| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON | The Linux Programming Interface |
SYSLOG(2) Linux Programmer's Manual SYSLOG(2)
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.
EINVAL 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).
ENOSYS This syslog() system call is not available, because the kernel
was compiled with the CONFIG_PRINTK kernel-configuration
option disabled.
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 CAP_SYS_ADMIN or
CAP_SYSLOG capability).
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.
syslog(3), capabilities(7)
This page is part of release 3.51 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/.
Linux 2012-11-29 SYSLOG(2)
HTML rendering created 2013-05-17 by Michael Kerrisk, author of The Linux Programming Interface, maintainer of the Linux man-pages project
Hosting by jambit GmbH