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

NAME

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

SYNOPSIS

       /* The glibc interface */
       #include <sys/klog.h>

       int klogctl(int type, char *bufp, int len);

       /* The handcrafted system call */
       #include <unistd.h>
       #include <linux/unistd.h>

       _syscall3(int, syslog, int, type, char *, bufp, int, len);

       int syslog(int type, char *bufp, int len);

DESCRIPTION

       If you need the libc function syslog(), (that 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
	*/

       Only function 3 is allowed to non-root processes.

       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 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 executes the `clear ring buffer' command.

       The call syslog (5,dummy,idummy) only executes the `clear ring buffer' 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 (initially DEFAULT_CONSOLE_LOGLEVEL (7), but set to 10 if the kernel commandline 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,idummy) 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 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

       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

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

       EINVAL Bad parameters.

       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							    2001-11-25								 SYSLOG(2)