syslog, klogctl - read and/or clear kernel message ring buffer; set
int syslog(int type, char *bufp, int len);
/* No wrapper provided in glibc */
/* The glibc interface */
int klogctl(int type, char *bufp, int len);
If you need the C library function syslog
() (which talks to
), 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
argument determines the action taken by this function, as
0 -- Close the log. Currently a NOP.
1 -- Open the log. Currently a NOP.
2 -- Read from the log.
3 -- Read all messages remaining in the ring buffer.
4 -- Read and clear all messages remaining in the ring buffer
5 -- Clear ring buffer.
6 -- Disable printk to console
7 -- Enable printk to console
8 -- Set level of messages printed to console
9 -- Return number of unread characters in the log buffer
10 -- Return size of the log buffer
Type 9 was added in Linux 2.4.10; type 10 in Linux 2.6.6.
In Linux kernels before 2.6.37, only command types 3 and 10 are allowed to
unprivileged processes. Since Linux 2.6.37, command types 3 and 10 are only
allowed to unprivileged processes 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
capability (now deprecated for this purpose) or the (new)
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 loglevel). In early kernels, LOG_BUF_LEN
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. In recent kernels
the size can be queried with command type 10.
The call syslog(2,buf,len)
waits until this kernel log buffer is
nonempty, and then reads at most len
bytes into the buffer buf
It returns the number of bytes read. Bytes read from the log disappear from
the log buffer: the information can only be read once. This is the function
executed by the kernel when a user program reads /proc/kmsg
The call syslog(3,buf,len)
will read 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 (which does
not clear the buffer at all). It returns the number of bytes read.
The call syslog(4,buf,len)
does precisely the same, but also executes the
"clear ring buffer" command.
The call syslog(5,dummy,dummy)
executes just the "clear ring
buffer" command. (In each call where buf
is shown as
"dummy", the value of the argument is ignored by the call.)
The call syslog(6,dummy,dummy)
sets the console log level to minimum, so
that no messages are printed to the console.
The call syslog(7,dummy,dummy)
sets the console log level to default, so
that messages are printed to the console.
The call syslog(8,dummy,level)
sets the console log level to
, which must be an integer between 1 and 8 (inclusive). See the
section for details.
The call syslog(9,dummy,dummy)
returns the number of bytes currently
available to be read on the kernel log buffer.
The call syslog(10,dummy,dummy)
returns the total size of the kernel log
The kernel routine printk
() will only print a message on the console, if
it has a loglevel less than the value of the variable console_loglevel
This variable initially has the value DEFAULT_CONSOLE_LOGLEVEL
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 the
. The calls syslog(type,dummy,dummy)
equal to 6 or 7, set it 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>
is a digit in the range 1-7, in which case the level is
. The conventional meaning of the loglevel is defined in
#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 */
equal to 2, 3, or 4, a successful call to syslog
returns the number of bytes read. For type
the number of bytes currently available to be read on the kernel log buffer.
() 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.
- 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).
- This syslog() system call is not available, because
the kernel was compiled with the CONFIG_PRINTK kernel-configuration
- 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
- System call was interrupted by a signal; nothing was read.
(This can be seen only during a trace.)
This system call is Linux-specific and should not be used in programs intended
to be portable.
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.
This page is part of release 3.44 of the Linux man-pages
description of the project, and information about reporting bugs, can be found