SYSLOG
Section: Linux Programmer's Manual (3)
Updated: 2021-03-22
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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
syslogd(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 arguments required by the
format,
except that the two-character sequence
%m
will be replaced by
the error message string
strerror(errno).
The format string need not include a terminating newline character.
The function
vsyslog()
performs the same task as
syslog()
with the difference 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 connection 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 created 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 logging the message.
This lets the configuration file specify that messages 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 processes)
- 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 following 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 5.11 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- openlog()
-
- syslog() and vsyslog()
-
- closelog()
-
- Values for option
-
- Values for facility
-
- Values for level
-
- ATTRIBUTES
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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Time: 06:22:44 GMT, May 09, 2021