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

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

       #include <sys/klog.h>        /* Definition of SYSLOG_* constants */
       #include <sys/syscall.h>     /* Definition of SYS_* constants */
       #include <unistd.h>

       int syscall(SYS_syslog, int type, char *bufp, int len);

       /* The glibc interface */
       #include <sys/klog.h>

       int klogctl(int type, char *bufp, int len);

       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

       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 (SYSLOG_ACTION_SIZE_UNREAD).

              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

       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
       KERN_CRIT              2        Critical conditions
       KERN_ERR               3        Error conditions
       KERN_WARNING           4        Warning conditions
       KERN_NOTICE            5        Normal but
       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.

       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.

       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

       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

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

       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

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

       This page is part of release 5.12 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                              2021-03-22                          SYSLOG(2)