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 wrapper function is called klogctl().

       The type argument determines the action taken by this function, as follows:

	     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

       Type 9 was added in Linux 2.4.10; type 10 in Linux 2.6.6.

       In Linux kernels before 2.6.37, only command types 3 and 10 are allowed to unprivileged processes.  Since Linux 2.6.37, command types 3 and
       10  are only allowed to unprivileged processes 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 capability.

   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).

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

       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 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.

SEE ALSO

       syslog(3), capabilities(7)

COLOPHON

       This page is part of release 3.44 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								    2012-05-05								 SYSLOG(2)