syslog(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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

NAME         top

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

SYNOPSIS         top

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

       /* 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).

   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 executed 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_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_console_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
              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 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
       /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 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.

       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         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 error.

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).

       ERESTARTSYS
              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 5.11 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                          2021-03-22                      SYSLOG(2)

Pages that refer to this page: dmesg(1)syscalls(2)proc(5)systemd.exec(5)bootparam(7)capabilities(7)rsyslogd(8)