SYSLOG(3) Library Functions Manual SYSLOG(3)
syslog, syslog_r, vsyslog, vsyslog_r, openlog, openlog_r, closelog,
closelog_r, setlogmask, setlogmask_r - control system log
syslog(int priority, const char *message, ...);
syslog_r(int priority, struct syslog_data *data, const char *message,
vsyslog(int priority, const char *message, va_list args);
vsyslog_r(int priority, struct syslog_data *data, const char *message,
openlog(const char *ident, int logopt, int facility);
openlog_r(const char *ident, int logopt, int facility,
struct syslog_data *data);
closelog_r(struct syslog_data *data);
setlogmask_r(int maskpri, struct syslog_data *data);
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
The syslog_r() function is a reentrant version of the syslog() function.
It takes a pointer to a syslog_data structure which is used to store
information. This parameter must be initialized before syslog_r() is
called. The SYSLOG_DATA_INIT constant is used for this purpose.
The vsyslog() function is an alternate form in which the arguments have
already been captured using the variable-length argument facilities of
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:
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
LOG_CRON The cron daemon, cron(8).
LOG_DAEMON System daemons, such as dhcpd(8), that are not provided for
explicitly by other facilities.
LOG_FTP The file transfer protocol daemon, ftpd(8).
LOG_KERN Messages generated by the kernel. These cannot be
generated by any user processes.
LOG_LPR The line printer spooling system: lpr(1), lpc(8), lpd(8),
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
The level (ORed with the facility) is selected from the following ordered
(high to low) list:
LOG_EMERG A panic condition. This is normally broadcast to all
LOG_ALERT A condition that should be corrected immediately, such as a
corrupted system database.
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 specially.
LOG_INFO Informational messages.
LOG_DEBUG Messages that contain information normally of use only when
debugging a program.
The vsyslog_r() function is used the same way as vsyslog() except that it
takes an additional pointer to a syslog_data structure. It is a
reentrant version of the vsyslog() function described above.
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(8) it will
attempt to write the message to the console (/dev/console).
LOG_NDELAY 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. This option must be used
in programs that call chroot(2) where the new root does not
have its own log socket.
LOG_ODELAY Delay opening the connection to syslogd(8) until the first
message is logged. This is the opposite of LOG_NDELAY and
is the default behaviour when neither option is specified.
LOG_PERROR Write the message to standard error output as well as to
the system log.
LOG_PID 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 openlog_r() function is the reentrant version of the openlog()
function. It takes an additional pointer to a syslog_data structure.
This function must be used in conjunction with the other reentrant
The closelog() function can be used to close the log file. closelog_r()
does the same thing but in a reentrant way and takes an additional
pointer to a syslog_data structure.
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 priorities up to
and including toppri is given by the macro LOG_UPTO(toppri). The default
allows all priorities to be logged.
The setlogmask_r() function is the reentrant version of setlogmask(). It
takes an additional pointer to a syslog_data structure.
The closelog(), closelog_r(), openlog(), openlog_r(), syslog(),
syslog_r(), vsyslog(), and vsyslog_r() functions return no value.
The routines setlogmask() and setlogmask_r() always return 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");
For the reentrant functions:
struct syslog_data sdata = SYSLOG_DATA_INIT;
syslog_r(LOG_INFO|LOG_LOCAL2, &sdata, "foobar error: %m");
These functions appeared in 4.2BSD. The reentrant functions appeared in
It is important never to pass a string with user-supplied data as a
format without using `%s'. An attacker can put format specifiers in the
string to mangle the stack, leading to a possible security hole. This
holds true even if the string has been built ``by hand'' using a function
like snprintf(), as the resulting string may still contain user-supplied
conversion specifiers for later interpolation by syslog().
Always be sure to use the proper secure idiom:
syslog(priority, "%s", string);
syslog_r() and the other reentrant functions should only be used where
reentrancy is required (for instance, in a signal handler). syslog()
being not reentrant, only syslog_r() should be used here. For more
information about reentrancy and signal handlers, see signal(3).
OpenBSD 5.9 July 10, 2014 OpenBSD 5.9
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