SYSLOG(2)						     Linux Programmer's Manual							 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 con-
       sole_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 (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> as fol-
       lows:

       #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).

       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.)

       ENOSYS This  syslog() system call is not available, because the kernel was compiled with the CONFIG_PRINTK kernel-configuration option dis-
	      abled.

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 3.25 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/.

Linux								    2008-06-20								 SYSLOG(2)