| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON | The Linux Programming Interface |
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 wrapper function is called
klogctl().
The type argument determines the action taken by this function, as follows:
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) CAP_SYSLOG capability.
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 or CAP_SYSLOG 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.
syslog(3), capabilities(7)
This page is part of release 3.41 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 2012-05-05 SYSLOG(2)
HTML rendering created 2012-05-11 by Michael Kerrisk,
author of
The Linux Programming Interface,
maintainer of the
Linux man-pages project