SYSLOG(3) Linux Programmer's Manual SYSLOG(3)
closelog, openlog, syslog, vsyslog - send messages to the system logger
void openlog(const char *ident, int option, int facility);
void syslog(int priority, const char *format, ...);
void vsyslog(int priority, const char *format, va_list ap);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
Since glibc 2.19:
Glibc 2.19 and earlier:
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
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-
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() 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
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
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-
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)
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
For an explanation of the terms used in this section, see
|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
Never pass a string with user-supplied data as a format, use the fol-
syslog(priority, "%s", string);
journalctl(1), logger(1), setlogmask(3), syslog.conf(5), syslogd(8)
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Linux 2017-09-15 SYSLOG(3)