'\" t
.\" Copyright (C) 1995 Andries Brouwer (aeb@cwi.nl)
.\" and Copyright (C) 2012, 2014 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Written 11 June 1995 by Andries Brouwer <aeb@cwi.nl>
.\" 2008-02-15, Jeremy Kerr <jk@ozlabs.org>
.\"     Add info on command type 10; add details on types 6, 7, 8, & 9.
.\" 2008-02-15, Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Update LOG_BUF_LEN details; update RETURN VALUE section.
.\"
.TH syslog 2 2024-05-02 "Linux man-pages (unreleased)"
.SH NAME
syslog, klogctl \- read and/or clear kernel message ring buffer;
set console_loglevel
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.BR "#include <sys/klog.h>" "        /* Definition of " SYSLOG_* " constants */"
.BR "#include <sys/syscall.h>" "     /* Definition of " SYS_* " constants */"
.B #include <unistd.h>
.P
.BI "int syscall(SYS_syslog, int " type ", char *" bufp ", int " len );
.P
/* The glibc interface */
.B #include <sys/klog.h>
.P
.BI "int klogctl(int " type ", char *" bufp ", int " len );
.fi
.SH DESCRIPTION
.IR Note :
Probably, you are looking for the C library function
.BR syslog (),
which talks to
.BR syslogd (8);
see
.BR syslog (3)
for details.
.P
This page describes the kernel
.BR syslog ()
system call, which is used to control the kernel
.IR printk ()
buffer; the glibc wrapper function for the system call is called
.BR klogctl ().
.SS The kernel log buffer
The kernel has a cyclic buffer of length
.B LOG_BUF_LEN
in which messages given as arguments to the kernel function
.BR printk ()
are stored (regardless of their log level).
In early kernels,
.B 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
.RB ( CONFIG_LOG_BUF_SHIFT ,
default value dependent on the architecture).
.\" Under "General setup" ==> "Kernel log buffer size"
.\" For Linux 2.6, precisely the option seems to have appeared in Linux 2.5.55.
Since Linux 2.6.6, the size can be queried with command type 10 (see below).
.SS Commands
The \fItype\fP argument determines the action taken by this function.
The list below specifies the values for
.IR 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.
.TP
.BR SYSLOG_ACTION_CLOSE " (0)"
Close the log.
Currently a NOP.
.TP
.BR SYSLOG_ACTION_OPEN " (1)"
Open the log.
Currently a NOP.
.TP
.BR SYSLOG_ACTION_READ " (2)"
Read from the log.
The call
waits until the kernel log buffer is nonempty, and then reads
at most \fIlen\fP bytes into the buffer pointed to by
.IR 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
.IR /proc/kmsg .
.TP
.BR SYSLOG_ACTION_READ_ALL " (3)"
Read all messages remaining in the ring buffer,
placing them in the buffer pointed to by
.IR bufp .
The call reads the last \fIlen\fP
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.
.TP
.BR SYSLOG_ACTION_READ_CLEAR " (4)"
Read and clear all messages remaining in the ring buffer.
The call does precisely the same as for a
.I type
of 3, but also executes the "clear ring buffer" command.
.TP
.BR SYSLOG_ACTION_CLEAR " (5)"
The call executes just the "clear ring buffer" command.
The
.I bufp
and
.I len
arguments are ignored.
.IP
This command does not really clear the ring buffer.
Rather, it sets a kernel bookkeeping variable that
determines the results returned by commands 3
.RB ( SYSLOG_ACTION_READ_ALL )
and 4
.RB ( SYSLOG_ACTION_READ_CLEAR ).
This command has no effect on commands 2
.RB ( SYSLOG_ACTION_READ )
and 9
.RB ( SYSLOG_ACTION_SIZE_UNREAD ).
.TP
.BR SYSLOG_ACTION_CONSOLE_OFF " (6)"
The command saves the current value of
.I console_loglevel
and then sets
.I console_loglevel
to
.IR minimum_console_loglevel ,
so that no messages are printed to the console.
Before Linux 2.6.32,
.\" commit 1aaad49e856ce41adc07d8ae0c8ef35fc4483245
the command simply sets
.I console_loglevel
to
.IR minimum_console_loglevel .
See the discussion of
.IR /proc/sys/kernel/printk ,
below.
.IP
The
.I bufp
and
.I len
arguments are ignored.
.TP
.BR SYSLOG_ACTION_CONSOLE_ON " (7)"
If a previous
.B SYSLOG_ACTION_CONSOLE_OFF
command has been performed,
this command restores
.I console_loglevel
to the value that was saved by that command.
Before Linux 2.6.32,
.\" commit 1aaad49e856ce41adc07d8ae0c8ef35fc4483245
this command simply sets
.I console_loglevel
to
.IR default_console_loglevel .
