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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 C library 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 wrapper
function is called klogctl().
The type 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
CAP_SYS_ADMIN 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 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. 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
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 or len 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 level,
which must be an integer between 1 and 8 (inclusive). See the loglevel
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
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,dummy) 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 */
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
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
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
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().
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