man2/syslog.2

   1 .\" Copyright (C) 1995 Andries Brouwer (aeb@cwi.nl)
   2 .\"
   3 .\" Permission is granted to make and distribute verbatim copies of this
   4 .\" manual provided the copyright notice and this permission notice are
   5 .\" preserved on all copies.
   6 .\"
   7 .\" Permission is granted to copy and distribute modified versions of this
   8 .\" manual under the conditions for verbatim copying, provided that the
   9 .\" entire resulting derived work is distributed under the terms of a
  10 .\" permission notice identical to this one.
  11 .\"
  12 .\" Since the Linux kernel and libraries are constantly changing, this
  13 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
  14 .\" responsibility for errors or omissions, or for damages resulting from
  15 .\" the use of the information contained herein.  The author(s) may not
  16 .\" have taken the same level of care in the production of this manual,
  17 .\" which is licensed free of charge, as they might when working
  18 .\" professionally.
  19 .\"
  20 .\" Formatted or processed versions of this manual, if unaccompanied by
  21 .\" the source, must acknowledge the copyright and authors of this work.
  22 .\"
  23 .\" Written 11 June 1995 by Andries Brouwer <aeb@cwi.nl>
  24 .\" 2008-02-15, Jeremy Kerr <jk@ozlabs.org>
  25 .\"     Add info on command type 10; add details on types 6, 7, 8, & 9.
  26 .\" 2008-02-15, Michael Kerrisk <mtk.manpages@gmail.com>
  27 .\"     Update LOG_BUF_LEN details; update RETURN VALUE section.
  28 .\"
  29 .TH SYSLOG 2  2012-11-29 "Linux" "Linux Programmer's Manual"
  30 .SH NAME
  31 syslog, klogctl \- read and/or clear kernel message ring buffer;
  32 set console_loglevel
  33 .SH SYNOPSIS
  34 .nf
  35 .BI "int syslog(int " type ", char *" bufp ", int " len );
  36 .B  "                /* No wrapper provided in glibc */"
  37 .sp
  38 /* The glibc interface */
  39 .br
  40 .B "#include <sys/klog.h>"
  41 .sp
  42 .BI "int klogctl(int " type ", char *" bufp ", int " len );
  43 .fi
  44 .SH DESCRIPTION
  45 If you need the C library function
  46 .BR syslog ()
  47 (which talks to
  48 .BR syslogd (8)),
  49 then look at
  50 .BR syslog (3).
  51 The system call of this name is about controlling the kernel
  52 .IR printk ()
  53 buffer, and the glibc wrapper function is called
  54 .BR klogctl ().
  55 .SS The kernel log buffer
  56 The kernel has a cyclic buffer of length
  57 .B LOG_BUF_LEN
  58 in which messages given as arguments to the kernel function
  59 .BR printk ()
  60 are stored (regardless of their loglevel).
  61 In early kernels,
  62 .B LOG_BUF_LEN
  63 had the value 4096;
  64 from kernel 1.3.54, it was 8192;
  65 from kernel 2.1.113 it was 16384;
  66 since 2.4.23/2.6 the value is a kernel configuration option
  67 .RB ( CONFIG_LOG_BUF_SHIFT ).
  68 .\" Under "General setup" ==> "Kernel log buffer size"
  69 .\" For 2.6, precisely the option seems to have appeared in 2.5.55.
  70 In recent kernels the size can be queried with command type 10 (see below).
  71 .SS Commands
  72 The \fItype\fP argument determines the action taken by this function.
  73 The list below specifies the values for
  74 .IR type .
  75 The symbolic names are defined in the kernel source,
  76 but are not exported to user space;
  77 you will either need to use the numbers, or define the names yourself.
  78 .TP
  79 .BR SYSLOG_ACTION_CLOSE " (0)"
  80 Close the log.
  81 Currently a NOP.
  82 .TP
  83 .BR SYSLOG_ACTION_OPEN " (1)"
  84 Open the log.
  85 Currently a NOP.
  86 .TP
  87 .BR SYSLOG_ACTION_READ " (2)"
  88 Read from the log.
  89 The call
  90 waits until the kernel log buffer is nonempty, and then reads
  91 at most \fIlen\fP bytes into the buffer pointed to by
  92 .IR bufp .
