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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  sys-
       logd(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 (regard-
       less 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_CON-
       SOLE_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  sys-
       log(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  con-
       ventional  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 out-
	      side the range 1 to 8).

       EPERM  An attempt was made to change console_loglevel or	clear the ker-
	      nel message ring buffer by a process without  sufficient	privi-
	      lege (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
	      disabled.

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 ani-
       mals.   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)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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