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

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

       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);

       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 version is
       called klogctl().

       The type argument determines the action taken by this function.

       Quoting from kernel/printk.c:
        * Commands to sys_syslog:
        *      0 -- Close the log.  Currently a NOP.
        *      1 -- Open the log. Currently a NOP.
        *      2 -- Read from the log.
        *      3 -- Read all messages remaining in the ring buffer.
        *      4 -- Read and clear all messages remaining in the ring buffer
        *      5 -- Clear ring buffer.
        *      6 -- Disable printk to console
        *      7 -- Enable printk to console
        *      8 -- Set level of messages printed to console
        *      9 -- Return number of unread characters in the log buffer
        *     10 -- Return size of the log buffer

       Only command types 3 and 10 are allowed to unprivileged processes.
       Type 9 was added in 2.4.10; type 10 in 2.6.6.

   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.  In
       recent kernels the size can be queried with command type 10.

       The call syslog(2,buf,len) waits until this kernel log buffer is
       nonempty, and then reads at most len bytes into the buffer buf.  It
       returns the number of bytes read.  Bytes read from the log disappear
       from the log buffer: the information can only be read once.  This is
       the function executed by the kernel when a user program reads

       The call syslog(3,buf,len) will read 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 (which does not
       clear the buffer at all).  It returns the number of bytes read.

       The call syslog(4,buf,len) does precisely the same, but also executes
       the "clear ring buffer" command.

       The call syslog(5,dummy,dummy) executes just the "clear ring buffer"
       command.  (In each call where buf or len is shown as "dummy", the value
       of the argument is ignored by the call.)

       The call syslog(6,dummy,dummy) sets the console log level to minimum,
       so that no messages are printed to the console.

       The call syslog(7,dummy,dummy) sets the console log level to default,
       so that messages are printed to the console.

       The call syslog(8,dummy,level) sets the console log level to level,
       which must be an integer between 1 and 8 (inclusive).  See the loglevel
       section for details.

       The call syslog(9,dummy,dummy) returns the number of bytes currently
       available to be read on the kernel log buffer.

       The call syslog(10,dummy,dummy) returns the total size of the kernel
       log buffer.

   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 the call syslog(8,dummy,value).
       The calls syslog(type,dummy,dummy) with type equal to 6 or 7, set it to
       1 (kernel panics only) or 7 (all except debugging messages),

       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

       #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             */

       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

       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).

       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

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

       ENOSYS This syslog() system call is not available, because the kernel
              was compiled with the CONFIG_PRINTK kernel-configuration option

       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
       animals.  In libc4 and libc5 the number of this call was defined by
       SYS_klog.  In glibc 2.0 the syscall is baptized klogctl().


       This page is part of release 3.25 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can
       be found at

Linux                             2008-06-20                         SYSLOG(2)


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