4 Contact: Kay Sievers <kay@vrfy.org>
5 Description: The /dev/kmsg character device node provides userspace access
6 to the kernel's printk buffer.
9 Every write() to the opened device node places a log entry in
10 the kernel's printk buffer.
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 next 8 bits the syslog facility number.
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.
24 Every read() from the opened device node receives one record
25 of the kernel's printk buffer.
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.
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.
36 Messages in the record ring buffer get overwritten as whole,
37 there are never partial messages received by read().
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.
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.
50 The device supports seek with the following parameters:
52 seek to the first entry in the buffer
54 seek after the last entry in the buffer
56 seek after the last record available at the time
57 the last SYSLOG_ACTION_CLEAR was issued.
59 Other seek operations or offsets are not supported because of
60 the special behavior this device has. The device allows to read
61 or write only whole variable length messages (records) that are
62 stored in a ring buffer.
64 Because of the non-standard behavior also the error values are
65 non-standard. -ESPIPE is returned for non-zero offset. -EINVAL
66 is returned for other operations, e.g. SEEK_CUR. This behavior
67 and values are historical and could not be modified without the
68 risk of breaking userspace.
70 The output format consists of a prefix carrying the syslog
71 prefix including priority and facility, the 64 bit message
72 sequence number and the monotonic timestamp in microseconds,
73 and a flag field. All fields are separated by a ','.
75 Future extensions might add more comma separated values before
76 the terminating ';'. Unknown fields and values should be
79 The human readable text string starts directly after the ';'
80 and is terminated by a '\n'. Untrusted values derived from
81 hardware or other facilities are printed, therefore
82 all non-printable characters and '\' itself in the log message
83 are escaped by "\x00" C-style hex encoding.
85 A line starting with ' ', is a continuation line, adding
86 key/value pairs to the log message, which provide the machine
87 readable context of the message, for reliable processing in
91 7,160,424069,-;pci_root PNP0A03:00: host bridge window [io 0x0000-0x0cf7] (ignored)
93 DEVICE=+acpi:PNP0A03:00
94 6,339,5140900,-;NET: Registered protocol family 10
95 30,340,5690716,-;udevd[80]: starting version 181
97 The DEVICE= key uniquely identifies devices the following way:
101 +sound:card0 - subsystem:devname
103 The flags field carries '-' by default. A 'c' indicates a
104 fragment of a line. Note, that these hints about continuation
105 lines are not necessarily correct, and the stream could be
106 interleaved with unrelated messages, but merging the lines in
107 the output usually produces better human readable results. A
108 similar logic is used internally when messages are printed to
109 the console, /proc/kmsg or the syslog() syscall.
111 By default, kernel tries to avoid fragments by concatenating
112 when it can and fragments are rare; however, when extended
113 console support is enabled, the in-kernel concatenation is
114 disabled and /dev/kmsg output will contain more fragments. If
115 the log consumer performs concatenation, the end result
116 should be the same. In the future, the in-kernel concatenation
117 may be removed entirely and /dev/kmsg users are recommended to
118 implement fragment handling.
120 Users: dmesg(1), userspace kernel log consumers