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+What: /dev/kmsg
+Date: Mai 2012
+KernelVersion: 3.5
+Contact: Kay Sievers <>
+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 higher 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 to the first entry in the buffer
+ seek after the last entry in the buffer
+ seek after the last record available at the time
+ the last SYSLOG_ACTION_CLEAR was issued.
+ 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.
+ The values are separated by a ','. Future extensions might
+ add more comma separated values before the terminating ';'.
+ Unknown 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 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)
+ 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
+Users: dmesg(1), userspace kernel log consumers