syslog(2) - Linux manual page (original) (raw)
syslog(2) System Calls Manual syslog(2)
NAME top
syslog, klogctl - read and/or clear kernel message ring buffer;
set console_loglevelLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/klog.h>** /* Definition of **SYSLOG_*** constants */
**#include <sys/syscall.h>** /* Definition of **SYS_*** constants */
**#include <unistd.h>**
**int syscall(SYS_syslog, int** _type_**, char ***_bufp_**, int** _size_**);**
/* The glibc interface */
**#include <sys/klog.h>**
**int klogctl(int** _type_**, char ***_bufp_**, int** _size_**);**DESCRIPTION top
_Note_: Probably, you are looking for the C library function
**syslog**(), which talks to **syslogd**(8); see [syslog(3)](../man3/syslog.3.html) for details.
This page describes the kernel **syslog**() system call, which is used
to control the kernel _printk_() buffer; the glibc wrapper function
for the system call is called **klogctl**().The kernel log buffer The kernel has a cyclic buffer of size LOG_BUF_LEN in which messages given as arguments to the kernel function printk() are stored (regardless of their log level). In early kernels, LOG_BUF_LEN had the value 4096; from Linux 1.3.54, it was 8192; from Linux 2.1.113, it was 16384; since Linux 2.4.23/2.6, the value is a kernel configuration option (CONFIG_LOG_BUF_SHIFT, default value dependent on the architecture). Since Linux 2.6.6, the size can be queried with command type 10 (see below).
Commands The type argument determines the action taken by this function. The list below specifies the values for type. The symbolic names are defined in the kernel source, but are not exported to user space; you will either need to use the numbers, or define the names yourself.
**SYSLOG_ACTION_CLOSE** (0)
Close the log. Currently a NOP.
**SYSLOG_ACTION_OPEN** (1)
Open the log. Currently a NOP.
**SYSLOG_ACTION_READ** (2)
Read from the log. The call waits until the kernel log
buffer is nonempty, and then reads at most _len_ bytes into
the buffer pointed to by _bufp_. The call returns the number
of bytes read. Bytes read from the log disappear from the
log buffer: the information can be read only once. This is
the function executed by the kernel when a user program
reads _/proc/kmsg_.
**SYSLOG_ACTION_READ_ALL** (3)
Read all messages remaining in the ring buffer, placing
them in the buffer pointed to by _bufp_. The call reads the
last _len_ bytes from the log buffer (nondestructively), but
will not read more than was written into the buffer since
the last "clear ring buffer" command (see command 5
below)). The call returns the number of bytes read.
**SYSLOG_ACTION_READ_CLEAR** (4)
Read and clear all messages remaining in the ring buffer.
The call does precisely the same as for a _type_ of 3, but
also executes the "clear ring buffer" command.
**SYSLOG_ACTION_CLEAR** (5)
The call executes just the "clear ring buffer" command.
The _bufp_ and _size_ arguments are ignored.
This command does not really clear the ring buffer.
Rather, it sets a kernel bookkeeping variable that
determines the results returned by commands 3
(**SYSLOG_ACTION_READ_ALL**) and 4 (**SYSLOG_ACTION_READ_CLEAR**).
This command has no effect on commands 2
(**SYSLOG_ACTION_READ**) and 9 (**SYSLOG_ACTION_SIZE_UNREAD**).
**SYSLOG_ACTION_CONSOLE_OFF** (6)
The command saves the current value of _consoleloglevel_ and
then sets _consoleloglevel_ to _minimumconsoleloglevel_, so
that no messages are printed to the console. Before Linux
2.6.32, the command simply sets _consoleloglevel_ to
_minimumconsoleloglevel_. See the discussion of
_/proc/sys/kernel/printk_, below.
The _bufp_ and _size_ arguments are ignored.
**SYSLOG_ACTION_CONSOLE_ON** (7)
If a previous **SYSLOG_ACTION_CONSOLE_OFF** command has been
performed, this command restores _consoleloglevel_ to the
value that was saved by that command. Before Linux 2.6.32,
this command simply sets _consoleloglevel_ to
_defaultconsoleloglevel_. See the discussion of
_/proc/sys/kernel/printk_, below.
The _bufp_ and _size_ arguments are ignored.
**SYSLOG_ACTION_CONSOLE_LEVEL** (8)
The call sets _consoleloglevel_ to the value given in _size_,
which must be an integer between 1 and 8 (inclusive). The
kernel silently enforces a minimum value of
_minimumconsoleloglevel_ for _size_. See the _log level_
section for details. The _bufp_ argument is ignored.
