Date: Mai 2012
Contact: Kay Sievers <email@example.com>
Description: The /dev/kmsg character device node provides userspace access
to the kernel's printk buffer.
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,
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
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
7,160,424069,-;pci_root PNP0A03:00: host bridge window [io 0x0000-0x0cf7] (ignored)
6,339,5140900,-;NET: Registered protocol family 10
30,340,5690716,-;udevd: 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. All following fragments are flagged with
'+'. Note, that these hints about continuation lines are not
neccessarily 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.
Users: dmesg(1), userspace kernel log consumers