man-pages/man2/syslog.2

.\" Copyright (C) 1995 Andries Brouwer (aeb@cwi.nl)
.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
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.\" 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  2012-11-29 "Linux" "Linux Programmer's Manual"
.SH NAME
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 */"
.sp
/* The glibc interface */
.br
.B "#include <sys/klog.h>"
.sp
.BI "int klogctl(int " type ", char *" bufp ", int " len );
.fi
.SH DESCRIPTION
If you need the C library function
.BR syslog ()
(which talks to
.BR syslogd (8)),
then look at
.BR syslog (3).
The system call of this name is about controlling the kernel
.IR printk ()
buffer, and the glibc wrapper function 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 loglevel).
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 2.4.23/2.6 the value is a kernel configuration option
.RB ( CONFIG_LOG_BUF_SHIFT ).
.\" Under "General setup" ==> "Kernel log buffer size"
.\" For 2.6, precisely the option seems to have appeared in 2.5.55.
In recent kernels 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 only be read 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 then 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)"
Disable printk to console.
The call sets the console log level to the minimum,
so that no messages are printed to the console.
The
.I bufp
and
.I len
arguments are ignored.
.TP
.BR SYSLOG_ACTION_CONSOLE_ON " (7)"
The call sets the console log level to the default,
so that messages are printed to the console.
The
.I bufp
and
.I len
arguments are ignored.
.TP
.BR SYSLOG_ACTION_CONSOLE_LEVEL " (8)"
The call sets the console log level to the value given in
.IR len ,
which must be an integer between 1 and 8 (inclusive).
See the
.B loglevel
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.
.fi
.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 The loglevel
The kernel routine
.BR printk ()
will only print a message on the
console, if it has a loglevel less than the value of the variable
.IR console_loglevel .
This variable initially has the value
.B 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 a
.BR syslog ()
call with a
.I type
of 8.
Calls to
.BR syslog ()
with
.I type
equal to 6 or 7 set the variable 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
.I "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"
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.

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
.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 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
intended to be portable.
.SH NOTES
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 syslog (3),
.BR capabilities (7)