Based on kernel version 4.13.3. Page generated on 2017-09-23 13:54 EST.
1 What: /dev/kmsg 2 Date: Mai 2012 3 KernelVersion: 3.5 4 Contact: Kay Sievers <firstname.lastname@example.org> 5 Description: 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 61 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. 67 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 71 all non-printable characters and '\' itself in the log message 72 are escaped by "\x00" C-style hex encoding. 73 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: 80 7,160,424069,-;pci_root PNP0A03:00: host bridge window [io 0x0000-0x0cf7] (ignored) 81 SUBSYSTEM=acpi 82 DEVICE=+acpi:PNP0A03:00 83 6,339,5140900,-;NET: Registered protocol family 10 84 30,340,5690716,-;udevd: starting version 181 85 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 92 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 95 necessarily correct, and the stream could be interleaved with 96 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. 100 101 By default, kernel tries to avoid fragments by concatenating 102 when it can and fragments are rare; however, when extended 103 console support is enabled, the in-kernel concatenation is 104 disabled and /dev/kmsg output will contain more fragments. If 105 the log consumer performs concatenation, the end result 106 should be the same. In the future, the in-kernel concatenation 107 may be removed entirely and /dev/kmsg users are recommended to 108 implement fragment handling. 109 110 Users: dmesg(1), userspace kernel log consumers