openlog (3)
Leading comments
Written Feb 1994 by Steve Greenland (stevegr@neosoft.com) and Copyright 2001, 2017 Michael Kerrisk <mtk.manpages@gmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is ...
NAME
closelog, openlog, syslog, vsyslog - send messages to the system loggerSYNOPSIS
#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(): _BSD_SOURCE
DESCRIPTION
closelog() closes the descriptor being used to write to the system logger. The use of closelog() is optional.
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().
Values for
option
and
facility
are given 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()
generates a log message, which will be distributed by
syslogd(8).
The
priority
argument is formed by ORing the
facility
and the
level
values (explained below).
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).
A trailing newline may be added if needed.
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.
The subsections below list the parameters used to set the values of
option, facility, and priority.
option
The option argument to openlog() is an OR of any of these:- LOG_CONS
- Write directly to system console if there is an error while sending to system logger.
- LOG_NDELAY
- Open the connection immediately (normally, the connection is opened when the first message is logged).
- 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.) Print to stderr as well.
- LOG_PID
- Include PID with each message.
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
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);