1SYSLOG(2) Linux Programmer's Manual SYSLOG(2)
2
6 syslog, klogctl - read and/or clear kernel message ring buffer; set
7 console_loglevel
8
10 int syslog(int type, char *bufp, int len);
11 /* No wrapper provided in glibc */
12 /* The glibc interface */
14 #include <sys/klog.h>
15 int klogctl(int type, char *bufp, int len);
17
19 Note: Probably, you are looking for the C library function syslog(),
20 which talks to syslogd(8); see syslog(3) for details.
21 This page describes the kernel syslog() system call, which is used to
23 control the kernel printk() buffer; the glibc wrapper function for the
24 system call is called klogctl().
25 The kernel log buffer
27 The kernel has a cyclic buffer of length LOG_BUF_LEN in which messages
28 given as arguments to the kernel function printk() are stored (regard‐
29 less of their log level). In early kernels, LOG_BUF_LEN had the value
30 4096; from kernel 1.3.54, it was 8192; from kernel 2.1.113, it was
31 16384; since kernel 2.4.23/2.6, the value is a kernel configuration
32 option (CONFIG_LOG_BUF_SHIFT, default value dependent on the architec‐
33 ture). Since Linux 2.6.6, the size can be queried with command type 10
34 (see below).
35 Commands
37 The type argument determines the action taken by this function. The
38 list below specifies the values for type. The symbolic names are
39 defined in the kernel source, but are not exported to user space; you
40 will either need to use the numbers, or define the names yourself.
41 SYSLOG_ACTION_CLOSE (0)
43 Close the log. Currently a NOP.
44 SYSLOG_ACTION_OPEN (1)
46 Open the log. Currently a NOP.
47 SYSLOG_ACTION_READ (2)
49 Read from the log. The call waits until the kernel log buffer
50 is nonempty, and then reads at most len bytes into the buffer
51 pointed to by bufp. The call returns the number of bytes read.
52 Bytes read from the log disappear from the log buffer: the
53 information can be read only once. This is the function exe‐
54 cuted by the kernel when a user program reads /proc/kmsg.
55 SYSLOG_ACTION_READ_ALL (3)
57 Read all messages remaining in the ring buffer, placing them in
58 the buffer pointed to by bufp. The call reads the last len
59 bytes from the log buffer (nondestructively), but will not read
60 more than was written into the buffer since the last "clear ring
61 buffer" command (see command 5 below)). The call returns the
62 number of bytes read.
63 SYSLOG_ACTION_READ_CLEAR (4)
65 Read and clear all messages remaining in the ring buffer. The
66 call does precisely the same as for a type of 3, but also exe‐
67 cutes the "clear ring buffer" command.
68 SYSLOG_ACTION_CLEAR (5)
70 The call executes just the "clear ring buffer" command. The
71 bufp and len arguments are ignored.
72 This command does not really clear the ring buffer. Rather, it
74 sets a kernel bookkeeping variable that determines the results
75 returned by commands 3 (SYSLOG_ACTION_READ_ALL) and 4 (SYS‐
76 LOG_ACTION_READ_CLEAR). This command has no effect on commands
77 2 (SYSLOG_ACTION_READ) and 9 (SYSLOG_ACTION_SIZE_UNREAD).
78 SYSLOG_ACTION_CONSOLE_OFF (6)
80 The command saves the current value of console_loglevel and then
81 sets console_loglevel to minimum_console_loglevel, so that no
82 messages are printed to the console. Before Linux 2.6.32, the
83 command simply sets console_loglevel to minimum_con‐
84 sole_loglevel. See the discussion of /proc/sys/kernel/printk,
85 below.
86 The bufp and len arguments are ignored.
88 SYSLOG_ACTION_CONSOLE_ON (7)
90 If a previous SYSLOG_ACTION_CONSOLE_OFF command has been per‐
91 formed, this command restores console_loglevel to the value that
92 was saved by that command. Before Linux 2.6.32, this command
93 simply sets console_loglevel to default_console_loglevel. See
94 the discussion of /proc/sys/kernel/printk, below.
95 The bufp and len arguments are ignored.
97 SYSLOG_ACTION_CONSOLE_LEVEL (8)
99 The call sets console_loglevel to the value given in len, which
100 must be an integer between 1 and 8 (inclusive). The kernel
101 silently enforces a minimum value of minimum_console_loglevel
102 for len. See the log level section for details. The bufp argu‐
103 ment is ignored.
