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 libc 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 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 */
Only function 3 is allowed to non-root processes. (Function 9 was added in 2.4.10.)
The kernel log buffer
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
printk
() are stored (regardless
of their loglevel).
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
,idummy
) only executes the
`clear ring buffer' command.
The loglevel
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
,idummy
) 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 */
In case of error, −1 is returned, and errno
is set. Otherwise, for type
equal to 2, 3 or 4,
syslog
() returns the number of
bytes read, and otherwise 0.
Bad parameters.
An attempt was made to change console_loglevel or clear the kernel message ring buffer by a process without root permissions.
ERESTARTSYS
System 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. In libc4 and libc5 the number of
this call was defined by SYS_klog
. In glibc 2.0 the
syscall is baptized klogctl
().
Copyright (C) 1995 Andries Brouwer (aeb@cwi.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 <aeb@cwi.nl> |