SYSLOG(2) Linux Programmer's Manual SYSLOG(2)
NAME
syslog, klogctl - read and/or clear kernel message ring buffer; set
console_loglevel
SYNOPSIS
int syslog(int type, char *bufp, int len);
/* No wrapper provided in glibc */
/* The glibc interface */
#include <sys/klog.h>
int klogctl(int type, char *bufp, int len);
DESCRIPTION
Note: Probably, you are looking for the C library function syslog(),
which talks to syslogd(8); see syslog(3) 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 length LOG_BUF_LEN in which messages
given as arguments to the kernel function printk() are stored (regard-
less of their log level). In early kernels, LOG_BUF_LEN had the value
4096; from kernel 1.3.54, it was 8192; from kernel 2.1.113, it was
16384; since kernel 2.4.23/2.6, the value is a kernel configuration
option (CONFIG_LOG_BUF_SHIFT, default value dependent on the architec-
ture). 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 exe-
cuted 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 exe-
cutes the "clear ring buffer" command.
SYSLOG_ACTION_CLEAR (5)
The call executes just the "clear ring buffer" command. The
bufp and len 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 (SYS-
LOG_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 console_loglevel and then
sets console_loglevel to minimum_console_loglevel, so that no
messages are printed to the console. Before Linux 2.6.32, the
command simply sets console_loglevel to minimum_con-
sole_loglevel. See the discussion of /proc/sys/kernel/printk,
below.
The bufp and len arguments are ignored.
SYSLOG_ACTION_CONSOLE_ON (7)
If a previous SYSLOG_ACTION_CONSOLE_OFF command has been per-
formed, this command restores console_loglevel to the value that
was saved by that command. Before Linux 2.6.32, this command
simply sets console_loglevel to default_console_loglevel. See
the discussion of /proc/sys/kernel/printk, below.
The bufp and len arguments are ignored.
SYSLOG_ACTION_CONSOLE_LEVEL (8)
The call sets console_loglevel to the value given in len, which
must be an integer between 1 and 8 (inclusive). The kernel
silently enforces a minimum value of minimum_console_loglevel
for len. See the log level section for details. The bufp argu-
ment 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 (SYS-
LOG_ACTION_READ). The bufp and len 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 len arguments are ignored.
All commands except 3 and 10 require privilege. In Linux kernels
before 2.6.37, command types 3 and 10 are allowed to unprivileged pro-
cesses; since Linux 2.6.37, these commands are allowed to unprivileged
processes only if /proc/sys/kernel/dmesg_restrict 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 val-
ues that influence kernel printk() behavior when printing or logging
error messages. The four values are:
console_loglevel
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 console_loglevel can be set (to a value in the range
1-8) by a syslog() call with a type of 8.
default_message_loglevel
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 a
defined by the kernel configuration option CONFIG_DEFAULT_MES-
SAGE_LOGLEVEL, which defaults to 4.
minimum_console_loglevel
The value in this field is the minimum value to which con-
sole_loglevel can be set.
default_console_loglevel
This is the default value for console_loglevel.
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
default_message_loglevel. 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 console_loglevel.
RETURN VALUE
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 is set to indicate the
error.
ERRORS
EINVAL Bad arguments (e.g., bad type; or for type 2, 3, or 4, buf is
NULL, or len is less than zero; or for type 8, the level is out-
side 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 console_loglevel or clear the ker-
nel message ring buffer by a process without sufficient privi-
lege (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.)
CONFORMING TO
This system call is Linux-specific and should not be used in programs
intended to be portable.
NOTES
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 ani-
mals.
SEE ALSO
dmesg(1), syslog(3), capabilities(7)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 SYSLOG(2)
SYSLOG(3) Linux Programmer's Manual SYSLOG(3)
NAME
closelog, openlog, syslog, vsyslog - send messages to the system logger
SYNOPSIS
#include <syslog.h>
void openlog(const char *ident, int option, int facility);
void syslog(int priority, const char *format, ...);
void closelog(void);
void vsyslog(int priority, const char *format, va_list ap);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
vsyslog():
Since glibc 2.19:
_DEFAULT_SOURCE
Glibc 2.19 and earlier:
_BSD_SOURCE
DESCRIPTION
openlog()
openlog() opens a connection to the system logger for a program.
The string pointed to by ident is prepended to every message, and is
typically set to the program name. If ident is NULL, the program name
is used. (POSIX.1-2008 does not specify the behavior when ident is
NULL.)
The option argument specifies flags which control the operation of
openlog() and subsequent calls to syslog(). The facility argument
establishes a default to be used if none is specified in subsequent
calls to syslog(). The values that may be specified for option and
facility are described below.
