1syslog(2) System Calls Manual syslog(2)
2
6 syslog, klogctl - read and/or clear kernel message ring buffer; set
7 console_loglevel
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10 Standard C library (libc, -lc)
11
13 #include <sys/klog.h> /* Definition of SYSLOG_* constants */
14 #include <sys/syscall.h> /* Definition of SYS_* constants */
15 #include <unistd.h>
16 int syscall(SYS_syslog, int type, char *bufp, int len);
18 /* The glibc interface */
20 #include <sys/klog.h>
21 int klogctl(int type, char *bufp, int len);
23
25 Note: Probably, you are looking for the C library function syslog(),
26 which talks to syslogd(8); see syslog(3) for details.
27 This page describes the kernel syslog() system call, which is used to
29 control the kernel printk() buffer; the glibc wrapper function for the
30 system call is called klogctl().
31 The kernel log buffer
33 The kernel has a cyclic buffer of length LOG_BUF_LEN in which messages
34 given as arguments to the kernel function printk() are stored (regard‐
35 less of their log level). In early kernels, LOG_BUF_LEN had the value
36 4096; from Linux 1.3.54, it was 8192; from Linux 2.1.113, it was 16384;
37 since Linux 2.4.23/2.6, the value is a kernel configuration option
38 (CONFIG_LOG_BUF_SHIFT, default value dependent on the architecture).
39 Since Linux 2.6.6, the size can be queried with command type 10 (see
40 below).
41 Commands
43 The type argument determines the action taken by this function. The
44 list below specifies the values for type. The symbolic names are de‐
45 fined in the kernel source, but are not exported to user space; you
46 will either need to use the numbers, or define the names yourself.
47 SYSLOG_ACTION_CLOSE (0)
49 Close the log. Currently a NOP.
50 SYSLOG_ACTION_OPEN (1)
52 Open the log. Currently a NOP.
53 SYSLOG_ACTION_READ (2)
55 Read from the log. The call waits until the kernel log buffer
56 is nonempty, and then reads at most len bytes into the buffer
57 pointed to by bufp. The call returns the number of bytes read.
58 Bytes read from the log disappear from the log buffer: the in‐
59 formation can be read only once. This is the function executed
60 by the kernel when a user program reads /proc/kmsg.
61 SYSLOG_ACTION_READ_ALL (3)
63 Read all messages remaining in the ring buffer, placing them in
64 the buffer pointed to by bufp. The call reads the last len
65 bytes from the log buffer (nondestructively), but will not read
66 more than was written into the buffer since the last "clear ring
67 buffer" command (see command 5 below)). The call returns the
68 number of bytes read.
69 SYSLOG_ACTION_READ_CLEAR (4)
71 Read and clear all messages remaining in the ring buffer. The
72 call does precisely the same as for a type of 3, but also exe‐
73 cutes the "clear ring buffer" command.
74 SYSLOG_ACTION_CLEAR (5)
76 The call executes just the "clear ring buffer" command. The
77 bufp and len arguments are ignored.
78 This command does not really clear the ring buffer. Rather, it
80 sets a kernel bookkeeping variable that determines the results
81 returned by commands 3 (SYSLOG_ACTION_READ_ALL) and 4 (SYS‐
82 LOG_ACTION_READ_CLEAR). This command has no effect on commands
83 2 (SYSLOG_ACTION_READ) and 9 (SYSLOG_ACTION_SIZE_UNREAD).
84 SYSLOG_ACTION_CONSOLE_OFF (6)
86 The command saves the current value of console_loglevel and then
87 sets console_loglevel to minimum_console_loglevel, so that no
88 messages are printed to the console. Before Linux 2.6.32, the
89 command simply sets console_loglevel to minimum_con‐
90 sole_loglevel. See the discussion of /proc/sys/kernel/printk,
91 below.
92 The bufp and len arguments are ignored.
94 SYSLOG_ACTION_CONSOLE_ON (7)
96 If a previous SYSLOG_ACTION_CONSOLE_OFF command has been per‐
97 formed, this command restores console_loglevel to the value that
98 was saved by that command. Before Linux 2.6.32, this command
99 simply sets console_loglevel to default_console_loglevel. See
100 the discussion of /proc/sys/kernel/printk, below.
101 The bufp and len arguments are ignored.
103 SYSLOG_ACTION_CONSOLE_LEVEL (8)
105 The call sets console_loglevel to the value given in len, which
106 must be an integer between 1 and 8 (inclusive). The kernel
107 silently enforces a minimum value of minimum_console_loglevel
108 for len. See the log level section for details. The bufp argu‐
109 ment is ignored.
