SYSLOG(2)

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 version is called klogctl(). The type argument determines the action taken by this function. Quoting from kernel/printk.c:
/*
 * Commands to sys_syslog:
 *
 *      0 -- Close the log.  Currently a NOP.
 *      1 -- Open the log. Currently a NOP.
 *      2 -- Read from the log.
 *      3 -- Read all messages remaining in the ring buffer.
 *      4 -- Read and clear all messages remaining in the ring buffer
 *      5 -- Clear ring buffer.
 *      6 -- Disable printk to console
 *      7 -- Enable printk to console
 *      8 -- Set level of messages printed to console
 *      9 -- Return number of unread characters in the log buffer
 *     10 -- Return size of the log buffer
 */
Only command types 3 and 10 are allowed to unprivileged processes. Type 9 was added in 2.4.10; type 10 in 2.6.6.

  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. In recent kernels the size can be queried with command type 10. The call syslog(2,buf,len) waits until this kernel log buffer is nonempty, and then reads at most len bytes into the buffer buf. It returns the number of bytes read. Bytes read from the log disappear from the log buffer: the information can only be read once. This is the function executed by the kernel when a user program reads /proc/kmsg. The call syslog(3,buf,len) will read 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 (which does not clear the buffer at all). It returns the number of bytes read. The call syslog(4,buf,len) does precisely the same, but also executes the "clear ring buffer" command. The call syslog(5,dummy,dummy) executes just the "clear ring buffer" command. (In each call where buf or len is shown as "dummy", the value of the argument is ignored by the call.) The call syslog(6,dummy,dummy) sets the console log level to minimum, so that no messages are printed to the console. The call syslog(7,dummy,dummy) sets the console log level to default, so that messages are printed to the console. The call syslog(8,dummy,level) sets the console log level to level, which must be an integer between 1 and 8 (inclusive). See the loglevel section for details. The call syslog(9,dummy,dummy) returns the number of bytes currently available to be read on the kernel log buffer. The call syslog(10,dummy,dummy) returns the total size of the kernel log buffer.

  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 the call syslog(8,dummy,value). The calls syslog(type,dummy,dummy) with type equal to 6 or 7, set it 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 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. In libc4 and libc5 the number of this call was defined by SYS_klog. In glibc 2.0 the syscall is baptized klogctl().

SEE ALSO
syslog(3)

COLOPHON
This page is part of release 3.32 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/.