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Sys::Syslog - Perl interface to the UNIX syslog(3) calls


Version 0.27


  1. use Sys::Syslog; # all except setlogsock(), or:
  2. use Sys::Syslog qw(:DEFAULT setlogsock); # default set, plus setlogsock()
  3. use Sys::Syslog qw(:standard :macros); # standard functions, plus macros
  4. openlog $ident, $logopt, $facility; # don't forget this
  5. syslog $priority, $format, @args;
  6. $oldmask = setlogmask $mask_priority;
  7. closelog;


Sys::Syslog is an interface to the UNIX syslog(3) program. Call syslog() with a string priority and a list of printf() args just like syslog(3) .

You can find a kind of FAQ in THE RULES OF SYS::SYSLOG. Please read it before coding, and again before asking questions.


Sys::Syslog exports the following Exporter tags:

  • :standard exports the standard syslog(3) functions:

    1. openlog closelog setlogmask syslog
  • :extended exports the Perl specific functions for syslog(3) :

    1. setlogsock
  • :macros exports the symbols corresponding to most of your syslog(3) macros and the LOG_UPTO() and LOG_MASK() functions. See CONSTANTS for the supported constants and their meaning.

By default, Sys::Syslog exports the symbols from the :standard tag.


  • openlog($ident, $logopt, $facility)

    Opens the syslog. $ident is prepended to every message. $logopt contains zero or more of the options detailed below. $facility specifies the part of the system to report about, for example LOG_USER or LOG_LOCAL0 : see Facilities for a list of well-known facilities, and your syslog(3) documentation for the facilities available in your system. Check SEE ALSO for useful links. Facility can be given as a string or a numeric macro.

    This function will croak if it can't connect to the syslog daemon.

    Note that openlog() now takes three arguments, just like openlog(3) .

    You should use openlog() before calling syslog() .


    • cons - This option is ignored, since the failover mechanism will drop down to the console automatically if all other media fail.

    • ndelay - Open the connection immediately (normally, the connection is opened when the first message is logged).

    • nofatal - When set to true, openlog() and syslog() will only emit warnings instead of dying if the connection to the syslog can't be established.

    • nowait - Don't wait for child processes that may have been created while logging the message. (The GNU C library does not create a child process, so this option has no effect on Linux.)

    • perror - Write the message to standard error output as well to the system log.

    • pid - Include PID with each message.


    Open the syslog with options ndelay and pid , and with facility LOCAL0 :

    1. openlog($name, "ndelay,pid", "local0");

    Same thing, but this time using the macro corresponding to LOCAL0 :

    1. openlog($name, "ndelay,pid", LOG_LOCAL0);
  • syslog($priority, $message)
  • syslog($priority, $format, @args)

    If $priority permits, logs $message or sprintf($format, @args) with the addition that %m in $message or $format is replaced with "$!" (the latest error message).

    $priority can specify a level, or a level and a facility. Levels and facilities can be given as strings or as macros. When using the eventlog mechanism, priorities DEBUG and INFO are mapped to event type informational , NOTICE and WARNIN to warning and ERR to EMERG to error .

    If you didn't use openlog() before using syslog() , syslog() will try to guess the $ident by extracting the shortest prefix of $format that ends in a ":" .


    1. syslog("info", $message); # informational level
    2. syslog(LOG_INFO, $message); # informational level
    3. syslog("info|local0", $message); # information level, Local0 facility
    4. syslog(LOG_INFO|LOG_LOCAL0, $message); # information level, Local0 facility
    • Note

      Sys::Syslog version v0.07 and older passed the $message as the formatting string to sprintf() even when no formatting arguments were provided. If the code calling syslog() might execute with older versions of this module, make sure to call the function as syslog($priority, "%s", $message) instead of syslog($priority, $message) . This protects against hostile formatting sequences that might show up if $message contains tainted data.

  • setlogmask($mask_priority)

    Sets the log mask for the current process to $mask_priority and returns the old mask. If the mask argument is 0, the current log mask is not modified. See Levels for the list of available levels. You can use the LOG_UPTO() function to allow all levels up to a given priority (but it only accept the numeric macros as arguments).


    Only log errors:

    1. setlogmask( LOG_MASK(LOG_ERR) );

    Log everything except informational messages:

    1. setlogmask( ~(LOG_MASK(LOG_INFO)) );

    Log critical messages, errors and warnings:


    Log all messages up to debug:

    1. setlogmask( LOG_UPTO(LOG_DEBUG) );
  • setlogsock($sock_type)
  • setlogsock($sock_type, $stream_location) (added in Perl 5.004_02)
  • setlogsock($sock_type, $stream_location, $sock_timeout) (added in 0.25)

    Sets the socket type to be used for the next call to openlog() or syslog() and returns true on success, undef on failure. The available mechanisms are:

    • "native" - use the native C functions from your syslog(3) library (added in Sys::Syslog 0.15).

