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Kay Sievers3b552b92012-05-08 18:50:50 +02001What: /dev/kmsg
2Date: Mai 2012
3KernelVersion: 3.5
4Contact: Kay Sievers <kay@vrfy.org>
5Description: The /dev/kmsg character device node provides userspace access
6 to the kernel's printk buffer.
7
8 Injecting messages:
9 Every write() to the opened device node places a log entry in
10 the kernel's printk buffer.
11
12 The logged line can be prefixed with a <N> syslog prefix, which
13 carries the syslog priority and facility. The single decimal
14 prefix number is composed of the 3 lowest bits being the syslog
15 priority and the higher bits the syslog facility number.
16
17 If no prefix is given, the priority number is the default kernel
18 log priority and the facility number is set to LOG_USER (1). It
19 is not possible to inject messages from userspace with the
20 facility number LOG_KERN (0), to make sure that the origin of
21 the messages can always be reliably determined.
22
23 Accessing the buffer:
24 Every read() from the opened device node receives one record
25 of the kernel's printk buffer.
26
27 The first read() directly following an open() always returns
28 first message in the buffer; there is no kernel-internal
29 persistent state; many readers can concurrently open the device
30 and read from it, without affecting other readers.
31
32 Every read() will receive the next available record. If no more
33 records are available read() will block, or if O_NONBLOCK is
34 used -EAGAIN returned.
35
36 Messages in the record ring buffer get overwritten as whole,
37 there are never partial messages received by read().
38
39 In case messages get overwritten in the circular buffer while
40 the device is kept open, the next read() will return -EPIPE,
41 and the seek position be updated to the next available record.
42 Subsequent reads() will return available records again.
43
44 Unlike the classic syslog() interface, the 64 bit record
45 sequence numbers allow to calculate the amount of lost
46 messages, in case the buffer gets overwritten. And they allow
47 to reconnect to the buffer and reconstruct the read position
48 if needed, without limiting the interface to a single reader.
49
50 The device supports seek with the following parameters:
51 SEEK_SET, 0
52 seek to the first entry in the buffer
53 SEEK_END, 0
54 seek after the last entry in the buffer
55 SEEK_DATA, 0
56 seek after the last record available at the time
57 the last SYSLOG_ACTION_CLEAR was issued.
58
59 The output format consists of a prefix carrying the syslog
60 prefix including priority and facility, the 64 bit message
Kay Sieversd39f3d72012-07-16 18:35:30 -070061 sequence number and the monotonic timestamp in microseconds,
62 and a flag field. All fields are separated by a ','.
63
64 Future extensions might add more comma separated values before
65 the terminating ';'. Unknown fields and values should be
66 gracefully ignored.
Kay Sievers3b552b92012-05-08 18:50:50 +020067
68 The human readable text string starts directly after the ';'
69 and is terminated by a '\n'. Untrusted values derived from
70 hardware or other facilities are printed, therefore
Kay Sieversd39f3d72012-07-16 18:35:30 -070071 all non-printable characters and '\' itself in the log message
72 are escaped by "\x00" C-style hex encoding.
Kay Sievers3b552b92012-05-08 18:50:50 +020073
74 A line starting with ' ', is a continuation line, adding
75 key/value pairs to the log message, which provide the machine
76 readable context of the message, for reliable processing in
77 userspace.
78
79 Example:
Kay Sieversd39f3d72012-07-16 18:35:30 -070080 7,160,424069,-;pci_root PNP0A03:00: host bridge window [io 0x0000-0x0cf7] (ignored)
Kay Sievers3b552b92012-05-08 18:50:50 +020081 SUBSYSTEM=acpi
82 DEVICE=+acpi:PNP0A03:00
Kay Sieversd39f3d72012-07-16 18:35:30 -070083 6,339,5140900,-;NET: Registered protocol family 10
84 30,340,5690716,-;udevd[80]: starting version 181
Kay Sievers3b552b92012-05-08 18:50:50 +020085
86 The DEVICE= key uniquely identifies devices the following way:
87 b12:8 - block dev_t
88 c127:3 - char dev_t
89 n8 - netdev ifindex
90 +sound:card0 - subsystem:devname
91
Kay Sieversd39f3d72012-07-16 18:35:30 -070092 The flags field carries '-' by default. A 'c' indicates a
93 fragment of a line. All following fragments are flagged with
94 '+'. Note, that these hints about continuation lines are not
Masanari Iida4e79162a2012-11-08 21:57:35 +090095 necessarily correct, and the stream could be interleaved with
Kay Sieversd39f3d72012-07-16 18:35:30 -070096 unrelated messages, but merging the lines in the output
97 usually produces better human readable results. A similar
98 logic is used internally when messages are printed to the
99 console, /proc/kmsg or the syslog() syscall.
100
Kay Sievers3b552b92012-05-08 18:50:50 +0200101Users: dmesg(1), userspace kernel log consumers