Ubuntu Manpage: syslog, klogctl - read and/or clear kernel message ring buffer; set console

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NAME

       syslog, klogctl - read and/or clear kernel message ring buffer; set console_loglevel

SYNOPSIS

       int syslog(int type, char *bufp, int len);
                       /* No wrapper provided in glibc */

       /* The glibc interface */
       #include <sys/klog.h>

       int klogctl(int type, char *bufp, int len);

DESCRIPTION

Note: Probably, you are looking for the C library function syslog(), which talks to syslogd(8); see
syslog(3) for details.

This page describes the kernel syslog() system call, which is used to control the kernel printk() buffer;
the glibc wrapper function for the system call is called klogctl().

The kernel log buffer
The kernel has a cyclic buffer of length LOG_BUF_LEN in which messages given as arguments to the kernel
function printk() are stored (regardless of their log level). In early kernels, LOG_BUF_LEN had the
value 4096; from kernel 1.3.54, it was 8192; from kernel 2.1.113, it was 16384; since kernel 2.4.23/2.6,
the value is a kernel configuration option (CONFIG_LOG_BUF_SHIFT, default value dependent on the
architecture). Since Linux 2.6.6, the size can be queried with command type 10 (see below).

Commands
The type argument determines the action taken by this function. The list below specifies the values for
type. The symbolic names are defined in the kernel source, but are not exported to user space; you will
either need to use the numbers, or define the names yourself.

SYSLOG_ACTION_CLOSE (0)
Close the log. Currently a NOP.

SYSLOG_ACTION_OPEN (1)
Open the log. Currently a NOP.

SYSLOG_ACTION_READ (2)
Read from the log. The call waits until the kernel log buffer is nonempty, and then reads at most
len bytes into the buffer pointed to by bufp. The call returns the number of bytes read. Bytes
read from the log disappear from the log buffer: the information can be read only once. This is
the function executed by the kernel when a user program reads /proc/kmsg.

SYSLOG_ACTION_READ_ALL (3)
Read all messages remaining in the ring buffer, placing them in the buffer pointed to by bufp.
The call reads the last len bytes from the log buffer (nondestructively), but will not read more
than was written into the buffer since the last "clear ring buffer" command (see command 5
below)). The call returns the number of bytes read.

SYSLOG_ACTION_READ_CLEAR (4)
Read and clear all messages remaining in the ring buffer. The call does precisely the same as for
a type of 3, but also executes the "clear ring buffer" command.

SYSLOG_ACTION_CLEAR (5)
The call executes just the "clear ring buffer" command. The bufp and len arguments are ignored.

This command does not really clear the ring buffer. Rather, it sets a kernel bookkeeping variable
that determines the results returned by commands 3 (SYSLOG_ACTION_READ_ALL) and 4
(SYSLOG_ACTION_READ_CLEAR). This command has no effect on commands 2 (SYSLOG_ACTION_READ) and 9
(SYSLOG_ACTION_SIZE_UNREAD).

SYSLOG_ACTION_CONSOLE_OFF (6)
The command saves the current value of console_loglevel and then sets console_loglevel to
minimum_console_loglevel, so that no messages are printed to the console. Before Linux 2.6.32,
the command simply sets console_loglevel to minimum_console_loglevel. See the discussion of
/proc/sys/kernel/printk, below.

The bufp and len arguments are ignored.

SYSLOG_ACTION_CONSOLE_ON (7)
If a previous SYSLOG_ACTION_CONSOLE_OFF command has been performed, this command restores
console_loglevel to the value that was saved by that command. Before Linux 2.6.32, this command
simply sets console_loglevel to default_console_loglevel. See the discussion of
/proc/sys/kernel/printk, below.

The bufp and len arguments are ignored.

SYSLOG_ACTION_CONSOLE_LEVEL (8)
The call sets console_loglevel to the value given in len, which must be an integer between 1 and 8
(inclusive). The kernel silently enforces a minimum value of minimum_console_loglevel for len.
See the log level section for details. The bufp argument is ignored.

SYSLOG_ACTION_SIZE_UNREAD (9) (since Linux 2.4.10)
The call returns the number of bytes currently available to be read from the kernel log buffer via
command 2 (SYSLOG_ACTION_READ). The bufp and len arguments are ignored.

