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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 syslogd(8)), then look at sys-
       log(3).	The system call of this name is about controlling the kernel printk() buffer, and
       the glibc wrapper function 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 (regardless of their loglevel).  In early  ker-
       nels, 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  (CON-
       FIG_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 your-
       self.

       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  (nondestruc-
	      tively),	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 book-
	      keeping  variable  that  determines  the	results  returned  by  commands  3  (SYS-
	      LOG_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 capa-
       bility.

   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 (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 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 or CAP_SYSLOG 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 por-
       table.

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.

SEE ALSO
       syslog(3), capabilities(7)

COLOPHON
       This  page  is  part of release 3.55 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					    2012-11-29					SYSLOG(2)
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