104 SYSLOG_ACTION_SIZE_UNREAD (9) (since Linux 2.4.10)
106 The call returns the number of bytes currently available to be
107 read from the kernel log buffer via command 2 (SYS‐
108 LOG_ACTION_READ). The bufp and len arguments are ignored.
109 SYSLOG_ACTION_SIZE_BUFFER (10) (since Linux 2.6.6)
111 This command returns the total size of the kernel log buffer.
112 The bufp and len arguments are ignored.
113 All commands except 3 and 10 require privilege. In Linux kernels
115 before 2.6.37, command types 3 and 10 are allowed to unprivileged pro‐
116 cesses; since Linux 2.6.37, these commands are allowed to unprivileged
117 processes only if /proc/sys/kernel/dmesg_restrict has the value 0.
118 Before Linux 2.6.37, "privileged" means that the caller has the
119 CAP_SYS_ADMIN capability. Since Linux 2.6.37, "privileged" means that
120 the caller has either the CAP_SYS_ADMIN capability (now deprecated for
121 this purpose) or the (new) CAP_SYSLOG capability.
122 /proc/sys/kernel/printk
124 /proc/sys/kernel/printk is a writable file containing four integer val‐
125 ues that influence kernel printk() behavior when printing or logging
126 error messages. The four values are:
127 console_loglevel
129 Only messages with a log level lower than this value will be
130 printed to the console. The default value for this field is
131 DEFAULT_CONSOLE_LOGLEVEL [22m(7), but it is set to 4 if the kernel
132 command line contains the word "quiet", 10 if the kernel command
133 line contains the word "debug", and to 15 in case of a kernel
134 fault (the 10 and 15 are just silly, and equivalent to 8). The
135 value of console_loglevel can be set (to a value in the range
136 1–8) by a syslog() call with a type of 8.
137 default_message_loglevel
139 This value will be used as the log level for printk() messages
140 that do not have an explicit level. Up to and including Linux
141 2.6.38, the hard-coded default value for this field was 4
142 (KERN_WARNING); since Linux 2.6.39, the default value is a
143 defined by the kernel configuration option CONFIG_DEFAULT_MES‐
144 SAGE_LOGLEVEL, which defaults to 4.
145 minimum_console_loglevel
147 The value in this field is the minimum value to which con‐
148 sole_loglevel can be set.
149 default_console_loglevel
151 This is the default value for console_loglevel.
152 The log level
154 Every printk() message has its own log level. If the log level is not
155 explicitly specified as part of the message, it defaults to
156 default_message_loglevel. The conventional meaning of the log level is
157 as follows:
158 Kernel constant Level value Meaning
160 KERN_EMERG 0 System is unusable
161 KERN_ALERT 1 Action must be taken immediately
162 KERN_CRIT 2 Critical conditions
163 KERN_ERR 3 Error conditions
164 KERN_WARNING 4 Warning conditions
165 KERN_NOTICE 5 Normal but significant condition
166 KERN_INFO 6 Informational
167 KERN_DEBUG 7 Debug-level messages
168 The kernel printk() routine will print a message on the console only if
170 it has a log level less than the value of console_loglevel.
171
173 For type equal to 2, 3, or 4, a successful call to syslog() returns the
174 number of bytes read. For type 9, syslog() returns the number of bytes
175 currently available to be read on the kernel log buffer. For type 10,
176 syslog() returns the total size of the kernel log buffer. For other
177 values of type, 0 is returned on success.
178 In case of error, -1 is returned, and errno is set to indicate the
180 error.
181
183 EINVAL Bad arguments (e.g., bad type; or for type 2, 3, or 4, buf is
184 NULL, or len is less than zero; or for type 8, the level is out‐
185 side the range 1 to 8).
186 ENOSYS This syslog() system call is not available, because the kernel
188 was compiled with the CONFIG_PRINTK kernel-configuration option
189 disabled.
190 EPERM An attempt was made to change console_loglevel or clear the ker‐
192 nel message ring buffer by a process without sufficient privi‐
193 lege (more precisely: without the CAP_SYS_ADMIN or CAP_SYSLOG
194 capability).
195 ERESTARTSYS
197 System call was interrupted by a signal; nothing was read.
198 (This can be seen only during a trace.)
199
201 This system call is Linux-specific and should not be used in programs
202 intended to be portable.
203
205 From the very start, people noted that it is unfortunate that a system
206 call and a library routine of the same name are entirely different ani‐
207 mals.
208
210 dmesg(1), syslog(3), capabilities(7)
211
213 This page is part of release 4.15 of the Linux man-pages project. A
214 description of the project, information about reporting bugs, and the
215 latest version of this page, can be found at
216 https://www.kernel.org/doc/man-pages/.
217Linux 2017-09-15 SYSLOG(2)