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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 syslog(3).  The system
       call of this name is about controlling 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 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
        */

       Only command types 3 and 10 are allowed to unprivileged processes.  Type 9 was added  in  2.4.10;  type  10  in
       2.6.6.

   The kernel log buffer
       The  kernel  has a cyclic buffer of length LOG_BUF_LEN in which messages given as arguments to the kernel func-
       tion 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 con-
       figuration 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 non-empty, 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 (non-destructively), 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 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 the call sys-
       log(8,dummy,value).  The calls syslog(type,dummy,dummy) 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 mean-
       ing 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, sys-
       log() returns the number of bytes currently available to be read on the kernel log buffer.  For type  10,  sys-
       log() 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).

       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 capability).

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

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

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.  In libc4 and libc5 the number of this call was defined by SYS_klog.  In
       glibc 2.0 the syscall is baptized klogctl().

SEE ALSO
       syslog(3)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-06-20                         SYSLOG(2)