  93 The call returns the number of bytes read.
  94 Bytes read from the log disappear from the log buffer:
  95 the information can only be read once.
  96 This is the function executed by the kernel when a user program reads
  97 .IR /proc/kmsg .
  98 .TP
  99 .BR SYSLOG_ACTION_READ_ALL " (3)"
 100 Read all messages remaining in the ring buffer,
 101 placing then in the buffer pointed to by
 102 .IR bufp .
 103 The call reads the last \fIlen\fP
 104 bytes from the log buffer (nondestructively),
 105 but will not read more than was written into the buffer since the
 106 last "clear ring buffer" command (see command 5 below)).
 107 The call returns the number of bytes read.
 108 .TP
 109 .BR SYSLOG_ACTION_READ_CLEAR " (4)"
 110 Read and clear all messages remaining in the ring buffer.
 111 The call does precisely the same as for a
 112 .I type
 113 of 3, but also executes the "clear ring buffer" command.
 114 .TP
 115 .BR SYSLOG_ACTION_CLEAR " (5)"
 116 The call executes just the "clear ring buffer" command.
 117 The
 118 .I bufp
 119 and
 120 .I len
 121 arguments are ignored.
 122 .IP
 123 This command does not really clear the ring buffer.
 124 Rather, it sets a kernel bookkeeping variable that
 125 determines the results returned by commands 3
 126 .RB ( SYSLOG_ACTION_READ_ALL )
 127 and 4
 128 .RB ( SYSLOG_ACTION_READ_CLEAR ).
 129 This command has no effect on commands 2
 130 .RB ( SYSLOG_ACTION_READ )
 131 and 9
 132 .RB ( SYSLOG_ACTION_SIZE_UNREAD ).
 133 .TP
 134 .BR SYSLOG_ACTION_CONSOLE_OFF " (6)"
 135 Disable printk to console.
 136 The call sets the console log level to the minimum,
 137 so that no messages are printed to the console.
 138 The
 139 .I bufp
 140 and
 141 .I len
 142 arguments are ignored.
 143 .TP
 144 .BR SYSLOG_ACTION_CONSOLE_ON " (7)"
 145 The call sets the console log level to the default,
 146 so that messages are printed to the console.
 147 The
 148 .I bufp
 149 and
 150 .I len
 151 arguments are ignored.
 152 .TP
 153 .BR SYSLOG_ACTION_CONSOLE_LEVEL " (8)"
 154 The call sets the console log level to the value given in
 155 .IR len ,
 156 which must be an integer between 1 and 8 (inclusive).
 157 See the
 158 .B loglevel
 159 section for details.
 160 The
 161 .I bufp
 162 argument is ignored.
 163 .TP
 164 .BR SYSLOG_ACTION_SIZE_UNREAD " (9) (since Linux 2.4.10)"
 165 The call
 166 returns the number of bytes currently available to be read
 167 from the kernel log buffer via command 2
 168 .RB ( SYSLOG_ACTION_READ ).
 169 The
 170 .I bufp
 171 and
 172 .I len
 173 arguments are ignored.
 174 .TP
 175 .BR SYSLOG_ACTION_SIZE_BUFFER " (10) (since Linux 2.6.6)"
 176 This command returns the total size of the kernel log buffer.
 177 The
 178 .I bufp
 179 and
 180 .I len
 181 arguments are ignored.
 182 .fi
 183 .PP
 184 All commands except 3 and 10 require privilege.
 185 In Linux kernels before 2.6.37,
 186 command types 3 and 10 are allowed to unprivileged processes;
 187 since Linux 2.6.37,
 188 these commands are allowed to unprivileged processes only if
 189 .IR /proc/sys/kernel/dmesg_restrict
 190 has the value 0.
 191 Before Linux 2.6.37, "privileged" means that the caller has the
 192 .BR CAP_SYS_ADMIN
 193 capability.
 194 Since Linux 2.6.37,
 195 "privileged" means that the caller has either the
 196 .BR CAP_SYS_ADMIN
 197 capability (now deprecated for this purpose) or the (new)
 198 .BR CAP_SYSLOG
 199 capability.
