klogctl(3)

1SYSLOG(2)                  Linux Programmer's Manual                 SYSLOG(2)
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NAME

6       syslog,  klogctl  -  read  and/or clear kernel message ring buffer; set
7       console_loglevel
8

SYNOPSIS

10       /* The glibc interface */
11       #include <sys/klog.h>
12
13       int klogctl(int type, char *bufp, int len);
14
15       /* The handcrafted system call */
16       #include <unistd.h>
17       #include <linux/unistd.h>
18       #include <errno.h>
19
20       _syscall3(int, syslog, int, type, char *, bufp, int, len)
21                 /* Using syscall(2) may be preferable; see intro(2) */
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23       int syslog(int type, char *bufp, int len);
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DESCRIPTION

26       If you need the libc function syslog(),  (that  talks  to  syslogd(8)),
27       then look at syslog(3).  The system call of this name is about control‐
28       ling the kernel printk()  buffer,  and  the  glibc  version  is  called
29       klogctl().
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31       The type argument determines the action taken by this function.
32
33       Quoting from kernel/printk.c:
34       /*
35        * Commands to sys_syslog:
36        *
37        *      0 -- Close the log.  Currently a NOP.
38        *      1 -- Open the log. Currently a NOP.
39        *      2 -- Read from the log.
40        *      3 -- Read up to the last 4k of messages in the ring buffer.
41        *      4 -- Read and clear last 4k of messages in the ring buffer
42        *      5 -- Clear ring buffer.
43        *      6 -- Disable printk's to console
44        *      7 -- Enable printk's to console
45        *      8 -- Set level of messages printed to console
46        *      9 -- Return number of unread characters in the log buffer
47        */
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49       Only  function  3  is  allowed  to non-root processes.  (Function 9 was
50       added in 2.4.10.)
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52       The kernel log buffer
53       The kernel has a cyclic  buffer  of  length  LOG_BUF_LEN  (4096,  since
54       1.3.54:  8192,  since 2.1.113: 16384; in recent kernels the size can be
55       set at compile time) in which messages given as argument to the  kernel
56       function printk() are stored (regardless of their loglevel).
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58       The  call  syslog()  (2,buf,len)  waits until this kernel log buffer is
59       nonempty, and then reads at most len bytes  into  the  buffer  buf.  It
60       returns  the  number  of  bytes read. Bytes read from the log disappear
61       from the log buffer: the information can only be read  once.   This  is
62       the  function  executed  by  the  kernel  when  a  user  program  reads
63       /proc/kmsg.
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65       The call syslog() (3,buf,len) will read the last len bytes from the log
66       buffer (nondestructively), but will not read more than was written into
67       the buffer since the last `clear ring buffer' command (which  does  not
68       clear the buffer at all).  It returns the number of bytes read.
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70       The  call  syslog()  (4,buf,len) does precisely the same, but also exe‐
71       cutes the `clear ring buffer' command.
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73       The call syslog() (5,dummy,idummy) only executes the `clear  ring  buf‐
74       fer' command.
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76       The loglevel
77       The  kernel  routine printk() will only print a message on the console,
78       if it has  a  loglevel  less  than  the  value  of  the  variable  con‐
79       sole_loglevel.   This  variable  initially  has  the value DEFAULT_CON‐
80       SOLE_LOGLEVEL (7), but is set to 10 if the kernel command line contains
81       the  word  `debug',  and to 15 in case of a kernel fault (the 10 and 15
82       are just silly, and equivalent to 8).  This variable is set (to a value
83       in the range 1-8) by the call syslog() (8,dummy,value).  The calls sys‐
84       log() (type,dummy,idummy) with type equal to 6 or 7, set it to 1  (ker‐
85       nel panics only) or 7 (all except debugging messages), respectively.
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87       Every  text  line  in  a  message  has  its own loglevel. This level is
88       DEFAULT_MESSAGE_LOGLEVEL - 1 (6) unless the line starts with <d>  where
89       d  is  a digit in the range 1-7, in which case the level is d. The con‐
90       ventional meaning of the loglevel is  defined  in  <linux/kernel.h>  as
91       follows:
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93       #define KERN_EMERG    "<0>"  /* system is unusable               */
94       #define KERN_ALERT    "<1>"  /* action must be taken immediately */
95       #define KERN_CRIT     "<2>"  /* critical conditions              */
96       #define KERN_ERR      "<3>"  /* error conditions                 */
97       #define KERN_WARNING  "<4>"  /* warning conditions               */
98       #define KERN_NOTICE   "<5>"  /* normal but significant condition */
99       #define KERN_INFO     "<6>"  /* informational                    */
100       #define KERN_DEBUG    "<7>"  /* debug-level messages             */
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RETURN VALUE

104       In case of error, -1 is returned, and errno is set. Otherwise, for type
105       equal to 2, 3 or 4, syslog() returns the number of bytes read, and oth‐
106       erwise 0.
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ERRORS

109       EINVAL Bad parameters.
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111       EPERM  An attempt was made to change console_loglevel or clear the ker‐
112              nel message ring buffer by a process without root permissions.
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114       ERESTARTSYS
115              System call was interrupted  by  a  signal;  nothing  was  read.
116              (This can be seen only during a trace.)
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CONFORMING TO

119       This  system  call is Linux specific and should not be used in programs
120       intended to be portable.
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NOTES

123       From the very start people noted that it  is  unfortunate  that  kernel
124       call  and  library routine of the same name are entirely different ani‐
125       mals.  In libc4 and libc5 the  number  of  this  call  was  defined  by
126       SYS_klog.  In glibc 2.0 the syscall is baptised klogctl().
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