sequence number and the monotonic timestamp in microseconds,		and a flag field. All fields are separated by a ','. 		Future extensions might add more comma separated values before		the terminating ';'. Unknown fields and values should be		gracefully ignored. 		The human readable text string starts directly after the ';'		and is terminated by a '\n'. Untrusted values derived from		hardware or other facilities are printed, therefore		all non-printable characters and '\' itself in the log message		are escaped by "\x00" C-style hex encoding. 		A line starting with ' ', is a continuation line, adding		key/value pairs to the log message, which provide the machine		readable context of the message, for reliable processing in		userspace. 		Example:: 		  7,160,424069,-;pci_root PNP0A03:00: host bridge window [io  0x0000-0x0cf7] (ignored)		   SUBSYSTEM=acpi		   DEVICE=+acpi:PNP0A03:00		  6,339,5140900,-;NET: Registered protocol family 10		  30,340,5690716,-;udevd[80]: starting version 181 		The DEVICE= key uniquely identifies devices the following way: 		  ============  =================		  b12:8         block dev_t		  c127:3        char dev_t		  n8            netdev ifindex		  +sound:card0  subsystem:devname		  ============  ================= 		The flags field carries '-' by default. A 'c' indicates a		fragment of a line. Note, that these hints about continuation		lines are not necessarily correct, and the stream could be		interleaved with unrelated messages, but merging the lines in		the output usually produces better human readable results. A		similar logic is used internally when messages are printed to		the console, /proc/kmsg or the syslog() syscall. 		By default, kernel tries to avoid fragments by concatenating		when it can and fragments are rare; however, when extended		console support is enabled, the in-kernel concatenation is		disabled and /dev/kmsg output will contain more fragments. If		the log consumer performs concatenation, the end result		should be the same. In the future, the in-kernel concatenation		may be removed entirely and /dev/kmsg users are recommended to		implement fragment handling. Users:		dmesg(1), userspace kernel log consumers