syslog, klogctl - read and/or clear kernel message ring buffer; set
console_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);
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 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).
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 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.
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.
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.
dmesg(1),
syslog(3),
capabilities(7)
This page is part of release 4.16 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/.