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 */