man2/syslog.2

   1 .\" Copyright (C) 1995 Andries Brouwer (aeb@cwi.nl)
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  23 .\" Written 11 June 1995 by Andries Brouwer <aeb@cwi.nl>
  24 .TH SYSLOG 2  2007-06-01 "Linux" "Linux Programmer's Manual"
  25 .SH NAME
  26 syslog, klogctl \- read and/or clear kernel message ring buffer;
  27 set console_loglevel
  28 .SH SYNOPSIS
  29 .nf
  30 .BI "int syslog(int " type ", char *" bufp ", int " len );
  31 .B  "                /* No wrapper provided in glibc */
  32 .sp
  33 /* The glibc interface */
  34 .br
  35 .B "#include <sys/klog.h>"
  36 .sp
  37 .BI "int klogctl(int " type ", char *" bufp ", int " len );
  38 .fi
  39 .SH DESCRIPTION
  40 If you need the libc function
  41 .BR syslog ()
  42 (which talks to
  43 .BR syslogd (8)),
  44 then look at
  45 .BR syslog (3).
  46 The system call of this name is about controlling the kernel
  47 .IR printk ()
  48 buffer, and the glibc version is called
  49 .BR klogctl ().
  50
  51 The \fItype\fP argument determines the action taken by this function.
  52
  53 Quoting from
  54 .IR kernel/printk.c :
  55 .nf
  56 /*
  57  * Commands to sys_syslog:
  58  *
  59  *      0 \-\- Close the log.  Currently a NOP.
  60  *      1 \-\- Open the log. Currently a NOP.
  61  *      2 \-\- Read from the log.
  62  *      3 \-\- Read up to the last 4k of messages in the ring buffer.
  63  *      4 \-\- Read and clear last 4k of messages in the ring buffer
  64  *      5 \-\- Clear ring buffer.
  65  *      6 \-\- Disable printk's to console
  66  *      7 \-\- Enable printk's to console
  67  *      8 \-\- Set level of messages printed to console
  68  *      9 \-\- Return number of unread characters in the log buffer
  69  */
  70 .fi
  71
  72 Only function 3 is allowed to non-root processes.
  73 (Function 9 was added in 2.4.10.)
  74
  75 .B The kernel log buffer
  76 .br
  77 The kernel has a cyclic buffer of length LOG_BUF_LEN
  78 (4096, since 1.3.54: 8192, since 2.1.113: 16384; in recent kernels
  79 the size can be set at compile time) in which messages given as argument
  80 to the kernel function
  81 .BR printk ()
  82 are stored
  83 (regardless of their loglevel).
  84
  85 The call
  86 .BR syslog ()
  87 .RI (2, buf , len )
  88 waits until this kernel log buffer is nonempty, and then reads
  89 at most \fIlen\fP bytes into the buffer \fIbuf\fP.
  90 It returns
  91 the number of bytes read.
  92 Bytes read from the log disappear from
  93 the log buffer: the information can only be read once.
  94 This is the function executed by the kernel when a user program
  95 reads
  96 .IR /proc/kmsg .
  97
  98 The call
  99 .BR syslog ()
 100 .RI (3, buf , len )
 101 will read the last \fIlen\fP bytes from the log buffer (nondestructively),
 102 but will not read more than was written into the buffer since the
 103 last `clear ring buffer' command (which does not clear the buffer at all).
 104 It returns the number of bytes read.
 105
 106 The call
 107 .BR syslog ()
 108 .RI (4, buf , len )
 109 does precisely the same, but also executes the `clear ring buffer' command.
 110
 111 The call
 112 .BR syslog ()
 113 .RI (5, dummy , idummy )
 114 only executes the `clear ring buffer' command.
 115
 116 .B The loglevel
 117 .br
 118 The kernel routine
 119 .BR printk ()
 120 will only print a message on the
 121 console, if it has a loglevel less than the value of the variable
 122 .IR console_loglevel .
 123 This variable initially has the value DEFAULT_CONSOLE_LOGLEVEL (7),
 124 but is set to 10 if the
 125 kernel command line contains the word `debug', and to 15 in case
 126 of a kernel fault (the 10 and 15 are just silly, and equivalent to 8).
 127 This variable is set (to a value in the range 1-8) by the call
 128 .BR syslog ()
 129 .RI (8, dummy , value ).
 130 The calls
 131 .BR syslog ()
 132 .RI ( type , dummy , idummy )
 133 with \fItype\fP equal to 6 or 7, set it to 1 (kernel panics only)
 134 or 7 (all except debugging messages), respectively.
 135
 136 Every text line in a message has its own loglevel.
 137 This level is
 138 DEFAULT_MESSAGE_LOGLEVEL \- 1 (6) unless the line starts with <d>
 139 where \fId\fP is a digit in the range 1-7, in which case the level
 140 is \fId\fP.
 141 The conventional meaning of the loglevel is defined in
 142 .I <linux/kernel.h>
 143 as follows:
 144
 145 .nf
 146 #define KERN_EMERG    "<0>"  /* system is unusable               */
 147 #define KERN_ALERT    "<1>"  /* action must be taken immediately */
 148 #define KERN_CRIT     "<2>"  /* critical conditions              */
 149 #define KERN_ERR      "<3>"  /* error conditions                 */
 150 #define KERN_WARNING  "<4>"  /* warning conditions               */
 151 #define KERN_NOTICE   "<5>"  /* normal but significant condition */
 152 #define KERN_INFO     "<6>"  /* informational                    */
 153 #define KERN_DEBUG    "<7>"  /* debug-level messages             */
 154 .fi
 155 .SH "RETURN VALUE"
 156 In case of error, \-1 is returned, and \fIerrno\fP is set.
 157 Otherwise,
 158 for \fItype\fP equal to 2, 3 or 4,
 159 .BR syslog ()
 160 returns the number
 161 of bytes read, and otherwise 0.
 162 .SH ERRORS
 163 .TP
 164 .B EINVAL
 165 Bad parameters.
 166 .TP
 167 .B EPERM
 168 An attempt was made to change console_loglevel or clear the kernel
 169 message ring buffer by a process without root permissions.
 170 .TP
 171 .B ERESTARTSYS
 172 System call was interrupted by a signal; nothing was read.
 173 (This can be seen only during a trace.)
 174 .SH "CONFORMING TO"
 175 This system call is Linux specific and should not be used in programs
 176 intended to be portable.
 177 .SH NOTES
 178 From the very start people noted that it is unfortunate that
 179 kernel call and library routine of the same name are entirely
 180 different animals.
 181 In libc4 and libc5 the number of this call was defined by
 182 .BR SYS_klog .
 183 In glibc 2.0 the syscall is baptized
 184 .BR klogctl ().
 185 .SH "SEE ALSO"
 186 .BR syslog (3)