SYSLOG_ACTION_SIZE_BUFFER (10) (since Linux 2.6.6)
This command returns the total size of the kernel log buffer. The bufp and len arguments are
ignored.

All commands except 3 and 10 require privilege. In Linux kernels before 2.6.37, command types 3 and 10
are allowed to unprivileged processes; since Linux 2.6.37, these commands are allowed to unprivileged
processes only if /proc/sys/kernel/dmesg_restrict has the value 0. Before Linux 2.6.37, "privileged"
means that the caller has the CAP_SYS_ADMIN capability. Since Linux 2.6.37, "privileged" means that the
caller has either the CAP_SYS_ADMIN capability (now deprecated for this purpose) or the (new) CAP_SYSLOG
capability.

/proc/sys/kernel/printk
/proc/sys/kernel/printk is a writable file containing four integer values that influence kernel printk()
behavior when printing or logging error messages. The four values are:

console_loglevel
Only messages with a log level lower than this value will be printed to the console. The default
value for this field is DEFAULT_CONSOLE_LOGLEVEL (7), but it is set to 4 if the kernel command
line contains the word "quiet", 10 if the kernel command line contains the word "debug", and to 15
in case of a kernel fault (the 10 and 15 are just silly, and equivalent to 8). The value of
console_loglevel can be set (to a value in the range 1–8) by a syslog() call with a type of 8.

default_message_loglevel
This value will be used as the log level for printk() messages that do not have an explicit level.
Up to and including Linux 2.6.38, the hard-coded default value for this field was 4
(KERN_WARNING); since Linux 2.6.39, the default value is a defined by the kernel configuration
option CONFIG_DEFAULT_MESSAGE_LOGLEVEL, which defaults to 4.

minimum_console_loglevel
The value in this field is the minimum value to which console_loglevel can be set.

default_console_loglevel
This is the default value for console_loglevel.

The log level
Every printk() message has its own log level. If the log level is not explicitly specified as part of
the message, it defaults to default_message_loglevel. The conventional meaning of the log level is as
follows:

Kernel constant Level value Meaning
KERN_EMERG 0 System is unusable
KERN_ALERT 1 Action must be taken immediately
KERN_CRIT 2 Critical conditions
KERN_ERR 3 Error conditions
KERN_WARNING 4 Warning conditions

KERN_NOTICE 5 Normal but significant condition
KERN_INFO 6 Informational
KERN_DEBUG 7 Debug-level messages

The kernel printk() routine will print a message on the console only if it has a log level less than the
value of console_loglevel.

RETURN VALUE

       For  type  equal to 2, 3, or 4, a successful call to syslog() returns the number of bytes read.  For type
       9, syslog() returns the number of bytes currently available to be read on the  kernel  log  buffer.   For
       type  10,  syslog()  returns  the  total  size  of the kernel log buffer.  For other values of type, 0 is
       returned on success.

       In case of error, -1 is returned, and errno is set to indicate the error.

ERRORS

       EINVAL Bad arguments (e.g., bad type; or for type 2, 3, or 4, buf is NULL, or len is less than  zero;  or
              for type 8, the level is outside the range 1 to 8).

       ENOSYS This syslog() system call is not available, because the kernel was compiled with the CONFIG_PRINTK
              kernel-configuration option disabled.

       EPERM  An attempt was made to change console_loglevel or clear  the  kernel  message  ring  buffer  by  a
              process  without  sufficient  privilege  (more  precisely: without the CAP_SYS_ADMIN or CAP_SYSLOG
              capability).

       ERESTARTSYS
              System call was interrupted by a signal; nothing was read.   (This  can  be  seen  only  during  a
              trace.)

CONFORMING TO

       This system call is Linux-specific and should not be used in programs intended to be portable.

NOTES

       From  the very start, people noted that it is unfortunate that a system call and a library routine of the
       same name are entirely different animals.

COLOPHON

       This page is part of release 5.10 of  the  Linux  man-pages  project.   A  description  of  the  project,
       information   about   reporting   bugs,   and   the  latest  version  of  this  page,  can  be  found  at
       https://www.kernel.org/doc/man-pages/.