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       #include <sys/klog.h>        /* Definition of SYSLOG_* constants */
11       #include <sys/syscall.h>     /* Definition of SYS_* constants */
12       #include <unistd.h>
13
14       int syscall(SYS_syslog, int type, char *bufp, int len);
15
16       /* The glibc interface */
17       #include <sys/klog.h>
18
19       int klogctl(int type, char *bufp, int len);
20

DESCRIPTION

22       Note: Probably, you are looking for the C  library  function  syslog(),
23       which talks to syslogd(8); see syslog(3) for details.
24
25       This  page  describes the kernel syslog() system call, which is used to
26       control the kernel printk() buffer; the glibc wrapper function for  the
27       system call is called klogctl().
28
29   The kernel log buffer
30       The  kernel has a cyclic buffer of length LOG_BUF_LEN in which messages
31       given as arguments to the kernel function printk() are stored  (regard‐
32       less  of their log level).  In early kernels, LOG_BUF_LEN had the value
33       4096; from kernel 1.3.54, it was 8192;  from  kernel  2.1.113,  it  was
34       16384; since kernel 2.4.23/2.6, the value is a kernel configuration op‐
35       tion (CONFIG_LOG_BUF_SHIFT, default value dependent  on  the  architec‐
36       ture).  Since Linux 2.6.6, the size can be queried with command type 10
37       (see below).
38
39   Commands
40       The type argument determines the action taken by  this  function.   The
41       list  below  specifies the values for type.  The symbolic names are de‐
42       fined in the kernel source, but are not exported  to  user  space;  you
43       will either need to use the numbers, or define the names yourself.
44
45       SYSLOG_ACTION_CLOSE (0)
46              Close the log.  Currently a NOP.
47
48       SYSLOG_ACTION_OPEN (1)
49              Open the log.  Currently a NOP.
50
51       SYSLOG_ACTION_READ (2)
52              Read  from  the log.  The call waits until the kernel log buffer
53              is nonempty, and then reads at most len bytes  into  the  buffer
54              pointed  to by bufp.  The call returns the number of bytes read.
55              Bytes read from the log disappear from the log buffer:  the  in‐
56              formation  can be read only once.  This is the function executed
57              by the kernel when a user program reads /proc/kmsg.
58
59       SYSLOG_ACTION_READ_ALL (3)
60              Read all messages remaining in the ring buffer, placing them  in
61              the  buffer  pointed  to  by  bufp.  The call reads the last len
62              bytes from the log buffer (nondestructively), but will not  read
63              more than was written into the buffer since the last "clear ring
64              buffer" command (see command 5 below)).  The  call  returns  the
65              number of bytes read.
66
67       SYSLOG_ACTION_READ_CLEAR (4)
68              Read  and  clear all messages remaining in the ring buffer.  The
69              call does precisely the same as for a type of 3, but  also  exe‐
70              cutes the "clear ring buffer" command.
71
72       SYSLOG_ACTION_CLEAR (5)
73              The  call  executes  just  the "clear ring buffer" command.  The
74              bufp and len arguments are ignored.
75
76              This command does not really clear the ring buffer.  Rather,  it
77              sets  a  kernel bookkeeping variable that determines the results
78              returned by commands  3  (SYSLOG_ACTION_READ_ALL)  and  4  (SYS‐
79              LOG_ACTION_READ_CLEAR).   This command has no effect on commands
80              2 (SYSLOG_ACTION_READ) and 9 (SYSLOG_ACTION_SIZE_UNREAD).
81
82       SYSLOG_ACTION_CONSOLE_OFF (6)
83              The command saves the current value of console_loglevel and then
84              sets  console_loglevel  to  minimum_console_loglevel, so that no
85              messages are printed to the console.  Before Linux  2.6.32,  the
86              command    simply    sets   console_loglevel   to   minimum_con‐
87              sole_loglevel.  See the discussion  of  /proc/sys/kernel/printk,
88              below.
89
90              The bufp and len arguments are ignored.
91
92       SYSLOG_ACTION_CONSOLE_ON (7)
93              If  a  previous  SYSLOG_ACTION_CONSOLE_OFF command has been per‐
94              formed, this command restores console_loglevel to the value that
95              was  saved  by  that command.  Before Linux 2.6.32, this command
96              simply sets console_loglevel to  default_console_loglevel.   See
97              the discussion of /proc/sys/kernel/printk, below.
98
99              The bufp and len arguments are ignored.
100
101       SYSLOG_ACTION_CONSOLE_LEVEL (8)
102              The  call sets console_loglevel to the value given in len, which
103              must be an integer between 1  and  8  (inclusive).   The  kernel
104              silently  enforces  a  minimum value of minimum_console_loglevel
105              for len.  See the log level section for details.  The bufp argu‐
106              ment is ignored.
107
108       SYSLOG_ACTION_SIZE_UNREAD (9) (since Linux 2.4.10)
109              The  call  returns the number of bytes currently available to be
110              read from the  kernel  log  buffer  via  command  2  (SYSLOG_AC‐
111              TION_READ).  The bufp and len arguments are ignored.
112
113       SYSLOG_ACTION_SIZE_BUFFER (10) (since Linux 2.6.6)
114              This  command  returns  the total size of the kernel log buffer.
115              The bufp and len arguments are ignored.
116
117       All commands except 3 and 10 require privilege.  In Linux  kernels  be‐
118       fore  2.6.37,  command  types 3 and 10 are allowed to unprivileged pro‐
119       cesses; since Linux 2.6.37, these commands are allowed to  unprivileged
120       processes only if /proc/sys/kernel/dmesg_restrict has the value 0.  Be‐
121       fore  Linux  2.6.37,  "privileged"  means  that  the  caller  has   the
122       CAP_SYS_ADMIN  capability.  Since Linux 2.6.37, "privileged" means that
123       the caller has either the CAP_SYS_ADMIN capability (now deprecated  for
124       this purpose) or the (new) CAP_SYSLOG capability.
125
126   /proc/sys/kernel/printk
127       /proc/sys/kernel/printk is a writable file containing four integer val‐
128       ues that influence kernel printk() behavior when  printing  or  logging
129       error messages.  The four values are:
130
131       console_loglevel
132              Only  messages  with  a  log level lower than this value will be
133              printed to the console.  The default value for this field is DE‐
134              FAULT_CONSOLE_LOGLEVEL  (7),  but  it  is set to 4 if the kernel
135              command line contains the word "quiet", 10 if the kernel command
136              line  contains  the  word "debug", and to 15 in case of a kernel
137              fault (the 10 and 15 are just silly, and equivalent to 8).   The
138              value  of  console_loglevel  can be set (to a value in the range
139              1–8) by a syslog() call with a type of 8.
140
141       default_message_loglevel
142              This value will be used as the log level for  printk()  messages
143              that  do  not have an explicit level.  Up to and including Linux
144              2.6.38, the hard-coded  default  value  for  this  field  was  4
145              (KERN_WARNING);  since  Linux 2.6.39, the default value is a de‐
146              fined by the  kernel  configuration  option  CONFIG_DEFAULT_MES‐
147              SAGE_LOGLEVEL, which defaults to 4.
148
149       minimum_console_loglevel
150              The  value  in  this  field  is  the minimum value to which con‐
151              sole_loglevel can be set.
152
153       default_console_loglevel
154              This is the default value for console_loglevel.
155
156   The log level
157       Every printk() message has its own log level.  If the log level is  not
158       explicitly  specified  as  part  of  the  message,  it  defaults to de‐
159       fault_message_loglevel.  The conventional meaning of the log  level  is
160       as follows:
161
162       Kernel constant   Level value   Meaning
163       KERN_EMERG             0        System is unusable
164       KERN_ALERT             1        Action must be taken
165                                       immediately
166       KERN_CRIT              2        Critical conditions
167       KERN_ERR               3        Error conditions
168       KERN_WARNING           4        Warning conditions
169       KERN_NOTICE            5        Normal but  signifi‐
170                                       cant condition
171       KERN_INFO              6        Informational
172       KERN_DEBUG             7        Debug-level messages
173
174       The kernel printk() routine will print a message on the console only if
175       it has a log level less than the value of console_loglevel.
176

