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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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syslog(2) System Calls Manual syslog(2)
syslog, klogctl - read and/or clear kernel message ring buffer;
set console_loglevel
Standard C library (libc, -lc)
#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 size);
/* The glibc interface */
#include <sys/klog.h>
int klogctl(int type, char *bufp, int size);
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 size 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 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 (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 size 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 size 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 size arguments are ignored.
SYSLOG_ACTION_CONSOLE_LEVEL (8)
The call sets console_loglevel to the value given in size,
which must be an integer between 1 and 8 (inclusive). The
kernel silently enforces a minimum value of
minimum_console_loglevel for size. 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 size 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 size 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 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 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.
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 size 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.)
Linux.
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 syslog(2)
Pages that refer to this page: dmesg(1), syscalls(2), proc(5), proc_kmsg(5), proc_sys_kernel(5), systemd.exec(5), bootparam(7), capabilities(7), rsyslogd(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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