syslog , vsyslog , openlog , closelog , setlogmask - control system log
void syslog(priority, message, ...);
void vsyslog(priority, message, args);
void openlog(ident, logopt, facility);
The syslog() function writes message to the system message logger. The message is then
written to the system console, log files, logged-in users, or forwarded to other machines
as appropriate. (See syslogd(8)).
The message is identical to a printf(3) format string, except that %m is replaced by the
current error message as denoted by the global variable errno. See strerror(3)). A
trailing newline is added if none is present.
The vsyslog() function is an alternate form in which the arguments have already been cap-
tured using the variable-length argument facilities of varargs(3).
The message is tagged with priority. Priorities are encoded as a facility and a level.
The facility describes the part of the system generating the message. The level is
selected from the following ordered (high to low) list:
LOG_EMERG A panic condition. This is normally broadcast to all users.
LOG_ALERT A condition that should be corrected immediately, such as a corrupted sys-
LOG_CRIT Critical conditions, e.g., hard device errors.
LOG_WARNING Warning messages.
LOG_NOTICE Conditions that are not error conditions, but should possibly be handled
LOG_INFO Informational messages.
LOG_DEBUG Messages that contain information normally of use only when debugging a
The openlog() function provides for more specialized processing of the messages sent by
syslog() and vsyslog(). The parameter ident is a string that will be prepended to every
message. The logopt argument is a bit field specifying logging options, which is formed
by OR'ing one or more of the following values:
LOG_CONS If syslog cannot pass the message to syslogd it will attempt to write the
message to the console (/dev/console).
LOG_NDELAY Open the connection to syslogd immediately. Normally the open is delayed
until the first message is logged. Useful for programs that need to manage
the order in which file descriptors are allocated.
LOG_PERROR Write the message to standard error output as well to the system log.
LOG_PID Log the process id with each message: useful for identifying instantiations
The facility parameter encodes a default facility to be assigned to all messages that do
not have an explicit facility encoded:
LOG_AUTH The authorization system: login(1), su(1), getty(8), etc.
LOG_AUTHPRIV The same as LOG_AUTH , but logged to a file readable only by selected indi-
LOG_CRON The clock daemon.
LOG_DAEMON System daemons, such as routed(8), that are not provided for explicitly by
LOG_KERN Messages generated by the kernel. These cannot be generated by any user
LOG_LPR The line printer spooling system: lpr(1), lpc(8), lpd(8), etc.
LOG_MAIL The mail system.
LOG_NEWS The network news system.
LOG_SYSLOG Messages generated internally by syslogd(8).
LOG_USER Messages generated by random user processes. This is the default facility
identifier if none is specified.
LOG_UUCP The uucp system.
LOG_LOCAL0 Reserved for local use. Similarly for LOG_LOCAL1 through LOG_LOCAL7.
The closelog function can be used to close the log file.
The setlogmask function sets the log priority mask to maskpri and returns the previous
mask. Calls to syslog with a priority not set in maskpri are rejected. The mask for an
individual priority pri is calculated by the macro LOG_MASK(pri). The mask for all prior-
ities up to and including toppri is given by the macro LOG_UPTO(toppri). The default
allows all priorities to be logged.
The routines closelog(), openlog(), syslog() and vsyslog() return no value.
The routine setlogmask() always returns the previous log mask level.
syslog(LOG_ALERT, "who: internal error 23");
openlog("ftpd", LOG_PID, LOG_DAEMON); setlogmask(LOG_UPTO(LOG_ERR)); sys-
log(LOG_INFO, "Connection from host %d", CallingHost);
syslog(LOG_INFO|LOG_LOCAL2, "foobar error: %m");
Under 2.11BSD the logfile /usr/adm/messages is used if a non networking kernel has been
booted. That file must be publically writeable in this case.
These functions appeared in 4.2BSD.
4.2 Berkeley Distribution April 1, 1995 SYSLOG(3)