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


       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);


       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  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

       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),

       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

       #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             */


       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


       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

              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 kernel-configuration option


       This system call is Linux-specific and should not be used  in  programs
       intended to be portable.


       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().




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