syslog man page on CentOS

Man page or keyword search:  
man Server   8420 pages
apropos Keyword Search (all sections)
Output format
CentOS logo
[printable version]

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 libc function syslog()  (which  talks  to  syslogd(8)),
       then look at syslog(3).	The system call of this name is about control‐
       ling 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 up to the last 4k of messages in the ring buffer.
	*      4 -- Read and clear last 4k of messages in the ring buffer
	*      5 -- Clear ring buffer.
	*      6 -- Disable printk's to console
	*      7 -- Enable printk's to console
	*      8 -- Set level of messages printed to console
	*      9 -- Return number of unread characters in the log buffer
	*/

       Only  function  3  is  allowed  to non-root processes.  (Function 9 was
       added in 2.4.10.)

       The kernel log buffer
       The kernel has a cyclic	buffer	of  length  LOG_BUF_LEN	 (4096,	 since
       1.3.54:	8192,  since 2.1.113: 16384; in recent kernels the size can be
       set at compile time) in which messages given as argument to the	kernel
       function printk() are stored (regardless of their loglevel).

       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 exe‐
       cutes the `clear ring buffer' command.

       The call syslog() (5,dummy,idummy) only executes the `clear  ring  buf‐
       fer' command.

       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,idummy) 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 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
       In  case	 of  error,  -1 is returned, and errno is set.	Otherwise, for
       type equal to 2, 3 or 4, syslog() returns the number of bytes read, and
       otherwise 0.

ERRORS
       EINVAL Bad parameters.

       EPERM  An attempt was made to change console_loglevel or clear the ker‐
	      nel message ring buffer by a process without root permissions.

       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	kernel
       call  and  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 baptised klogctl().

SEE ALSO
       syslog(3)

Linux 1.2.9			  2007-06-01			     SYSLOG(2)
[top]

List of man pages available for CentOS

Copyright (c) for man pages and the logo by the respective OS vendor.

For those who want to learn more, the polarhome community provides shell access and support.

[legal] [privacy] [GNU] [policy] [cookies] [netiquette] [sponsors] [FAQ]
Tweet
Polarhome, production since 1999.
Member of Polarhome portal.
Based on Fawad Halim's script.
....................................................................
Vote for polarhome
Free Shell Accounts :: the biggest list on the net