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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