SYSLOG
Section: Linux Programmer's Manual (2)
Updated: 2021-03-22
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NAME
syslog, klogctl - read and/or clear kernel message ring buffer;
set console_loglevel
SYNOPSIS
int syslog(int type, char *bufp, int len);
/* The glibc interface */
#include <sys/klog.h>
int klogctl(int type, char *bufp, int len);
DESCRIPTION
Note:
Probably, you are looking for the C library function
syslog(),
which talks to
syslogd(8);
see
syslog(3)
for details.
This page describes the kernel
syslog()
system call, which is used to control the kernel
printk()
buffer; the glibc wrapper function for the system call 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 log level).
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 kernel 2.4.23/2.6, the value is a kernel configuration option
(CONFIG_LOG_BUF_SHIFT,
default value dependent on the architecture).
Since Linux 2.6.6, 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 them 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)
-
The command saves the current value of
console_loglevel
and then sets
console_loglevel
to
minimum_console_loglevel,
so that no messages are printed to the console.
Before Linux 2.6.32,
the command simply sets
console_loglevel
to
minimum_console_loglevel.
See the discussion of
/proc/sys/kernel/printk,
below.
-
The
bufp
and
len
arguments are ignored.
- SYSLOG_ACTION_CONSOLE_ON (7)
-
If a previous
SYSLOG_ACTION_CONSOLE_OFF
command has been performed,
this command restores
console_loglevel
to the value that was saved by that command.
Before Linux 2.6.32,
this command simply sets
console_loglevel
to
default_console_loglevel.
See the discussion of
/proc/sys/kernel/printk,
below.
-
The
bufp
and
len
arguments are ignored.
- SYSLOG_ACTION_CONSOLE_LEVEL (8)
-
The call sets
console_loglevel
to the value given in
len,
which must be an integer between 1 and 8 (inclusive).
The kernel silently enforces a minimum value of
minimum_console_loglevel
for
len.
See the
log level
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.
/proc/sys/kernel/printk
/proc/sys/kernel/printk
is a writable file containing four integer values that influence kernel
printk()
behavior when printing or logging error messages.
The four values are:
- console_loglevel
-
Only messages with a log level lower than this value will
be printed to the console.
The default value for this field is
DEFAULT_CONSOLE_LOGLEVEL
(7), but it is set to
4 if the kernel command line contains the word "quiet",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).
The value of
console_loglevel
can be set (to a value in the range 1-8) by a
syslog()
call with a
type
of 8.
- default_message_loglevel
-
This value will be used as the log level for
printk()
messages that do not have an explicit level.
Up to and including Linux 2.6.38,
the hard-coded default value for this field was 4
(KERN_WARNING);
since Linux 2.6.39,
the default value is a defined by the kernel configuration option
CONFIG_DEFAULT_MESSAGE_LOGLEVEL,
which defaults to 4.
- minimum_console_loglevel
-
The value in this field is the minimum value to which
console_loglevel
can be set.
- default_console_loglevel
-
This is the default value for
console_loglevel.
The log level
Every
printk()
message has its own log level.
If the log level is not explicitly specified as part of the message,
it defaults to
default_message_loglevel.
The conventional meaning of the log level is as follows:
| Kernel constant | Level value | Meaning
|
| KERN_EMERG | 0 | System is unusable
|
| KERN_ALERT | 1 |
Action must be taken immediately
|
| KERN_CRIT | 2 | Critical conditions
|
| KERN_ERR | 3 | Error conditions
|
| KERN_WARNING | 4 | Warning conditions
|
| KERN_NOTICE | 5 |
Normal but significant condition
|
| KERN_INFO | 6 | Informational
|
| KERN_DEBUG | 7 | Debug-level messages
|
The kernel
printk()
routine will print a message on the
console only if it has a log level less than the value of
console_loglevel.
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
dmesg(1),
syslog(3),
capabilities(7)
COLOPHON
This page is part of release 5.11 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- The kernel log buffer
-
- Commands
-
- /proc/sys/kernel/printk
-
- The log level
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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Time: 06:22:48 GMT, May 09, 2021