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)):
closelog() closes the descriptor being used to write to the system log-
ger. 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 typi-
cally set to the program name. The option argument specifies flags
which control the operation of openlog() and subsequent calls to sys-
log(). 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 sys-
logd(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 dif-
ference 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.
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 connec-
tion is opened when the first message is logged).
LOG_NOWAIT Don't wait for child processes that may have been cre-
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 (DEPRECATED Use
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
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 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);
logger(1), setlogmask(3), syslog.conf(5), syslogd(8)
This page is part of release 3.35 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://man7.org/linux/man-pages/.
Linux 2008-11-12 SYSLOG(3)
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