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SYSLOG(3) Library Functions Manual SYSLOG(3) NAME syslog, vsyslog, openlog, closelog, setlogmask -- control system message log LIBRARY Standard C Library (libc, -lc) SYNOPSIS #include <syslog.h> void syslog(int priority, const char *message, ...); void openlog(const char *ident, int logopt, int facility); void closelog(void); int setlogmask(int maskpri); #include <syslog.h> #include <stdarg.h> void vsyslog(int priority, const char *message, va_list args); DESCRIPTION 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 al- ready been captured using the variable-length argument facilities of stdarg(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 system database. LOG_CRIT Critical conditions, e.g., hard device errors. LOG_ERR 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 openlog() function provides for more specialized processing of the mes- sages sent by syslog() and vsyslog(). The ident argument is a string that will be prepended to every message. It may be formatted as ident[N] in which case decimal number N replaces the process id within messages. 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 at- tempt 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 de- scriptors 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 of daemons. On FreeBSD, this option is en- abled by default and cannot be disabled. The facility argument encodes a default facility to be assigned to all mes- sages 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 individuals. LOG_CONSOLE Messages written to /dev/console by the kernel console output driver. 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 generated by any user processes. 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_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 de- fault 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 priorities up to and including toppri is given by the macro LOG_UPTO(toppri);. The default allows all priorities to be logged. RETURN VALUES The routines closelog(), openlog(), syslog() and vsyslog() return no value. The routine setlogmask() always returns the previous log mask level. EXAMPLES syslog(LOG_ALERT, "who: internal error 23"); openlog("ftpd", LOG_PID | LOG_NDELAY, LOG_FTP); setlogmask(LOG_UPTO(LOG_ERR)); syslog(LOG_INFO, "Connection from host %d", CallingHost); syslog(LOG_ERR|LOG_LOCAL2, "foobar error: %m"); SEE ALSO logger(1), syslogd(8) HISTORY These functions appeared in 4.2BSD. BUGS 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: syslog(priority, "%s", string); FreeBSD 15.1 March 8, 2026 SYSLOG(3)
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | EXAMPLES | SEE ALSO | HISTORY | BUGS
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