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);
                       /* No wrapper provided in glibc */

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

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

DESCRIPTION         top

       If you need the C library function syslog() (which talks to
       syslogd(8)), then look at syslog(3).  The system call of this name is
       about controlling the kernel printk() buffer, and the glibc wrapper
       function 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 loglevel).  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 2.4.23/2.6 the value is a kernel
       configuration option (CONFIG_LOG_BUF_SHIFT).  In recent kernels 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 then
              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)
              Disable printk to console.  The call sets the console log
              level to the minimum, so that no messages are printed to the
              console.  The bufp and len arguments are ignored.

       SYSLOG_ACTION_CONSOLE_ON (7)
              The call sets the console log level to the default, so that
              messages are printed to the console.  The bufp and len
              arguments are ignored.

       SYSLOG_ACTION_CONSOLE_LEVEL (8)
              The call sets the console log level to the value given in len,
              which must be an integer between 1 and 8 (inclusive).  See the
              loglevel 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.

   The loglevel
       The kernel routine printk() will only print a message on the console,
       if it has a loglevel less than the value of the variable
       console_loglevel.  This variable initially has the value
       DEFAULT_CONSOLE_LOGLEVEL (7), but is set to 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).  This variable
       is set (to a value in the range 1-8) by a syslog() call with a type
       of 8.  Calls to syslog() with type equal to 6 or 7 set the variable
       to 1 (kernel panics only) or 7 (all except debugging messages),
       respectively.

       Every text line in a message has its own loglevel.  This level is
       DEFAULT_MESSAGE_LOGLEVEL - 1 (6) unless the line starts with <d>
       where d is a digit in the range 1-7, in which case the level is d.
       The conventional meaning of the loglevel is defined in
       <linux/kernel.h> as follows:

       #define KERN_EMERG    "<0>"  /* system is unusable               */
       #define KERN_ALERT    "<1>"  /* action must be taken immediately */
       #define KERN_CRIT     "<2>"  /* critical conditions              */
       #define KERN_ERR      "<3>"  /* error conditions                 */
       #define KERN_WARNING  "<4>"  /* warning conditions               */
       #define KERN_NOTICE   "<5>"  /* normal but significant condition */
       #define KERN_INFO     "<6>"  /* informational                    */
       #define KERN_DEBUG    "<7>"  /* debug-level messages             */

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

       syslog(3), capabilities(7)

COLOPHON         top

       This page is part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                            2012-11-29                        SYSLOG(2)