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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
       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 syslog(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  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 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), capabilities(7)

COLOPHON

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