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@@ -0,0 +1,90 @@
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+What: /dev/kmsg
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+Date: Mai 2012
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+KernelVersion: 3.5
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+Contact: Kay Sievers <kay@vrfy.org>
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+Description: The /dev/kmsg character device node provides userspace access
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+ to the kernel's printk buffer.
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+
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+ Injecting messages:
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+ Every write() to the opened device node places a log entry in
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+ the kernel's printk buffer.
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+
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+ The logged line can be prefixed with a <N> syslog prefix, which
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+ carries the syslog priority and facility. The single decimal
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+ prefix number is composed of the 3 lowest bits being the syslog
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+ priority and the higher bits the syslog facility number.
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+
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+ If no prefix is given, the priority number is the default kernel
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+ log priority and the facility number is set to LOG_USER (1). It
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+ is not possible to inject messages from userspace with the
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+ facility number LOG_KERN (0), to make sure that the origin of
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+ the messages can always be reliably determined.
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+
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+ Accessing the buffer:
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+ Every read() from the opened device node receives one record
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+ of the kernel's printk buffer.
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+
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+ The first read() directly following an open() always returns
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+ first message in the buffer; there is no kernel-internal
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+ persistent state; many readers can concurrently open the device
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+ and read from it, without affecting other readers.
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+
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+ Every read() will receive the next available record. If no more
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+ records are available read() will block, or if O_NONBLOCK is
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+ used -EAGAIN returned.
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+
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+ Messages in the record ring buffer get overwritten as whole,
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+ there are never partial messages received by read().
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+
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+ In case messages get overwritten in the circular buffer while
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+ the device is kept open, the next read() will return -EPIPE,
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+ and the seek position be updated to the next available record.
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+ Subsequent reads() will return available records again.
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+
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+ Unlike the classic syslog() interface, the 64 bit record
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+ sequence numbers allow to calculate the amount of lost
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+ messages, in case the buffer gets overwritten. And they allow
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+ to reconnect to the buffer and reconstruct the read position
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+ if needed, without limiting the interface to a single reader.
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+
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+ The device supports seek with the following parameters:
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+ SEEK_SET, 0
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+ seek to the first entry in the buffer
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+ SEEK_END, 0
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+ seek after the last entry in the buffer
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+ SEEK_DATA, 0
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+ seek after the last record available at the time
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+ the last SYSLOG_ACTION_CLEAR was issued.
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+
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+ The output format consists of a prefix carrying the syslog
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+ prefix including priority and facility, the 64 bit message
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+ sequence number and the monotonic timestamp in microseconds.
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+ The values are separated by a ','. Future extensions might
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+ add more comma separated values before the terminating ';'.
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+ Unknown values should be gracefully ignored.
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+
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+ The human readable text string starts directly after the ';'
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+ and is terminated by a '\n'. Untrusted values derived from
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+ hardware or other facilities are printed, therefore
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+ all non-printable characters in the log message are escaped
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+ by "\x00" C-style hex encoding.
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+
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+ A line starting with ' ', is a continuation line, adding
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+ key/value pairs to the log message, which provide the machine
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+ readable context of the message, for reliable processing in
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+ userspace.
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+
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+ Example:
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+ 7,160,424069;pci_root PNP0A03:00: host bridge window [io 0x0000-0x0cf7] (ignored)
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+ SUBSYSTEM=acpi
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+ DEVICE=+acpi:PNP0A03:00
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+ 6,339,5140900;NET: Registered protocol family 10
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+ 30,340,5690716;udevd[80]: starting version 181
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+
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+ The DEVICE= key uniquely identifies devices the following way:
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+ b12:8 - block dev_t
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+ c127:3 - char dev_t
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+ n8 - netdev ifindex
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+ +sound:card0 - subsystem:devname
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+
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+Users: dmesg(1), userspace kernel log consumers
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Kay Sievers