The use of openlog() is optional; it will automatically be called by
syslog() if necessary, in which case ident will default to NULL.
syslog() and vsyslog()
syslog() generates a log message, which will be distributed by sys-
logd(8).
The priority argument is formed by ORing together a facility value and
a level value (described below). If no facility value is ORed into
priority, then the default value set by openlog() is used, or, if there
was no preceding openlog() call, a default of LOG_USER is employed.
The remaining arguments are a format, as in printf(3), and any argu-
ments required by the format, except that the two-character sequence %m
will be replaced by the error message string strerror(errno). The for-
mat string need not include a terminating newline character.
The function vsyslog() performs the same task as syslog() with the dif-
ference that it takes a set of arguments which have been obtained using
the stdarg(3) variable argument list macros.
closelog()
closelog() closes the file descriptor being used to write to the system
logger. The use of closelog() is optional.
Values for option
The option argument to openlog() is a bit mask constructed by ORing
together any of the following values:
LOG_CONS Write directly to the system console if there is an
error while sending to the system logger.
LOG_NDELAY Open the connection immediately (normally, the connec-
tion is opened when the first message is logged). This
may be useful, for example, if a subsequent chroot(2)
would make the pathname used internally by the logging
facility unreachable.
LOG_NOWAIT Don't wait for child processes that may have been cre-
ated while logging the message. (The GNU C library does
not create a child process, so this option has no effect
on Linux.)
LOG_ODELAY The converse of LOG_NDELAY; opening of the connection is
delayed until syslog() is called. (This is the default,
and need not be specified.)
LOG_PERROR (Not in POSIX.1-2001 or POSIX.1-2008.) Also log the
message to stderr.
LOG_PID Include the caller's PID with each message.
Values for facility
The facility argument is used to specify what type of program is log-
ging the message. This lets the configuration file specify that mes-
sages from different facilities will be handled differently.
LOG_AUTH security/authorization messages
LOG_AUTHPRIV security/authorization messages (private)
LOG_CRON clock daemon (cron and at)
LOG_DAEMON system daemons without separate facility value
LOG_FTP ftp daemon
LOG_KERN kernel messages (these can't be generated from user pro-
cesses)
LOG_LOCAL0 through LOG_LOCAL7
reserved for local use
LOG_LPR line printer subsystem
LOG_MAIL mail subsystem
LOG_NEWS USENET news subsystem
LOG_SYSLOG messages generated internally by syslogd(8)
LOG_USER (default)
generic user-level messages
LOG_UUCP UUCP subsystem
Values for level
This determines the importance of the message. The levels are, in
order of decreasing importance:
LOG_EMERG system is unusable
LOG_ALERT action must be taken immediately
LOG_CRIT critical conditions
LOG_ERR error conditions
LOG_WARNING warning conditions
LOG_NOTICE normal, but significant, condition
LOG_INFO informational message
LOG_DEBUG debug-level message
The function setlogmask(3) can be used to restrict logging to specified
levels only.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
+----------------------+---------------+--------------------+
|Interface | Attribute | Value |
+----------------------+---------------+--------------------+
|openlog(), closelog() | Thread safety | MT-Safe |
+----------------------+---------------+--------------------+
|syslog(), vsyslog() | Thread safety | MT-Safe env locale |
+----------------------+---------------+--------------------+
CONFORMING TO
The functions openlog(), closelog(), and syslog() (but not vsyslog())
are specified in SUSv2, POSIX.1-2001, and POSIX.1-2008.
POSIX.1-2001 specifies only the LOG_USER and LOG_LOCAL* values for
facility. However, with the exception of LOG_AUTHPRIV and LOG_FTP, the
other facility values appear on most UNIX systems.
The LOG_PERROR value for option is not specified by POSIX.1-2001 or
POSIX.1-2008, but is available in most versions of UNIX.
NOTES
The argument ident in the call of openlog() is probably stored as-is.
Thus, if the string it points to is changed, syslog() may start
prepending the changed string, and if the string it points to ceases to
exist, the results are undefined. Most portable is to use a string
constant.
Never pass a string with user-supplied data as a format, use the fol-
lowing instead:
syslog(priority, "%s", string);
SEE ALSO
journalctl(1), logger(1), setlogmask(3), syslog.conf(5), syslogd(8)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 SYSLOG(3)
SYSLOG(3P) POSIX Programmer's Manual SYSLOG(3P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAME
syslog -- log a message
SYNOPSIS
#include <syslog.h>
void syslog(int priority, const char *message, ... /* argument */);
DESCRIPTION
Refer to closelog().
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 SYSLOG(3P)
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