+'\" t
.\" Copyright (C) 1995 Andries Brouwer (aeb@cwi.nl)
+.\" and Copyright (C) 2012, 2014 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
+.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
-.\"
+.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
-.\"
+.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
.\"
.\" Written 11 June 1995 by Andries Brouwer <aeb@cwi.nl>
-.TH SYSLOG 2 2001-11-25 "Linux 1.2.9" "Linux Programmer's Manual"
+.\" 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 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
-syslog, klogctl \- read and/or clear kernel message ring buffer; set console_loglevel
+syslog, klogctl \- read and/or clear kernel message ring buffer;
+set console_loglevel
.SH SYNOPSIS
.nf
+.BI "int syslog(int " type ", char *" bufp ", int " len );
+.B " /* No wrapper provided in glibc */"
+.PP
/* The glibc interface */
-.br
.B "#include <sys/klog.h>"
-.sp
+.PP
.BI "int klogctl(int " type ", char *" bufp ", int " len );
-.sp
-/* The handcrafted system call */
-.br
-.B #include <unistd.h>
-.br
-.B #include <linux/unistd.h>
-.br
-.B #include <errno.h>
-.sp
-.B _syscall3(int, syslog, int, type, char *, bufp, int, len)
- /* Using \fBsyscall\fP(2) may be preferable; see \fBintro\fP(2) */
-.sp
-.BI "int syslog(int " type ", char *" bufp ", int " len );
.fi
.SH DESCRIPTION
-If you need the libc function
+.IR Note :
+Probably, you are looking for the C library function
.BR syslog (),
-(that talks to
-.BR syslogd (8)),
-then look at
-.BR syslog (3).
-The system call of this name is about controlling the kernel
+which talks to
+.BR syslogd (8);
+see
+.BR syslog (3)
+for details.
+.PP
+This page describes the kernel
+.BR syslog ()
+system call, which is used to control the kernel
.IR printk ()
-buffer, and the glibc version is called
+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 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
+.RB ( CONFIG_LOG_BUF_SHIFT ,
+default value dependent on the architecture).
+.\" Under "General setup" ==> "Kernel log buffer size"
+.\" For 2.6, precisely the option seems to have appeared in 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.
-
-Quoting from
-.IR kernel/printk.c :
-.nf
-/*
- * Commands to sys_syslog:
- *
- * 0 \-\- Close the log. Currently a NOP.
- * 1 \-\- Open the log. Currently a NOP.
- * 2 \-\- Read from the log.
- * 3 \-\- Read up to the last 4k of messages in the ring buffer.
- * 4 \-\- Read and clear last 4k of messages in the ring buffer
- * 5 \-\- Clear ring buffer.
- * 6 \-\- Disable printk's to console
- * 7 \-\- Enable printk's to console
- * 8 \-\- Set level of messages printed to console
- * 9 \-\- Return number of unread characters in the log buffer
- */
-.fi
-
-Only function 3 is allowed to non-root processes.
-(Function 9 was added in 2.4.10.)
-
-.B The kernel log buffer
-.br
-The kernel has a cyclic buffer of length LOG_BUF_LEN
-(4096, since 1.3.54: 8192, since 2.1.113: 16384; in recent kernels
-the size can be set at compile time) in which messages given as argument
-to the kernel function \fIprintk\fP() are stored
-(regardless of their loglevel).
-
+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
-.BR syslog ()
-.RI (2, buf , len )
-waits until this kernel log buffer is nonempty, and then reads
-at most \fIlen\fP bytes into the buffer \fIbuf\fP. It returns
-the number of bytes read. Bytes read from the log disappear from
-the log buffer: the information can only be read once.
-This is the function executed by the kernel when a user program
-reads
+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 .
-
-The call
-.BR syslog ()
-.RI (3, buf , len )
-will read the last \fIlen\fP bytes from the log buffer (nondestructively),
+.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 (which does not clear the buffer at all).
-It returns the number of bytes read.
-
-The call
-.BR syslog ()
-.RI (4, buf , len )
-does precisely the same, but also executes the `clear ring buffer' command.
-
+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
+.IR minimum_console_loglevel
+for
+.IR len .
+See the
+.IR "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.
+.PP
+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
+.IR /proc/sys/kernel/dmesg_restrict
+has the value 0.
+Before Linux 2.6.37, "privileged" means that the caller has the
+.BR CAP_SYS_ADMIN
+capability.
+Since Linux 2.6.37,
+"privileged" means that the caller has either the
+.BR CAP_SYS_ADMIN
+capability (now deprecated for this purpose) or the (new)
+.BR 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
+.IR console_loglevel
+can be set (to a value in the range 1\(en8) by a
.BR syslog ()
-.RI (5, dummy , idummy )
-only executes the `clear ring buffer' command.
-
-.B The loglevel
-.br
-The kernel routine \fIprintk\fP() will only print a message on the
-console, if it has a loglevel less than the value of the variable
+call with a
+.I type
+of 8.
+.TP
+.I default_message_loglevel
+This value will be used as the log level for
+.IR 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 a 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 .
-This variable initially has the value DEFAULT_CONSOLE_LOGLEVEL (7),
-but is set to 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).
-This variable is set (to a value in the range 1-8) by the call
+.\"
+.\"
+.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 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
+.TE
+.sp 1
+The kernel
+.IR 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 ()
-.RI (8, dummy , value ).
-The calls
+returns the number
+of bytes read.
+For \fItype\fP 9,
.BR syslog ()
-.RI ( type , dummy , idummy )
-with \fItype\fP equal to 6 or 7, set it to 1 (kernel panics only)
-or 7 (all except debugging messages), respectively.
-
-Every text line in a message has its own loglevel. This level is
-DEFAULT_MESSAGE_LOGLEVEL \- 1 (6) unless the line starts with <d>
-where \fId\fP is a digit in the range 1-7, in which case the level
-is \fId\fP. The conventional meaning of the loglevel is defined in
-.I <linux/kernel.h>
-as follows:
-
-.nf
-#define KERN_EMERG "<0>" /* system is unusable */
-#define KERN_ALERT "<1>" /* action must be taken immediately */
-#define KERN_CRIT "<2>" /* critical conditions */
-#define KERN_ERR "<3>" /* error conditions */
-#define KERN_WARNING "<4>" /* warning conditions */
-#define KERN_NOTICE "<5>" /* normal but significant condition */
-#define KERN_INFO "<6>" /* informational */
-#define KERN_DEBUG "<7>" /* debug-level messages */
-.fi
-
-.SH "RETURN VALUE"
-In case of error, \-1 is returned, and \fIerrno\fP is set. Otherwise,
-for \fItype\fP equal to 2, 3 or 4, \fBsyslog\fP() returns the number
-of bytes read, and otherwise 0.
+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.
+.PP
+In case of error, \-1 is returned,
+and \fIerrno\fP is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
-Bad parameters.
+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
+.BR CONFIG_PRINTK
+kernel-configuration option disabled.
.TP
.B EPERM
-An attempt was made to change console_loglevel or clear the kernel
-message ring buffer by a process without root permissions.
+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
+.BR 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 "CONFORMING TO"
-This system call is Linux specific and should not be used in programs
+.SH CONFORMING TO
+This system call is Linux-specific and should not be used in programs
intended to be portable.
.SH NOTES
-From the very start people noted that it is unfortunate that
-kernel call and library routine of the same name are entirely
+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 baptised
-.BR klogctl ().
-
-.SH "SEE ALSO"
-.BR syslog (3)
+.\" 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)
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