See the discussion of
.IR /proc/sys/kernel/printk ,
below.
.IP
The
.I bufp
and
.I len
arguments are ignored.
.TP
.BR SYSLOG_ACTION_CONSOLE_LEVEL " (8)"
The call sets
.I console_loglevel
to the value given in
.IR len ,
which must be an integer between 1 and 8 (inclusive).
The kernel silently enforces a minimum value of
.I minimum_console_loglevel
for
.IR len .
See the
.I log level
section for details.
The
.I bufp
argument is ignored.
.TP
.BR 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
.RB ( SYSLOG_ACTION_READ ).
The
.I bufp
and
.I len
arguments are ignored.
.TP
.BR SYSLOG_ACTION_SIZE_BUFFER " (10) (since Linux 2.6.6)"
This command returns the total size of the kernel log buffer.
The
.I bufp
and
.I len
arguments are ignored.
.P
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
.I /proc/sys/kernel/dmesg_restrict
has the value 0.
Before Linux 2.6.37, "privileged" means that the caller has the
.B CAP_SYS_ADMIN
capability.
Since Linux 2.6.37,
"privileged" means that the caller has either the
.B CAP_SYS_ADMIN
capability (now deprecated for this purpose) or the (new)
.B CAP_SYSLOG
capability.
.\"
.\"
.SS /proc/sys/kernel/printk
.I /proc/sys/kernel/printk
is a writable file containing four integer values that influence kernel
.I printk()
behavior when printing or logging error messages.
The four values are:
.TP
.I 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
.B DEFAULT_CONSOLE_LOGLEVEL
(7), but it is set to
4 if the kernel command line contains the word "quiet",\" since Linux 2.4
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
.I console_loglevel
can be set (to a value in the range 1\[en]8) by a
.BR syslog ()
call with a
.I type
of 8.
.TP
.I default_message_loglevel
This value will be used as the log level for
.I 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
.RB ( KERN_WARNING );
since Linux 2.6.39,
.\" commit 5af5bcb8d37f99ba415a1adc6da71051b84f93a5
the default value is defined by the kernel configuration option
.BR CONFIG_DEFAULT_MESSAGE_LOGLEVEL ,
which defaults to 4.
.TP
.I minimum_console_loglevel
The value in this field is the minimum value to which
.I console_loglevel
can be set.
.TP
.I default_console_loglevel
This is the default value for
.IR console_loglevel .
.\"
.\"
.SS The log level
Every
.IR printk ()
message has its own log level.
If the log level is not explicitly specified as part of the message,
it defaults to
.IR default_message_loglevel .
The conventional meaning of the log level is as follows:
.TS
lB lB lB
lB c l.
Kernel constant	Level value	Meaning
KERN_EMERG	0	System is unusable
KERN_ALERT	1	T{
Action must be taken immediately
T}
KERN_CRIT	2	Critical conditions
KERN_ERR	3	Error conditions
KERN_WARNING	4	Warning conditions
KERN_NOTICE	5	T{
Normal but significant condition
T}
KERN_INFO	6	Informational
KERN_DEBUG	7	Debug-level messages
.TE
.P
The kernel
.I printk()
routine will print a message on the
console only if it has a log level less than the value of
.IR console_loglevel .
.SH RETURN VALUE
For \fItype\fP equal to 2, 3, or 4, a successful call to
.BR syslog ()
returns the number
of bytes read.
For \fItype\fP 9,
.BR syslog ()
returns the number of bytes currently
available to be read on the kernel log buffer.
For \fItype\fP 10,
.BR syslog ()
returns the total size of the kernel log buffer.
For other values of \fItype\fP, 0 is returned on success.
.P
In case of error, \-1 is returned,
and \fIerrno\fP is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
Bad arguments (e.g.,
bad
.IR type ;
or for
.I type
2, 3, or 4,
.I buf
is NULL,
or
.I len
is less than zero; or for
.I type
8, the
.I level
is outside the range 1 to 8).
.TP
.B ENOSYS
This
.BR syslog ()
system call is not available, because the kernel was compiled with the
.B CONFIG_PRINTK
kernel-configuration option disabled.
.TP
.B EPERM
An attempt was made to change
.I console_loglevel
or clear the kernel
message ring buffer by a process without sufficient privilege
(more precisely: without the
.B CAP_SYS_ADMIN
or
.B CAP_SYSLOG
capability).
.TP
.B ERESTARTSYS
System call was interrupted by a signal; nothing was read.
(This can be seen only during a trace.)
.SH STANDARDS
Linux.
.SH 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 animals.
.\" In libc4 and libc5 the number of this call was defined by
.\" .BR SYS_klog .
.\" In glibc 2.0 the syscall is baptized
.\" .BR klogctl ().
.SH SEE ALSO
.BR dmesg (1),
.BR syslog (3),
.BR capabilities (7)