syslog, openlog, closelog, setlogmask, vsyslog - control system log
openlog(ident, logopt, facility)
syslog(priority, message, parameters ... )
vsyslog(priority, message, args)
Syslog arranges to write message onto the system log maintained by sys‐
logd(8). The message is tagged with priority. The message looks like
a printf(3) string except that %m is replaced by the current error mes‐
sage (as referenced by errno). A trailing newline is added if needed.
An alternate form, in which the arguments have already been captured
using the variable-length argument facilities of varargs(3), is avail‐
able under the name vsyslog.
This message will be read by syslogd(8) and written to the system con‐
sole, log files, or forwarded to syslogd on another host as appropri‐
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 an ordered list:
A condition that should be corrected immediately, such as a cor‐
rupted system database.
Critical conditions, e.g., hard device errors.
Messages that contain information normally of use only when
debugging a program.
A panic condition. This is normally broadcast to all users.
Conditions that are not error conditions, but should possibly be
If syslog cannot pass the message to syslogd, it will attempt to write
the message to the console (``/dev/console'') if the LOG_CONS option is
set (see below).
If special processing is needed, openlog can be called to initialize
the log file. The parameter ident is a string that is prepended to
every message. Logopt is a bit field indicating logging options. Cur‐
rent values for logopt are:
If unable to send the message to syslogd, write it to the con‐
Open the connection to syslogd immediately. Normally the open
is delayed until the first message is logged. Useful for pro‐
grams that need to manage the order in which file descriptors
Write the message to stderr as well to the system log.
Log the process id with each message: useful for identifying
instantiations of daemons.
The facility parameter encodes a default facility to be assigned to all
messages that do not have an explicit facility encoded:
The authorization system: login(1), su(1), getty(8), etc.
The clock daemon.
System daemons, such as ftpd(8), routed(8), etc, that are not
provided for explicitly by other facilities.
Messages generated by the kernel. These cannot be generated by
any user processes.
The line printer spooling system: lpr(1), lpc(8), lpd(8), etc.
The mail system.
The network news system.
Messages generated internally by syslogd(8).
Messages generated by random user processes. This is the
default facility identifier if none is specified.
The uucp system.
Reserved for local use. Similarly for LOG_LOCAL1 through
Closelog can be used to close the log file.
Setlogmask sets the log priority mask to maskpri and returns the previ‐
ous 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 priorities up to and including
toppri is given by the macro LOG_UPTO(toppri). The default allows all
priorities to be logged.
syslog(LOG_ALERT, "who: internal error 23");
openlog("ftpd", LOG_PID, LOG_DAEMON);
syslog(LOG_INFO, "Connection from host %d", CallingHost);
syslog(LOG_INFO|LOG_LOCAL2, "foobar error: %m");
SEE ALSOlogger(1), syslogd(8)4.2 Berkeley Distribution June 23, 1990 SYSLOG(3)