110 SYSLOG_ACTION_SIZE_UNREAD (9) (since Linux 2.4.10)
112 The call returns the number of bytes currently available to be
113 read from the kernel log buffer via command 2 (SYSLOG_AC‐
114 TION_READ). The bufp and len arguments are ignored.
115 SYSLOG_ACTION_SIZE_BUFFER (10) (since Linux 2.6.6)
117 This command returns the total size of the kernel log buffer.
118 The bufp and len arguments are ignored.
119 All commands except 3 and 10 require privilege. In Linux kernels be‐
121 fore Linux 2.6.37, command types 3 and 10 are allowed to unprivileged
122 processes; since Linux 2.6.37, these commands are allowed to unprivi‐
123 leged processes only if /proc/sys/kernel/dmesg_restrict has the value
124 0. Before Linux 2.6.37, "privileged" means that the caller has the
125 CAP_SYS_ADMIN capability. Since Linux 2.6.37, "privileged" means that
126 the caller has either the CAP_SYS_ADMIN capability (now deprecated for
127 this purpose) or the (new) CAP_SYSLOG capability.
128 /proc/sys/kernel/printk
130 /proc/sys/kernel/printk is a writable file containing four integer val‐
131 ues that influence kernel printk() behavior when printing or logging
132 error messages. The four values are:
133 console_loglevel
135 Only messages with a log level lower than this value will be
136 printed to the console. The default value for this field is DE‐
137 FAULT_CONSOLE_LOGLEVEL [22m(7), but it is set to 4 if the kernel
138 command line contains the word "quiet", 10 if the kernel command
139 line contains the word "debug", and to 15 in case of a kernel
140 fault (the 10 and 15 are just silly, and equivalent to 8). The
141 value of console_loglevel can be set (to a value in the range
142 1–8) by a syslog() call with a type of 8.
143 default_message_loglevel
145 This value will be used as the log level for printk() messages
146 that do not have an explicit level. Up to and including Linux
147 2.6.38, the hard-coded default value for this field was 4
148 (KERN_WARNING); since Linux 2.6.39, the default value is defined
149 by the kernel configuration option CONFIG_DEFAULT_MES‐
150 SAGE_LOGLEVEL, which defaults to 4.
151 minimum_console_loglevel
153 The value in this field is the minimum value to which con‐
154 sole_loglevel can be set.
155 default_console_loglevel
157 This is the default value for console_loglevel.
158 The log level
160 Every printk() message has its own log level. If the log level is not
161 explicitly specified as part of the message, it defaults to de‐
162 fault_message_loglevel. The conventional meaning of the log level is
163 as follows:
164 Kernel constant Level value Meaning
166 KERN_EMERG 0 System is unusable
167 KERN_ALERT 1 Action must be taken
168 immediately
169 KERN_CRIT 2 Critical conditions
170 KERN_ERR 3 Error conditions
171 KERN_WARNING 4 Warning conditions
172 KERN_NOTICE 5 Normal but signifi‐
173 cant condition
174 KERN_INFO 6 Informational
175 KERN_DEBUG 7 Debug-level messages
176 The kernel printk() routine will print a message on the console only if
178 it has a log level less than the value of console_loglevel.
179
181 For type equal to 2, 3, or 4, a successful call to syslog() returns the
182 number of bytes read. For type 9, syslog() returns the number of bytes
183 currently available to be read on the kernel log buffer. For type 10,
184 syslog() returns the total size of the kernel log buffer. For other
185 values of type, 0 is returned on success.
186 In case of error, -1 is returned, and errno is set to indicate the er‐
188 ror.
189
191 EINVAL Bad arguments (e.g., bad type; or for type 2, 3, or 4, buf is
192 NULL, or len is less than zero; or for type 8, the level is out‐
193 side the range 1 to 8).
194 ENOSYS This syslog() system call is not available, because the kernel
196 was compiled with the CONFIG_PRINTK kernel-configuration option
197 disabled.
198 EPERM An attempt was made to change console_loglevel or clear the ker‐
200 nel message ring buffer by a process without sufficient privi‐
201 lege (more precisely: without the CAP_SYS_ADMIN or CAP_SYSLOG
202 capability).
203 ERESTARTSYS
205 System call was interrupted by a signal; nothing was read.
206 (This can be seen only during a trace.)
207
209 Linux.
210
212 From the very start, people noted that it is unfortunate that a system
213 call and a library routine of the same name are entirely different ani‐
214 mals.
215
217 dmesg(1), syslog(3), capabilities(7)
218Linux man-pages 6.04 2023-03-30 syslog(2)