SYSLOG(2) Linux Programmer's Manual SYSLOG(2)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
If you need the C library function syslog() (which talks to sys‐
logd(8)), then look at syslog(3). The system call of this name is
about controlling the kernel printk() buffer, and the glibc wrapper
function 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 (regard‐
less of their loglevel). 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 2.4.23/2.6 the value is a kernel configuration option
(CONFIG_LOG_BUF_SHIFT). In recent kernels 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 exe‐
cuted by the kernel when a user program reads /proc/kmsg.
SYSLOG_ACTION_READ_ALL (3)
Read all messages remaining in the ring buffer, placing then 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 exe‐
cutes 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 (SYS‐
LOG_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)
Disable printk to console. The call sets the console log level
to the minimum, so that no messages are printed to the console.
The bufp and len arguments are ignored.
SYSLOG_ACTION_CONSOLE_ON (7)
The call sets the console log level to the default, so that mes‐
sages are printed to the console. The bufp and len arguments
are ignored.
SYSLOG_ACTION_CONSOLE_LEVEL (8)
The call sets the console log level to the value given in len,
which must be an integer between 1 and 8 (inclusive). See the
loglevel 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 (SYS‐
LOG_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 pro‐
cesses; 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.
The loglevel
The kernel routine printk() will only print a message on the console,
if it has a loglevel less than the value of the variable con‐
sole_loglevel. This variable initially has the value DEFAULT_CON‐
SOLE_LOGLEVEL (7), but is set to 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). This variable is set (to a value
in the range 1-8) by a syslog() call with a type of 8. Calls to sys‐
log() with type equal to 6 or 7 set the variable to 1 (kernel panics
only) or 7 (all except debugging messages), respectively.
Every text line in a message has its own loglevel. This level is
DEFAULT_MESSAGE_LOGLEVEL - 1 (6) unless the line starts with <d> where
d is a digit in the range 1-7, in which case the level is d. The con‐
ventional meaning of the loglevel is defined in <linux/kernel.h> as
follows:
#define KERN_EMERG "<0>" /* system is unusable */
#define KERN_ALERT "<1>" /* action must be taken immediately */
#define KERN_CRIT "<2>" /* critical conditions */
#define KERN_ERR "<3>" /* error conditions */
#define KERN_WARNING "<4>" /* warning conditions */
#define KERN_NOTICE "<5>" /* normal but significant condition */
#define KERN_INFO "<6>" /* informational */
#define KERN_DEBUG "<7>" /* debug-level messages */
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 out‐
side 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 ker‐
nel message ring buffer by a process without sufficient privi‐
lege (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 ani‐
mals.
SEE ALSOsyslog(3), capabilities(7)COLOPHON
This page is part of release 3.54 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2012-11-29 SYSLOG(2)
