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syslog, klogctl — read and/or clear kernel message ring buffer; set console_loglevel
int
syslog( |
int type, |
| char *bufp, | |
int len); |
/* No wrapper provided in glibc */
/* The glibc interface */
#include <sys/klog.h>
int
klogctl( |
int type, |
| char *bufp, | |
int len); |
If you need the C library function syslog() (which talks to syslogd(8)), then look at
syslog(3). The system call
of this name is about controlling the kernel printk() buffer, and the glibc wrapper
function is called klogctl().
The kernel has a cyclic buffer of length LOG_BUF_LEN in which messages given as
arguments to the kernel function printk() are stored (regardless of their
loglevel). In early kernels, 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 (CONFIG_LOG_BUF_SHIFT). In recent kernels
the size can be queried with command type 10 (see
below).
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 then 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 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_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)Disable printk to console. The call sets the
console log level to the minimum, so that no messages
are printed to the console. The bufp and len arguments are
ignored.
SYSLOG_ACTION_CONSOLE_ON
(7)The call sets the console log level to the
default, so that messages are printed to the console.
The bufp and
len arguments
are ignored.
SYSLOG_ACTION_CONSOLE_LEVEL
(8)The call sets the console log level to the value
given in len,
which must be an integer between 1 and 8 (inclusive).
See the loglevel 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 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 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.
The kernel routine printk() will only print a message on the
console, if it has a loglevel less than the value of the
variable 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 a syslog() call with a type of 8. Calls to
syslog() with 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 DEFAULT_MESSAGE_LOGLEVEL −
1 (6) unless the line starts with <d>
where d is a
digit in the range 1-7, in which case the level is
d. The
conventional meaning of the loglevel is defined in
<linux/kernel.h> as follows:
#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 */
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.
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 outside the
range 1 to 8).
This syslog() system
call is not available, because the kernel was compiled
with the CONFIG_PRINTK
kernel-configuration option disabled.
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).
ERESTARTSYSSystem call was interrupted by a signal; nothing was read. (This can be seen only during a trace.)
This system call is Linux-specific and should not be used in programs intended to be portable.
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.
This page is part of release 3.52 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man−pages/.
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Copyright (C) 1995 Andries Brouwer (aebcwi.nl) %%%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. Permission is granted to copy and distribute modified versions of this 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 the use of the information contained herein. The author(s) may not 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 <aebcwi.nl> 2008-02-15, Jeremy Kerr <jkozlabs.org> Add info on command type 10; add details on types 6, 7, 8, & 9. 2008-02-15, Michael Kerrisk <mtk.manpagesgmail.com> Update LOG_BUF_LEN details; update RETURN VALUE section. |