 200 .SS The loglevel
 201 The kernel routine
 202 .BR printk ()
 203 will only print a message on the
 204 console, if it has a loglevel less than the value of the variable
 205 .IR console_loglevel .
 206 This variable initially has the value
 207 .B DEFAULT_CONSOLE_LOGLEVEL
 208 (7), but is set to 10 if the
 209 kernel command line contains the word "debug", and to 15 in case
 210 of a kernel fault (the 10 and 15 are just silly, and equivalent to 8).
 211 This variable is set (to a value in the range 1-8) by a
 212 .BR syslog ()
 213 call with a
 214 .I type
 215 of 8.
 216 Calls to
 217 .BR syslog ()
 218 with
 219 .I type
 220 equal to 6 or 7 set the variable to 1 (kernel panics only)
 221 or 7 (all except debugging messages), respectively.
 222
 223 Every text line in a message has its own loglevel.
 224 This level is
 225 .I "DEFAULT_MESSAGE_LOGLEVEL \- 1"
 226 (6) unless the line starts with <d>
 227 where \fId\fP is a digit in the range 1-7, in which case the level
 228 is \fId\fP.
 229 The conventional meaning of the loglevel is defined in
 230 .I <linux/kernel.h>
 231 as follows:
 232
 233 .nf
 234 #define KERN_EMERG    "<0>"  /* system is unusable               */
 235 #define KERN_ALERT    "<1>"  /* action must be taken immediately */
 236 #define KERN_CRIT     "<2>"  /* critical conditions              */
 237 #define KERN_ERR      "<3>"  /* error conditions                 */
 238 #define KERN_WARNING  "<4>"  /* warning conditions               */
 239 #define KERN_NOTICE   "<5>"  /* normal but significant condition */
 240 #define KERN_INFO     "<6>"  /* informational                    */
 241 #define KERN_DEBUG    "<7>"  /* debug-level messages             */
 242 .fi
 243 .SH "RETURN VALUE"
 244 For \fItype\fP equal to 2, 3, or 4, a successful call to
 245 .BR syslog ()
 246 returns the number
 247 of bytes read.
 248 For \fItype\fP 9,
 249 .BR syslog ()
 250 returns the number of bytes currently
 251 available to be read on the kernel log buffer.
 252 For \fItype\fP 10,
 253 .BR syslog ()
 254 returns the total size of the kernel log buffer.
 255 For other values of \fItype\fP, 0 is returned on success.
 256
 257 In case of error, \-1 is returned,
 258 and \fIerrno\fP is set to indicate the error.
 259 .SH ERRORS
 260 .TP
 261 .B EINVAL
 262 Bad arguments (e.g.,
 263 bad
 264 .IR type ;
 265 or for
 266 .I type
 267 2, 3, or 4,
 268 .I buf
 269 is NULL,
 270 or
 271 .I len
 272 is less than zero; or for
 273 .I type
 274 8, the
 275 .I level
 276 is outside the range 1 to 8).
 277 .TP
 278 .B ENOSYS
 279 This
 280 .BR syslog ()
 281 system call is not available, because the kernel was compiled with the
 282 .BR CONFIG_PRINTK
 283 kernel-configuration option disabled.
 284 .TP
 285 .B EPERM
 286 An attempt was made to change console_loglevel or clear the kernel
 287 message ring buffer by a process without sufficient privilege
 288 (more precisely: without the
 289 .B CAP_SYS_ADMIN
 290 or
 291 .BR CAP_SYSLOG
 292 capability).
 293 .TP
 294 .B ERESTARTSYS
 295 System call was interrupted by a signal; nothing was read.
 296 (This can be seen only during a trace.)
 297 .SH "CONFORMING TO"
 298 This system call is Linux-specific and should not be used in programs
 299 intended to be portable.
 300 .SH NOTES
 301 From the very start people noted that it is unfortunate that
 302 a system call and a library routine of the same name are entirely
 303 different animals.
 304 .\" In libc4 and libc5 the number of this call was defined by
 305 .\" .BR SYS_klog .
 306 .\" In glibc 2.0 the syscall is baptized
 307 .\" .BR klogctl ().
 308 .SH "SEE ALSO"
 309 .BR syslog (3),
 310 .BR capabilities (7)