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SYSLOG(2) Linux Programmer's Manual SYSLOG(2)
syslog, klogctl - read and/or clear kernel message ring buffer; set con-
sole_loglevel
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);
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 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 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
*/
Only command types 3 and 10 are allowed to unprivileged processes. Type 9 was
added in 2.4.10; type 10 in 2.6.6.
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 /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 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 log
buffer.
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),
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 conventional
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 */
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.
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 disabled.
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 capability).
ERESTARTSYS
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().
syslog(3)
This page is part of release 3.32 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 2008-06-20 SYSLOG(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface