SYSLOG
Section: Linux Programmer's Manual (2)
Updated: 2008-06-20
NAME
syslog, klogctl - read and/or clear kernel message ring buffer;
set console_loglevel
SYNOPSIS
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);
DESCRIPTION
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 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
/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 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
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 */
RETURN VALUE
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.
ERRORS
- 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).
- 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.)
- ENOSYS
-
This
syslog()
system call is not available, because the kernel was compiled with the
CONFIG_PRINTK
kernel-configuration option disabled.
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
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().
SEE ALSO
syslog(3)
COLOPHON
This page is part of release 3.27 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- The kernel log buffer
-
- The loglevel
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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Time: 07:34:54 GMT, March 26, 2013