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);
If you need the C library function syslog() (which talks to syslogd(8)), then look at sys-
log(3). The system call of this name is about controlling the kernel printk() buffer, and
the glibc wrapper function 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 loglevel). 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 2.4.23/2.6 the value is a kernel configuration option (CON-
FIG_LOG_BUF_SHIFT). In recent kernels the size can be queried with command type 10 (see
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 then 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).
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
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.
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
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-
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 (ker-
nel 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>
#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).
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 3.53 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
Linux 2012-11-29 SYSLOG(2)