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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
       Note: Probably, you are looking for the C  library  function  syslog(),
       which talks to syslogd(8); see syslog(3) for details.
       This  page  describes the kernel syslog() system call, which is used to
       control the kernel printk() buffer; the glibc wrapper function for  the
       system call is called klogctl().
   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 log level).  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  kernel  2.4.23/2.6,  the value is a kernel configuration
       option (CONFIG_LOG_BUF_SHIFT, default value dependent on the  architec-
       ture).  Since Linux 2.6.6, the size can be queried with command type 10
       (see below).
   Commands
       The type argument determines the action taken by  this  function.   The
       list  below  specifies  the  values  for  type.  The symbolic names are
       defined in the kernel source, but are not exported to user  space;  you
       will either need to use the numbers, or define the names yourself.
       SYSLOG_ACTION_CLOSE (0)
              Close the log.  Currently a NOP.
       SYSLOG_ACTION_OPEN (1)
              Open the log.  Currently a NOP.
       SYSLOG_ACTION_READ (2)
              Read  from  the log.  The call waits until the kernel log buffer
              is nonempty, and then reads at most len bytes  into  the  buffer
              pointed  to by bufp.  The call returns the number of bytes read.
              Bytes read from the log  disappear  from  the  log  buffer:  the
              information  can  be  read only once.  This is the function exe-
              cuted by the kernel when a user program reads /proc/kmsg.
       SYSLOG_ACTION_READ_ALL (3)
              Read all messages remaining in the ring buffer, placing them  in
              the  buffer  pointed  to  by  bufp.  The call reads 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 (see command 5 below)).  The  call  returns  the
              number of bytes read.
       SYSLOG_ACTION_READ_CLEAR (4)
              Read  and  clear all messages remaining in the ring buffer.  The
              call does precisely the same as for a type of 3, but  also  exe-
              cutes the "clear ring buffer" command.
       SYSLOG_ACTION_CLEAR (5)
              The  call  executes  just  the "clear ring buffer" command.  The
              bufp and len arguments are ignored.
              This command does not really clear the ring buffer.  Rather,  it
              sets  a  kernel bookkeeping variable that determines the results
              returned by commands  3  (SYSLOG_ACTION_READ_ALL)  and  4  (SYS-
              LOG_ACTION_READ_CLEAR).   This command has no effect on commands
              2 (SYSLOG_ACTION_READ) and 9 (SYSLOG_ACTION_SIZE_UNREAD).
       SYSLOG_ACTION_CONSOLE_OFF (6)
              The command saves the current value of console_loglevel and then
              sets  console_loglevel  to  minimum_console_loglevel, so that no
              messages are printed to the console.  Before Linux  2.6.32,  the
              command    simply    sets   console_loglevel   to   minimum_con-
              sole_loglevel.  See the discussion  of  /proc/sys/kernel/printk,
              below.
              The bufp and len arguments are ignored.
       SYSLOG_ACTION_CONSOLE_ON (7)
              If  a  previous  SYSLOG_ACTION_CONSOLE_OFF command has been per-
              formed, this command restores console_loglevel to the value that
              was  saved  by  that command.  Before Linux 2.6.32, this command
              simply sets console_loglevel to  default_console_loglevel.   See
              the discussion of /proc/sys/kernel/printk, below.
              The bufp and len arguments are ignored.
       SYSLOG_ACTION_CONSOLE_LEVEL (8)
              The  call sets console_loglevel to the value given in len, which
              must be an integer between 1  and  8  (inclusive).   The  kernel
              silently  enforces  a  minimum value of minimum_console_loglevel
              for len.  See the log level section for details.  The bufp argu-
              ment is ignored.
       SYSLOG_ACTION_SIZE_UNREAD (9) (since Linux 2.4.10)
              The  call  returns the number of bytes currently available to be
              read  from  the  kernel  log  buffer   via   command   2   (SYS-
              LOG_ACTION_READ).  The bufp and len arguments are ignored.
       SYSLOG_ACTION_SIZE_BUFFER (10) (since Linux 2.6.6)
              This  command  returns  the total size of the kernel log buffer.
              The bufp and len arguments are ignored.
       All commands except 3 and  10  require  privilege.   In  Linux  kernels
       before  2.6.37, command types 3 and 10 are allowed to unprivileged pro-
       cesses; since Linux 2.6.37, these commands are allowed to  unprivileged
       processes  only  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.
   /proc/sys/kernel/printk
       /proc/sys/kernel/printk is a writable file containing four integer val-
       ues that influence kernel printk() behavior when  printing  or  logging
       error messages.  The four values are:
       console_loglevel
              Only  messages  with  a  log level lower than this value will be
              printed to the console.  The default value  for  this  field  is
              DEFAULT_CONSOLE_LOGLEVEL  (7),  but it is set to 4 if the kernel
              command line contains the word "quiet", 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).   The
              value  of  console_loglevel  can be set (to a value in the range
              1-8) by a syslog() call with a type of 8.
       default_message_loglevel
              This value will be used as the log level for  printk()  messages
              that  do  not have an explicit level.  Up to and including Linux
              2.6.38, the hard-coded  default  value  for  this  field  was  4
              (KERN_WARNING);  since  Linux  2.6.39,  the  default  value is a
              defined by the kernel configuration  option  CONFIG_DEFAULT_MES-
              SAGE_LOGLEVEL, which defaults to 4.
       minimum_console_loglevel
              The  value  in  this  field  is  the minimum value to which con-
              sole_loglevel can be set.
       default_console_loglevel
              This is the default value for console_loglevel.
   The log level
       Every printk() message has its own log level.  If the log level is  not
       explicitly   specified   as   part  of  the  message,  it  defaults  to
       default_message_loglevel.  The conventional meaning of the log level is
       as follows:
       Kernel constant   Level value   Meaning
       KERN_EMERG             0        System is unusable
       KERN_ALERT             1        Action must be taken immediately
       KERN_CRIT              2        Critical conditions
       KERN_ERR               3        Error conditions
       KERN_WARNING           4        Warning conditions
       KERN_NOTICE            5        Normal but significant condition
       KERN_INFO              6        Informational
       KERN_DEBUG             7        Debug-level messages
       The kernel printk() routine will print a message on the console only if
       it has a log level less than the value of console_loglevel.
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
              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 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.
SEE ALSO
       dmesg(1), syslog(3), capabilities(7)
COLOPHON
       This page is part of release 4.15 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.
Linux                             2017-09-15                         SYSLOG(2)