Updated: 2022/Sep/29

Please read Privacy Policy. It's for your privacy.

SYSLOG(3)                  Library Functions Manual                  SYSLOG(3)

     syslog, syslog_r, vsyslog, vsyslog_r, syslogp, syslogp_r, vsyslogp,
     vsyslogp_r, openlog, openlog_r, closelog, closelog_r, setlogmask,
     setlogmask_r - control system log

     Standard C Library (libc, -lc)

     #include <syslog.h>

     syslog(int priority, const char *message, ...);

     syslog_r(int priority, struct syslog_data *data, const char *message,

     syslogp(int priority, const char *msgid, const char *sdfmt,
         const char *message, ...);

     syslogp_r(int priority, struct syslog_data *data, const char *msgid,
         const char *sdfmt, 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(int maskpri);

     setlogmask_r(int maskpri, struct syslog_data *data);

     #include <stdarg.h>

     vsyslog(int priority, const char *message, va_list args);

     vsyslog_r(int priority, struct syslog_data *data, const char *message,
         va_list args);

     vsyslogp(int priority, const char *msgid, const char *sdfmt,
         const char *message, va_list args);

     vsyslogp_r(int priority, struct syslog_data *data, const char *msgid,
         const char *sdfmt, const char *message, va_list args);

     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 multithread-safe 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
     syslog_data structure and the SYSLOG_DATA_INIT constant are defined as:

           struct syslog_data {
                   int             log_file;
                   int             connected;
                   int             opened;
                   int             log_stat;
                   const char     *log_tag;
                   int             log_fac;
                   int             log_mask;

           #define SYSLOG_DATA_INIT { \
               .log_file = -1, \
               .log_fac = LOG_USER, \
               .log_mask = 0xff, \

     The structure is composed of the following elements:

           log_file   contains the file descriptor of the file where the
                      message is logged

           connected  indicates if connect has been done

           opened     indicates if openlog_r() has been called

           log_stat   status bits, set by openlog_r()

           log_tag    string to tag the entry with

           log_fac    facility code

           log_mask   mask of priorities to be logged

     The vsyslog() function is an alternative form in which the arguments have
     already been captured using the variable-length argument facilities of

     The syslogp() variants take additional arguments which correspond to new
     fields in the syslog-protocol message format.  All three arguments are
     evaluated as printf(3) format strings and any of them can be NULL.  This
     enables applications to use message IDs, structured data, and UTF-8
     encoded content in messages.

     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

     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 vsyslog_r() is used the same way as vsyslog() except that it takes an
     additional pointer to a syslog_data structure.  It is a multithread-safe
     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

     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_NLOG      Stops syslog from writing to the system log.  Only useful
                   with LOG_PERROR.

     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.  (This PID is placed
                   within brackets between the ident and the message.)

     LOG_PTRIM     Trim anything syslog added to the message before writing to
                   standard error output.

     The facility parameter 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
                   selected individuals.

     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 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 openlog_r() function is the multithread-safe 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 multithread-safe

     The closelog() function can be used to close the log file.

     The closelog_r() does the same thing as closelog(3) but in a multithread-
     safe 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 multithread-safe version of
     setlogmask().  It takes an additional pointer to a syslog_data structure.

     The routines closelog(), closelog_r(), openlog(), openlog_r(), syslog(),
     syslog_r(), vsyslog(), vsyslog_r(), syslogp(), syslogp_r(), vsyslogp(),
     and vsyslogp_r() 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");

           syslogp(LOG_INFO|LOG_LOCAL2, NULL, NULL, "foobar error: %m");

           syslogp(LOG_INFO, "ID%d", "[meta language=\"en-US\"]",
                   "event: %s", 42, EventDescription);

     For the multithread-safe functions:

           struct syslog_data sdata = SYSLOG_DATA_INIT;

           syslog_r(LOG_INFO|LOG_LOCAL2, &sdata, "foobar error: %m");

     logger(1), syslogd(8)

     The BSD syslog Protocol, RFC, 3164, August 2001.

     The syslog Protocol, Internet-Draft, draft-ietf-syslog-protocol-23,
     September 2007.

     These non-multithread-safe functions appeared in 4.2BSD.  The
     multithread-safe functions appeared in OpenBSD 3.1 and then in
     NetBSD 4.0.  The async-signal-safe functions appeared in NetBSD 4.0.  The
     syslog-protocol functions appeared in NetBSD 5.0.

     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 your stack, leading to a possible security hole.  This
     holds true even if you have built the string "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);

     With syslogp() the caller is responsible to use the right formatting for
     the message fields.  A msgid must only contain up to 32 ASCII characters.
     A sdfmt has strict rules for parenthesis and character quoting.  If the
     msgfmt contains UTF-8 characters, then it has to start with a Byte Order

NetBSD 10.99                    March 22, 2017                    NetBSD 10.99