klogctl
SYSLOG(2) Linux Programmer’s Manual SYSLOG(2)
NAME
syslog, klogctl - read and/or clear kernel message ring buffer; set
console_loglevel
SYNOPSIS
/* The glibc interface */
#include <sys/klog.h>
int klogctl(int type, char *bufp, int len);
/* The handcrafted system call */
#include <unistd.h>
#include <linux/unistd.h>
_syscall3(int, syslog, int, type, char *, bufp, int, len);
int syslog(int type, char *bufp, int len);
DESCRIPTION
If you need the libc function syslog(), (that talks to syslogd(8)),
then look at syslog(3). The system call of this name is about con-
trolling the kernel printk() buffer, and the glibc version is called
klogctl().
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
/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,idummy) only executes the ‘clear ring buffer’
command.
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 (initially DEFAULT_CONSOLE_LOGLEVEL (7), but set to 10
if the kernel commandline 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), 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
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 otherwise 0.
ERRORS
EPERM An attempt was made to change console_loglevel or clear the
kernel message ring buffer by a process without root permis-
sions.
EINVAL Bad parameters.
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 kernel
call and library routine of the same name are entirely different ani-
mals. In libc4 and libc5 the number of this call was defined by
SYS_klog. In glibc 2.0 the syscall is baptised klogctl.
SEE ALSO
syslog(3)
Linux 1.2.9 2001-11-25 SYSLOG(2)