**SYSLOG_ACTION_SIZE_UNREAD** (9) (since Linux 2.4.10)
The call returns the number of bytes currently available to
be read from the kernel log buffer via command 2
(**SYSLOG_ACTION_READ**). The _bufp_ and _size_ arguments are
ignored.
**SYSLOG_ACTION_SIZE_BUFFER** (10) (since Linux 2.6.6)
This command returns the total size of the kernel log
buffer. The _bufp_ and _size_ arguments are ignored.
All commands except 3 and 10 require privilege. In Linux kernels
before Linux 2.6.37, command types 3 and 10 are allowed to
unprivileged processes; since Linux 2.6.37, these commands are
allowed to unprivileged processes only if
_/proc/sys/kernel/dmesgrestrict_ has the value 0. Before Linux
2.6.37, "privileged" means that the caller has the **CAP_SYS_ADMIN**
capability. Since Linux 2.6.37, "privileged" means that the
caller has either the **CAP_SYS_ADMIN** capability (now deprecated for
this purpose) or the (new) **CAP_SYSLOG** capability./proc/sys/kernel/printk /proc/sys/kernel/printk is a writable file containing four integer values that influence kernel printk() behavior when printing or logging error messages. The four values are:
_consoleloglevel_
Only messages with a log level lower than this value will
be printed to the console. The default value for this
field is **DEFAULT_CONSOLE_LOGLEVEL** (7), but it is set to 4
if the kernel command line contains the word "quiet", 10 if
the kernel command line contains the word "debug", and to
15 in case of a kernel fault (the 10 and 15 are just silly,
and equivalent to 8). The value of _consoleloglevel_ can be
set (to a value in the range 1–8) by a **syslog**() call with a
_type_ of 8.
_defaultmessageloglevel_
This value will be used as the log level for _printk()_
messages that do not have an explicit level. Up to and
including Linux 2.6.38, the hard-coded default value for
this field was 4 (**KERN_WARNING**); since Linux 2.6.39, the
default value is defined by the kernel configuration option
**CONFIG_DEFAULT_MESSAGE_LOGLEVEL**, which defaults to 4.
_minimumconsoleloglevel_
The value in this field is the minimum value to which
_consoleloglevel_ can be set.
_defaultconsoleloglevel_
This is the default value for _consoleloglevel_.The log level Every printk() message has its own log level. If the log level is not explicitly specified as part of the message, it defaults to defaultmessageloglevel. The conventional meaning of the log level is as follows: Kernel constant Level value Meaning KERN_EMERG 0 System is unusable KERN_ALERT 1 Action must be taken immediately KERN_CRIT 2 Critical conditions KERN_ERR 3 Error conditions KERN_WARNING 4 Warning conditions KERN_NOTICE 5 Normal but significant condition KERN_INFO 6 Informational KERN_DEBUG 7 Debug-level messages
The kernel _printk()_ routine will print a message on the console
only if it has a log level less than the value of
_consoleloglevel_.RETURN VALUE top
For _type_ equal to 2, 3, or 4, a successful call to **syslog**()
returns the number of bytes read. For _type_ 9, **syslog**() returns
the number of bytes currently available to be read on the kernel
log buffer. For _type_ 10, **syslog**() returns the total size of the
kernel log buffer. For other values of _type_, 0 is returned on
success.
In case of error, -1 is returned, and _[errno](../man3/errno.3.html)_ is set to indicate the
error.ERRORS top
**EINVAL** Bad arguments (e.g., bad _type_; or for _type_ 2, 3, or 4, _buf_
is NULL, or _size_ is less than zero; or for _type_ 8, the
_level_ is outside the range 1 to 8).
**ENOSYS** This **syslog**() system call is not available, because the
kernel was compiled with the **CONFIG_PRINTK** kernel-
configuration option disabled.
**EPERM** An attempt was made to change _consoleloglevel_ or clear the
kernel message ring buffer by a process without sufficient
privilege (more precisely: without the **CAP_SYS_ADMIN** or
**CAP_SYSLOG** capability).
**ERESTARTSYS**
System call was interrupted by a signal; nothing was read.
(This can be seen only during a trace.)STANDARDS top
Linux.HISTORY top
From the very start, people noted that it is unfortunate that a
system call and a library routine of the same name are entirely
different animals.SEE ALSO top
[dmesg(1)](../man1/dmesg.1.html), [syslog(3)](../man3/syslog.3.html), [capabilities(7)](../man7/capabilities.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-11-17 syslog(2)
Pages that refer to this page:dmesg(1), syscalls(2), proc(5), proc_kmsg(5), proc_sys_kernel(5), systemd.exec(5), bootparam(7), capabilities(7), rsyslogd(8)