syslog (2)

Contents

NAME
SYNOPSIS
DESCRIPTION
The kernel log buffer
Commands
The loglevel
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
SEE ALSO
COLOPHON

NAME

syslog, klogctl - read and/or clear kernel message ring buffer; set console_loglevel

SYNOPSIS

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);

DESCRIPTION

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 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 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).

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 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 loglevel

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             */

RETURN VALUE

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.

ERRORS

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.)

CONFORMING TO

This system call is Linux-specific and should not be used in programs intended to be portable.

NOTES

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.

SEE ALSO

syslog(3), capabilities(7)

COLOPHON

This page is part of release 3.54 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/.