SYSLOG(3) Library Routines SYSLOG(3)
syslog, vsyslog, syslogmt, vsyslogmt, openlog, closelog, setlogmask -
control system log
void syslog (int priority, const char *message, ...);
void vsyslog (int priority, const char *message, va_list args);
void syslogmt (int priority, const char *message, ...);
void vsyslogmt (int priority, const char *message, va_list args);
void openlog (const char *ident, int logopt, int facility);
void closelog (void);
int setlogmask (int maskpri);
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
The vsyslog function is an alternate form in which the arguments have
already been captured using the variable-length argument facilities of
The syslogmt and vsyslogmt are multithread-safe variants of the syslog
and vsyslog routines. Note that the permissable list of format speci-
fiers for these two routines is much smaller than those for syslog or
vsyslog. The full list of format specifiers is given in the
sprintmt(3) manual page. Note that syslogmt and vsyslogmt will not log
to syslogd(8) unless openlog has been called. Since openlog is not
itself thread safe, the call to openlog must be made before the process
forks if any children will be using these routines. These two routines
are GNO-specific and therefore non-portable.
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:
A panic condition. This is normally broadcast to all
A condition that should be corrected immediately, such as
a corrupted system database.
Critical conditions, e.g., hard device errors.
Conditions that are not error conditions, but should pos-
sibly be handled specially.
Messages that contain information normally of use only
when debugging a program.
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:
If syslog cannot pass the message to syslogd(8) it will
attempt to write the message to the console .ttyco.
Open the connection to syslogd(8) 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.
Write the message to standard error output as well to the
Log the process id with each message: useful for identi-
fying 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 same as LOG_AUTH, but logged to a file readable only
by selected individuals.
The cron daemon: cron(8).
System daemons, such as routed(8), that are not provided
for explicitly by other facilities.
The file transfer protocol daemon: ftpd(8).
Messages generated by the kernel. These cannot be gener-
ated by any user processes.
The line printer spooling system: lpr(1), lpc(8), lpd(8),
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
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 cal-
culated 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.
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_NDELAY, LOG_FTP);
syslog(LOG_INFO, "Connection from host %d", CallingHost);
syslog(LOG_INFO|LOG_LOCAL2, "foobar error: %m");
syslogmt and vsyslogmt are thread safe; the remainder are not. openlog
should be called in the parent thread before creating the child
threads. This will ensure that any generated messages are properly
tagged with ident.
The current implementation does not take into account the possibility
that syslogd(8) may be killed and restarted. If this happens, all log-
ging from the current process will stop until a call is made to
closelog followed by openlog, which is of course not thread safe. This
could be handled better.
logger(1), sprintmt(3), syslogd(8)
These functions appeared in 4.2BSD.
GNO 4 January 1999 SYSLOG(3)
Man(1) output converted with