klogctl

       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  sys-
       logd(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 fol-
       lows:

             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/ker-
       nel/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  (regard-
       less  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
       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_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,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 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
       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
              nel  message  ring buffer by a process without sufficient privi-
              lege (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 ani-
       mals.   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
       This page is part of release 3.35 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.



Linux                             2011-09-07                         SYSLOG(2)
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