syslog(2) System Calls Manual syslog(2)
NAME
syslog, klogctl - read and/or clear kernel message ring buffer; set con-
sole_loglevel
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/klog.h> /* Definition of SYSLOG_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_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 (regard-
less of their log level). In early kernels, LOG_BUF_LEN had the value
4096; from Linux 1.3.54, it was 8192; from Linux 2.1.113, it was 16384;
since Linux 2.4.23/2.6, the value is a kernel configuration option (CON-
FIG_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 de-
fined 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 infor-
mation 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 exe-
cutes 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_AC-
TION_READ_CLEAR). This command has no effect on commands 2 (SYS-
LOG_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 per-
formed, 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_AC-
TION_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
Linux 2.6.37, command types 3 and 10 are allowed to unprivileged
processes; since Linux 2.6.37, these commands are allowed to unprivi-
leged 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 val-
ues that influence kernel printk() behavior when printing or logging er-
ror 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 DE-
FAULT_CONSOLE_LOGLEVEL (7), but it is set to 4 if the kernel com-
mand 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 defined
by the kernel configuration option CONFIG_DEFAULT_MES-
SAGE_LOGLEVEL, which defaults to 4.
minimum_console_loglevel
The value in this field is the minimum value to which con-
sole_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_mes-
sage_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 signifi-
cant 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 er-
ror.
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 out-
side 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 ker-
nel message ring buffer by a process without sufficient privilege
(more precisely: without the CAP_SYS_ADMIN or CAP_SYSLOG capabil-
ity).
ERESTARTSYS
System call was interrupted by a signal; nothing was read. (This
can be seen only during a trace.)
STANDARDS
Linux.
HISTORY
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 ani-
mals.
SEE ALSO
dmesg(1), syslog(3), capabilities(7)
Linux man-pages 6.9.1 2024-05-02 syslog(2)
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