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SYSLOG(3)                BSD Library Functions Manual                SYSLOG(3)

     closelog, openlog, setlogmask, syslog, vsyslog -- control system log

     Standard C Library (libc, -lc)

     #include <syslog.h>


     openlog(const char *ident, int logopt, int facility);

     setlogmask(int maskpri);

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

     #include <syslog.h>
     #include <stdarg.h>

     vsyslog(int priority, 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

     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.

     Newlines and other non-printable characters embedded in the message string are printed in an alternate
     format.  This prevents someone from using non-printable characters to construct misleading log messages
     in an output file.  Newlines are printed as "\n", tabs are printed as "\t".  Other control characters
     are printed using a caret ("^") representation, for example "^M" for carriage return.

     The vsyslog() function is an alternate form in which the arguments have already 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 messages sent by syslog() and
     vsyslog().  The parameter ident is a string that will be prepended to every message.  The logopt argu-ment argument
     ment is a bit field specifying logging options, which is formed by OR'ing one or more of the following

     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.

     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.

     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 facili-ties. 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: cups-lpd(8), cupsd(8), etc.

     LOG_MAIL      The mail system.

     LOG_NEWS      The network news 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 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.

     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");

     #include <syslog.h>
     #include <stdarg.h>

     These include files are necessary for all functions.

     asl(3), logger(1), compat(5), syslogd(8)

     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 con-tain contain
     tain user-supplied conversion specifiers for later interpolation by syslog().

     Always use the proper secure idiom:

           syslog(LOG_ERR, "%s", string);

BSD                              June 4, 1993                              BSD

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