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

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 4.15 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/.