closelog, openlog, syslog, vsyslog - send messages to the system logger
Standard C library (
libc,
-lc)
#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);
vsyslog():
Since glibc 2.19:
_DEFAULT_SOURCE
glibc 2.19 and earlier:
_BSD_SOURCE
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() 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() closes the file descriptor being used to write to the system
logger. The use of
closelog() is optional.
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.
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
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.
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 |
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.
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);
journalctl(1),
logger(1),
setlogmask(3),
syslog.conf(5),
syslogd(8)