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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 libc function syslog() (which talks to syslogd(8)),
then look at syslog(3). The system call of this name is about
controlling the kernel printk() buffer, and the glibc version is called
The type argument determines the action taken by this function.
Quoting from kernel/printk.c:
* Commands to sys_syslog:
* 0 -- Close the log. Currently a NOP.
* 1 -- Open the log. Currently a NOP.
* 2 -- Read from the log.
* 3 -- Read up to the last 4k of messages in the ring buffer.
* 4 -- Read and clear last 4k of messages in the ring buffer
* 5 -- Clear ring buffer.
* 6 -- Disable printk’s to console
* 7 -- Enable printk’s to console
* 8 -- Set level of messages printed to console
* 9 -- Return number of unread characters in the log buffer
Only function 3 is allowed to non-root processes. (Function 9 was
added in 2.4.10.)
The kernel log buffer
The kernel has a cyclic buffer of length LOG_BUF_LEN (4096, since
1.3.54: 8192, since 2.1.113: 16384; in recent kernels the size can be
set at compile time) in which messages given as argument to the kernel
function printk() are stored (regardless of their loglevel).
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
The call syslog() (3,buf,len) will read the last len bytes from the log
buffer (non-destructively), 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,idummy) only executes the "clear ring
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 (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 the call syslog() (8,dummy,value).
The calls syslog() (type,dummy,idummy) with type equal to 6 or 7, set
it to 1 (kernel panics only) or 7 (all except debugging messages),
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
conventional meaning of the loglevel is defined in <linux/kernel.h> as
#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 */
In case of error, -1 is returned, and errno is set. Otherwise, for
type equal to 2, 3 or 4, syslog() returns the number of bytes read, and
EINVAL Bad parameters.
EPERM An attempt was made to change console_loglevel or clear the
kernel message ring buffer by a process without root
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. In libc4 and libc5 the number of this call was defined by
SYS_klog. In glibc 2.0 the syscall is baptized klogctl().
This page is part of release 2.77 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/.