RETURN VALUE

178       For type equal to 2, 3, or 4, a successful call to syslog() returns the
179       number of bytes read.  For type 9, syslog() returns the number of bytes
180       currently available to be read on the kernel log buffer.  For type  10,
181       syslog()  returns  the  total size of the kernel log buffer.  For other
182       values of type, 0 is returned on success.
183
184       In case of error, -1 is returned, and errno is set to indicate the  er‐
185       ror.
186

ERRORS

188       EINVAL Bad  arguments  (e.g.,  bad type; or for type 2, 3, or 4, buf is
189              NULL, or len is less than zero; or for type 8, the level is out‐
190              side the range 1 to 8).
191
192       ENOSYS This  syslog()  system call is not available, because the kernel
193              was compiled with the CONFIG_PRINTK kernel-configuration  option
194              disabled.
195
196       EPERM  An attempt was made to change console_loglevel or clear the ker‐
197              nel message ring buffer by a process without  sufficient  privi‐
198              lege  (more  precisely:  without the CAP_SYS_ADMIN or CAP_SYSLOG
199              capability).
200
201       ERESTARTSYS
202              System call was interrupted  by  a  signal;  nothing  was  read.
203              (This can be seen only during a trace.)
204

CONFORMING TO

206       This  system  call is Linux-specific and should not be used in programs
207       intended to be portable.
208

NOTES

210       From the very start, people noted that it is unfortunate that a  system
211       call and a library routine of the same name are entirely different ani‐
212       mals.
213

SEE ALSO

215       dmesg(1), syslog(3), capabilities(7)
216

COLOPHON

218       This page is part of release 5.12 of the Linux  man-pages  project.   A
219       description  of  the project, information about reporting bugs, and the
220       latest    version    of    this    page,    can     be     found     at
221       https://www.kernel.org/doc/man-pages/.
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225Linux                             2021-03-22                         SYSLOG(2)
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