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SYSLOG(2)		   Linux Programmer's Manual		     SYSLOG(2)

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

       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);

       Note: Probably, you are looking for the C  library  function  syslog(),
       which talks to syslogd(8); see syslog(3)	for details.

       This  page  describes the kernel	syslog() system	call, which is used to
       control the kernel printk() buffer; the glibc wrapper function for  the
       system call 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  (regard-
       less  of	their log level).  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 kernel 2.4.23/2.6, the value is a kernel configuration op-
       tion (CONFIG_LOG_BUF_SHIFT, default value dependent  on	the  architec-
       ture).  Since Linux 2.6.6, the size can be queried with command type 10
       (see below).

       The type	argument determines the	action taken by	 this  function.   The
       list  below  specifies the values for type.  The	symbolic names are de-
       fined in	the kernel source, but are not exported	 to  user  space;  you
       will either need	to use the numbers, or define the names	yourself.

	      Close the	log.  Currently	a NOP.

	      Open the log.  Currently a NOP.

	      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  in-
	      formation	 can be	read only once.	 This is the function executed
	      by the kernel when a user	program	reads /proc/kmsg.

	      Read all messages	remaining in the ring buffer, placing them  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.

	      Read  and	 clear all messages remaining in the ring buffer.  The
	      call does	precisely the same as for a type of 3, but  also  exe-
	      cutes the	"clear ring buffer" command.

	      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	 (SYS-
	      LOG_ACTION_READ_CLEAR).	This command has no effect on commands

	      The command saves	the current value of console_loglevel and then
	      sets  console_loglevel  to  minimum_console_loglevel, so that no
	      messages are printed to the console.  Before Linux  2.6.32,  the
	      command	 simply	   sets	  console_loglevel   to	  minimum_con-
	      sole_loglevel.  See the discussion  of  /proc/sys/kernel/printk,

	      The bufp and len arguments are ignored.

	      If  a  previous  SYSLOG_ACTION_CONSOLE_OFF command has been per-
	      formed, this command restores console_loglevel to	the value that
	      was  saved  by  that command.  Before Linux 2.6.32, this command
	      simply sets console_loglevel to  default_console_loglevel.   See
	      the discussion of	/proc/sys/kernel/printk, below.

	      The bufp and len arguments are ignored.

	      The  call	sets console_loglevel to the value given in len, which
	      must be an integer between 1  and	 8  (inclusive).   The	kernel
	      silently	enforces  a  minimum value of minimum_console_loglevel
	      for len.	See the	log level section for details.	The bufp argu-
	      ment 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_AC-
	      TION_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  be-
       fore  2.6.37,  command  types 3 and 10 are allowed to unprivileged pro-
       cesses; since Linux 2.6.37, these commands are allowed to  unprivileged
       processes only if /proc/sys/kernel/dmesg_restrict has the value 0.  Be-
       fore  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.

       /proc/sys/kernel/printk is a writable file containing four integer val-
       ues that	influence kernel printk() behavior when	 printing  or  logging
       error messages.	The four values	are:

	      Only  messages  with  a  log level lower than this value will be
	      printed to the console.  The default value for this field	is DE-
	      FAULT_CONSOLE_LOGLEVEL  (7),  but	 it  is	set to 4 if the	kernel
	      command line contains the	word "quiet", 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).   The
	      value  of	 console_loglevel  can be set (to a value in the range
	      1-8) by a	syslog() call with a type of 8.

	      This value will be used as the log level for  printk()  messages
	      that  do	not have an explicit level.  Up	to and including Linux
	      2.6.38, the hard-coded  default  value  for  this	 field	was  4
	      (KERN_WARNING);  since  Linux 2.6.39, the	default	value is a de-
	      fined by the  kernel  configuration  option  CONFIG_DEFAULT_MES-
	      SAGE_LOGLEVEL, which defaults to 4.

	      The  value  in  this  field  is  the minimum value to which con-
	      sole_loglevel can	be set.

	      This is the default value	for console_loglevel.

   The log level
       Every printk() message has its own log level.  If the log level is  not
       explicitly  specified  as  part	of  the	 message,  it  defaults	to de-
       fault_message_loglevel.	The conventional meaning of the	log  level  is
       as follows:

       Kernel constant	 Level value   Meaning
       KERN_EMERG	      0	       System is unusable
       KERN_ALERT	      1	       Action must be taken immediately
       KERN_CRIT	      2	       Critical	conditions
       KERN_ERR		      3	       Error conditions
       KERN_WARNING	      4	       Warning conditions
       KERN_NOTICE	      5	       Normal but significant condition
       KERN_INFO	      6	       Informational
       KERN_DEBUG	      7	       Debug-level messages

       The kernel printk() routine will	print a	message	on the console only if
       it has a	log level less than the	value of console_loglevel.

       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  er-

       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).

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

       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 or CAP_SYSLOG

	      System call was interrupted  by  a  signal;  nothing  was	 read.
	      (This can	be seen	only during a trace.)

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

       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-

       syslog(3), capabilities(7)

       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at

Linux				  2014-07-08			     SYSLOG(2)


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