syslog, klogctl

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  sys‐
       logd(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 (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
       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 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
       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 out‐
              side the range 1 to 8).

       ENOSYS This  syslog()  system call is not available, because the kernel
              NULL, or len is less than zero; or for type 8, the level is out‐
              side 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 ker‐
              nel message ring buffer by a process without  sufficient  privi‐
              lege (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.)

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)

COLOPHON
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       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.