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 version is
called klogctl().
The type argument
determines the action taken by this function.
Quoting from kernel/printk.c:
/* * 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 all messages remaining in the ring buffer. * 4 −− Read and clear all messages remaining in the ring buffer * 5 −− Clear ring buffer. * 6 −− Disable printk to console * 7 −− Enable printk to console * 8 −− Set level of messages printed to console * 9 −− Return number of unread characters in the log buffer * 10 −− Return size of the log buffer */
Only command types 3 and 10 are allowed to unprivileged processes. Type 9 was added in 2.4.10; type 10 in 2.6.6.
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. In recent kernels the size can be queried with
command type 10.
The call syslog(2,buf,len) waits
until this kernel log buffer is nonempty, and then reads at
most len bytes into
the buffer buf.
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 /proc/kmsg.
The call syslog(3,buf,len) will read
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 (which does not clear the buffer at
all). It returns the number of bytes read.
The call syslog(4,buf,len) does
precisely the same, but also executes the "clear ring
buffer" command.
The call syslog(5,dummy,dummy)
executes just the "clear ring buffer" command. (In each
call where buf or
len is shown as
"dummy", the value of the argument is ignored by the
call.)
The call syslog(6,dummy,dummy) sets
the console log level to minimum, so that no messages are
printed to the console.
The call syslog(7,dummy,dummy) sets
the console log level to default, so that messages are
printed to the console.
The call syslog(8,dummy,level) sets
the console log level to level, which must be an
integer between 1 and 8 (inclusive). See the loglevel section for
details.
The call syslog(9,dummy,dummy)
returns the number of bytes currently available to be read
on the kernel log buffer.
The call syslog(10,dummy,dummy)
returns the total size of the kernel log buffer.
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 the call syslog(8,dummy,value). The
calls syslog(type,dummy,dummy)
with type 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 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).
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
capability).
ERESTARTSYSSystem call was interrupted by a signal; nothing was read. (This can be seen only during a trace.)
This syslog() system
call is not available, because the kernel was compiled
with the CONFIG_PRINTK
kernel-configuration option disabled.
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. In libc4 and libc5 the number of
this call was defined by SYS_klog. In glibc 2.0 the
syscall is baptized klogctl().
This page is part of release 3.24 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) 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. 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. |