klogctl(3) — Linux manual page


SYSLOG(2)                 Linux Programmer's Manual                SYSLOG(2)

NAME         top

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

SYNOPSIS         top

       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         top

       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 (regardless 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 architecture).  Since Linux 2.6.6, the size can be
       queried with command type 10 (see below).

       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.

              Close the log.  Currently a NOP.

              Open the log.  Currently a NOP.

              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
              executed by the kernel when a user program reads /proc/kmsg.

              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.

              Read and clear all messages remaining in the ring buffer.  The
              call does precisely the same as for a type of 3, but also
              executes the "clear ring buffer" command.

              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
              (SYSLOG_ACTION_READ_CLEAR).  This command has no effect on
              commands 2 (SYSLOG_ACTION_READ) and 9

              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_console_loglevel.  See the discussion of
              /proc/sys/kernel/printk, below.

              The bufp and len arguments are ignored.

              If a previous SYSLOG_ACTION_CONSOLE_OFF command has been
              performed, 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.

              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 argument 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
              (SYSLOG_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
       processes; 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 is a writable file containing four integer
       values that influence kernel printk() behavior when printing or
       logging error messages.  The four values are:

              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.

              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_MESSAGE_LOGLEVEL, which defaults to 4.

              The value in this field is the minimum value to which
              console_loglevel can be set.

              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         top

       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

ERRORS         top

       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).

              System call was interrupted by a signal; nothing was read.
              (This can be seen only during a trace.)

CONFORMING TO         top

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

NOTES         top

       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.

SEE ALSO         top

       dmesg(1), syslog(3), capabilities(7)

COLOPHON         top

       This page is part of release 5.08 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

Linux                            2020-08-13                        SYSLOG(2)