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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
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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);
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 log buffer
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 kernel 1.3.54, it was 8192; from
kernel 2.1.113, it was 16384; since kernel 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).
Commands
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 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.
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)
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.
SYSLOG_ACTION_CONSOLE_ON (7)
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.
SYSLOG_ACTION_CONSOLE_LEVEL (8)
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.
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.
/proc/sys/kernel/printk
/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_loglevel
Only 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_loglevel
This 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 a
defined by the kernel configuration option
CONFIG_DEFAULT_MESSAGE_LOGLEVEL, which defaults to 4.
minimum_console_loglevel
The value in this field is the minimum value to which
console_loglevel can be set.
default_console_loglevel
This is the default value for console_loglevel.
The log level
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.
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.
dmesg(1), syslog(3), capabilities(7)
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 SYSLOG(2)
Pages that refer to this page: dmesg(1), syscalls(2), proc(5), systemd.exec(5), bootparam(7), capabilities(7), rsyslogd(8)
Copyright and license for this manual page
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