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

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

SYNOPSIS
       #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);

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  (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).

   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 ex‐
              ecuted 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 executes 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 (SYSLOG_ACTION_READ_CLEAR).  This command has no effect on commands 2 (SYSLOG_ACTION_READ)  and  9  (SYSLOG_ACTION_SIZE_UN‐
              READ).

       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_console_loglevel.  See the discussion of  /proc/sys/kernel/printk,  be‐
              low.

              The bufp and len arguments are ignored.

       SYSLOG_ACTION_CONSOLE_ON (7)
              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.

       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 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 argu‐
              ments 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
       /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:

       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 de‐
              fault 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 console_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  avail‐
       able  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 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).

       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 animals.

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

Linux                                                                          2021-03-22                                                                      SYSLOG(2)