SYSLOG(3) BSD Library Functions Manual SYSLOG(3)NAME
syslog, vsyslog, openlog, closelog, setlogmask — control system log
Standard C Library (libc, -lc)
syslog(int priority, const char *message, ...);
vsyslog(int priority, const char *message, va_list args);
openlog(const char *ident, int logopt, int 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
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 gen‐
erating 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
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 possi‐
bly be handled specially.
LOG_INFO Informational messages.
LOG_DEBUG 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 ident argument 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
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.
LOG_PERROR Write the message to standard error output as well to the
LOG_PID Log the process id with each message: useful for identify‐
ing instantiations of daemons.
The facility argument 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
LOG_CONSOLE Messages written to /dev/console by the kernel console out‐
LOG_CRON The cron daemon: cron(8).
LOG_DAEMON System daemons, such as routed(8), that are not provided
for explicitly by other facilities.
LOG_FTP The file transfer protocol daemons: ftpd(8), tftpd(8).
LOG_KERN Messages generated by the kernel. These cannot be gener‐
ated 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_NTP The network time protocol system.
LOG_SECURITY Security subsystems, such as ipfw(4).
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 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 calcu‐
lated 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
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");
SEE ALSOlogger(1), syslogd(8)HISTORY
These functions appeared in 4.2BSD.
Never pass a string with user-supplied data as a format without using
‘%s’. An attacker can put format specifiers in the string to mangle your
stack, leading to a possible security hole. This holds true even if the
string was built using a function like snprintf(), as the resulting
string may still contain user-supplied conversion specifiers for later
interpolation by syslog().
Always use the proper secure idiom:
BSD December 30, 2004 BSD