diff options
Diffstat (limited to 'Documentation/ABI/testing/dev-kmsg')
| -rw-r--r-- | Documentation/ABI/testing/dev-kmsg | 53 |
1 files changed, 35 insertions, 18 deletions
diff --git a/Documentation/ABI/testing/dev-kmsg b/Documentation/ABI/testing/dev-kmsg index fff817efa508..a377b6c093c9 100644 --- a/Documentation/ABI/testing/dev-kmsg +++ b/Documentation/ABI/testing/dev-kmsg @@ -6,13 +6,14 @@ 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. + priority and the next 8 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 @@ -21,6 +22,7 @@ Description: The /dev/kmsg character device node provides userspace access 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. @@ -48,6 +50,7 @@ Description: The /dev/kmsg character device node provides userspace access if needed, without limiting the interface to a single reader. The device supports seek with the following parameters: + SEEK_SET, 0 seek to the first entry in the buffer SEEK_END, 0 @@ -56,6 +59,17 @@ Description: The /dev/kmsg character device node provides userspace access seek after the last record available at the time the last SYSLOG_ACTION_CLEAR was issued. + Other seek operations or offsets are not supported because of + the special behavior this device has. The device allows to read + or write only whole variable length messages (records) that are + stored in a ring buffer. + + Because of the non-standard behavior also the error values are + non-standard. -ESPIPE is returned for non-zero offset. -EINVAL + is returned for other operations, e.g. SEEK_CUR. This behavior + and values are historical and could not be modified without the + risk of breaking userspace. + 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, @@ -76,27 +90,30 @@ Description: The /dev/kmsg character device node provides userspace access 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) - SUBSYSTEM=acpi - DEVICE=+acpi:PNP0A03:00 - 6,339,5140900,-;NET: Registered protocol family 10 - 30,340,5690716,-;udevd[80]: starting version 181 + Example:: + + 7,160,424069,-;pci_root PNP0A03:00: host bridge window [io 0x0000-0x0cf7] (ignored) + SUBSYSTEM=acpi + 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 + + ============ ================= + b12:8 block dev_t + c127:3 char dev_t + n8 netdev ifindex + +sound:card0 subsystem:devname + ============ ================= The flags field carries '-' by default. A 'c' indicates a - fragment of a line. All following fragments are flagged with - '+'. Note, that these hints about continuation lines are not - necessarily correct, and the stream could be interleaved with - unrelated messages, but merging the lines in the output - usually produces better human readable results. A similar - logic is used internally when messages are printed to the - console, /proc/kmsg or the syslog() syscall. + fragment of a line. Note, that these hints about continuation + lines are not necessarily correct, and the stream could be + interleaved with unrelated messages, but merging the lines in + the output usually produces better human readable results. A + similar logic is used internally when messages are printed to + the console, /proc/kmsg or the syslog() syscall. By default, kernel tries to avoid fragments by concatenating when it can and fragments are rare; however, when extended |