syslog(2) - phpMan
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 */
int klogctl(int type, char *bufp, int len);
Note: Probably, you are looking for the C library function syslog(), which talks to sys‐
logd(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 ker‐
nels, 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).
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 your‐
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 (nondestruc‐
tively), 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
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
This command does not really clear the ring buffer. Rather, it sets a kernel book‐
keeping variable that determines the results returned by commands 3 (SYS‐
LOG_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 con‐
sole_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 mini‐
mum_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_con‐
sole_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 mini‐
mum_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
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 capa‐
/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:
Only messages with a log level lower than this value will be printed to the con‐
sole. 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.
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
The value in this field is the minimum value to which console_loglevel can be set.
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 speci‐
fied 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).
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 por‐
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.
This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.
Linux 2014-07-08 SYSLOG(2)