syslog, klogctl - read and/or clear kernel message ring buffer; set console_loglevel
Standard C library (libc, -lc)
#include <sys/klog.h> /* Definition of SYSLOG_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_syslog, int type, char *bufp, int len);
/* The glibc interface */
#include <sys/klog.h>
int klogctl(int type, char *bufp, int len);Note: Probably, you are looking for the C library function
syslog(), which talks to syslogd(8);
see syslog(3) for details.
This page describes the kernel syslog() system call,
which is used to control the kernel printk() buffer; the glibc
wrapper function for the system call 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 log level). In early kernels, LOG_BUF_LEN had the value 4096; from Linux 1.3.54, it was 8192; from Linux 2.1.113, it was 16384; since Linux 2.4.23/2.6, the value is a kernel configuration option (CONFIG_LOG_BUF_SHIFT, default value dependent on the architecture). Since Linux 2.6.6, 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.
Close the log. Currently a NOP.
Open the log. Currently a NOP.
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.
Read all messages remaining in the ring buffer, placing them 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.
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.
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).
The command saves the current value of console_loglevel and
then sets console_loglevel to
minimum_console_loglevel, so that no messages are printed to
the console. Before Linux 2.6.32, the command simply sets
console_loglevel to minimum_console_loglevel. See the
discussion of /proc/sys/kernel/printk, below.
The bufp and len arguments are ignored.
If a previous SYSLOG_ACTION_CONSOLE_OFF command has
been performed, this command restores console_loglevel to the
value that was saved by that command. Before Linux 2.6.32, this command
simply sets console_loglevel to
default_console_loglevel. See the discussion of
/proc/sys/kernel/printk, below.
The bufp and len arguments are ignored.
The call sets console_loglevel to the value given in
len, which must be an integer between 1 and 8 (inclusive). The
kernel silently enforces a minimum value of
minimum_console_loglevel for len. See the log
level section for details. The bufp argument is
ignored.
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.
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 Linux 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.
/proc/sys/kernel/printk is a writable file containing four
integer values that influence kernel printk() behavior when
printing or logging error messages. The four values are:
console_loglevelOnly messages with a log level lower than this value will be printed
to the console. The default value for this field is
DEFAULT_CONSOLE_LOGLEVEL (7), but it is set to 4 if the
kernel command line contains the word "quiet", 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). The value of
console_loglevel can be set (to a value in the range 1–8) by a
syslog() call with a type of 8.
default_message_loglevelThis value will be used as the log level for printk()
messages that do not have an explicit level. Up to and including Linux
2.6.38, the hard-coded default value for this field was 4
(KERN_WARNING); since Linux 2.6.39, the default value
is defined by the kernel configuration option
CONFIG_DEFAULT_MESSAGE_LOGLEVEL, which defaults to
4.
minimum_console_loglevelThe value in this field is the minimum value to which
console_loglevel can be set.
default_console_loglevelThis is the default value for console_loglevel.
Every printk() message has its own log level. If the log
level is not explicitly specified as part of the message, it defaults to
default_message_loglevel. The conventional meaning of the log
level is as follows:
| Kernel constant | Level value | Meaning |
| KERN_EMERG | 0 | System is unusable |
| KERN_ALERT | 1 | Action must be taken immediately |
| KERN_CRIT | 2 | Critical conditions |
| KERN_ERR | 3 | Error conditions |
| KERN_WARNING | 4 | Warning conditions |
| KERN_NOTICE | 5 | Normal but significant condition |
| KERN_INFO | 6 | Informational |
| KERN_DEBUG | 7 | Debug-level messages |
The kernel printk() routine will print a message on the
console only if it has a log level less than the value of
console_loglevel.
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).
System call was interrupted by a signal; nothing was read. (This can be seen only during a trace.)
Linux.
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.
dmesg(1), syslog(3), capabilities(7)