SYSLOG
Linux Programmer's Manual (2)
2012-11-29
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 wrapper function is called
klogctl().
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
(
CONFIG_LOG_BUF_SHIFT).
In recent kernels the size can be queried with command type 10 (see below).
Commands
The
type argument determines the action taken by this function.
The list below specifies the values for
type.
The symbolic names are defined in the kernel source,
but are not exported to user space;
you will either need to use the numbers, or define the names yourself.
- SYSLOG_ACTION_CLOSE (0)
-
Close the log.
Currently a NOP.
- SYSLOG_ACTION_OPEN (1)
-
Open the log.
Currently a NOP.
- SYSLOG_ACTION_READ (2)
-
Read from the log.
The call
waits until the kernel log buffer is nonempty, and then reads
at most len bytes into the buffer pointed to by
bufp.
The call returns the number of bytes read.
Bytes read from the log disappear from the log buffer:
the information can be read only once.
This is the function executed by the kernel when a user program reads
/proc/kmsg.
- SYSLOG_ACTION_READ_ALL (3)
-
Read all messages remaining in the ring buffer,
placing then in the buffer pointed to by
bufp.
The call reads 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 (see command 5 below)).
The call returns the number of bytes read.
- SYSLOG_ACTION_READ_CLEAR (4)
-
Read and clear all messages remaining in the ring buffer.
The call does precisely the same as for a
type
of 3, but also executes the "clear ring buffer" command.
- SYSLOG_ACTION_CLEAR (5)
-
The call executes just the "clear ring buffer" command.
The
bufp
and
len
arguments are ignored.
-
This command does not really clear the ring buffer.
Rather, it sets a kernel bookkeeping variable that
determines the results returned by commands 3
(SYSLOG_ACTION_READ_ALL)
and 4
(SYSLOG_ACTION_READ_CLEAR).
This command has no effect on commands 2
(SYSLOG_ACTION_READ)
and 9
(SYSLOG_ACTION_SIZE_UNREAD).
- SYSLOG_ACTION_CONSOLE_OFF (6)
-
Disable printk to console.
The call sets the console log level to the minimum,
so that no messages are printed to the console.
The
bufp
and
len
arguments are ignored.
- SYSLOG_ACTION_CONSOLE_ON (7)
-
The call sets the console log level to the default,
so that messages are printed to the console.
The
bufp
and
len
arguments are ignored.
- SYSLOG_ACTION_CONSOLE_LEVEL (8)
-
The call sets the console log level to the value given in
len,
which must be an integer between 1 and 8 (inclusive).
See the
loglevel
section for details.
The
bufp
argument is ignored.
- SYSLOG_ACTION_SIZE_UNREAD (9) (since Linux 2.4.10)
-
The call
returns the number of bytes currently available to be read
from the kernel log buffer via command 2
(SYSLOG_ACTION_READ).
The
bufp
and
len
arguments are ignored.
- SYSLOG_ACTION_SIZE_BUFFER (10) (since Linux 2.6.6)
-
This command returns the total size of the kernel log buffer.
The
bufp
and
len
arguments are ignored.
All commands except 3 and 10 require privilege.
In Linux kernels before 2.6.37,
command types 3 and 10 are allowed to unprivileged processes;
since Linux 2.6.37,
these commands are allowed to unprivileged processes only 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 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 a
syslog()
call with a
type
of 8.
Calls to
syslog()
with
type
equal to 6 or 7 set the variable 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).
- 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.)
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.
SEE ALSO
syslog(3),
capabilities(7)
COLOPHON
This page is part of release 3.51 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
-
- Commands
-
- The loglevel
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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Time: 20:26:23 GMT, May 21, 2013