    • "eventlog" - send messages to the Win32 events logger (Win32 only; added in Sys::Syslog 0.19).

    • "tcp" - connect to a TCP socket, on the syslog/tcp or syslogng/tcp service. If defined, the second parameter is used as a hostname to connect to.

    • "udp" - connect to a UDP socket, on the syslog/udp service. If defined, the second parameter is used as a hostname to connect to, and the third parameter as the timeout used to check for UDP response.

    • "inet" - connect to an INET socket, either TCP or UDP, tried in that order. If defined, the second parameter is used as a hostname to connect to.

    • "unix" - connect to a UNIX domain socket (in some systems a character special device). The name of that socket is the second parameter or, if you omit the second parameter, the value returned by the _PATH_LOG macro (if your system defines it), or /dev/log or /dev/conslog, whatever is writable.

    • "stream" - connect to the stream indicated by the pathname provided as the optional second parameter, or, if omitted, to /dev/conslog. For example Solaris and IRIX system may prefer "stream" instead of "unix" .

    • "pipe" - connect to the named pipe indicated by the pathname provided as the optional second parameter, or, if omitted, to the value returned by the _PATH_LOG macro (if your system defines it), or /dev/log (added in Sys::Syslog 0.21).

    • "console" - send messages directly to the console, as for the "cons" option of openlog() .

    A reference to an array can also be passed as the first parameter. When this calling method is used, the array should contain a list of mechanisms which are attempted in order.

    The default is to try native , tcp , udp , unix , pipe, stream , console . Under systems with the Win32 API, eventlog will be added as the first mechanism to try if Win32::EventLog is available.

    Giving an invalid value for $sock_type will croak .


    Select the UDP socket mechanism:

    1. setlogsock("udp");

    Select the native, UDP socket then UNIX domain socket mechanisms:

    1. setlogsock(["native", "udp", "unix"]);
    • Note

      Now that the "native" mechanism is supported by Sys::Syslog and selected by default, the use of the setlogsock() function is discouraged because other mechanisms are less portable across operating systems. Authors of modules and programs that use this function, especially its cargo-cult form setlogsock("unix") , are advised to remove any occurence of it unless they specifically want to use a given mechanism (like TCP or UDP to connect to a remote host).

  • closelog()

    Closes the log file and returns true on success.


The First Rule of Sys::Syslog is: You do not call setlogsock .

The Second Rule of Sys::Syslog is: You do not call setlogsock .

The Third Rule of Sys::Syslog is: The program crashes, dies, calls closelog , the log is over.

The Fourth Rule of Sys::Syslog is: One facility, one priority.

The Fifth Rule of Sys::Syslog is: One log at a time.

The Sixth Rule of Sys::Syslog is: No syslog before openlog .

The Seventh Rule of Sys::Syslog is: Logs will go on as long as they have to.

The Eighth, and Final Rule of Sys::Syslog is: If this is your first use of Sys::Syslog, you must read the doc.


An example:

  1. openlog($program, 'cons,pid', 'user');
  2. syslog('info', '%s', 'this is another test');
  3. syslog('mail|warning', 'this is a better test: %d', time);
  4. closelog();
  5. syslog('debug', 'this is the last test');

Another example:

  1. openlog("$program $$", 'ndelay', 'user');
  2. syslog('notice', 'fooprogram: this is really done');

Example of use of %m :

  1. $! = 55;
  2. syslog('info', 'problem was %m'); # %m == $! in syslog(3)

Log to UDP port on $remotehost instead of logging locally:

  1. setlogsock("udp", $remotehost);
  2. openlog($program, 'ndelay', 'user');
  3. syslog('info', 'something happened over here');



  • LOG_AUDIT - audit daemon (IRIX); falls back to LOG_AUTH

  • LOG_AUTH - security/authorization messages

  • LOG_AUTHPRIV - security/authorization messages (private)

  • LOG_CONSOLE - /dev/console output (FreeBSD); falls back to LOG_USER

  • LOG_CRON - clock daemons (cron and at)

  • LOG_DAEMON - system daemons without separate facility value

  • LOG_FTP - FTP daemon

  • LOG_KERN - kernel messages

  • LOG_INSTALL - installer subsystem (Mac OS X); falls back to LOG_USER

  • LOG_LAUNCHD - launchd - general bootstrap daemon (Mac OS X); falls back to LOG_DAEMON

  • LOG_LFMT - logalert facility; falls back to LOG_USER

  • LOG_LOCAL0 through LOG_LOCAL7 - reserved for local use

  • LOG_LPR - line printer subsystem

  • LOG_MAIL - mail subsystem

  • LOG_NETINFO - NetInfo subsystem (Mac OS X); falls back to LOG_DAEMON

  • LOG_NEWS - USENET news subsystem

  • LOG_NTP - NTP subsystem (FreeBSD, NetBSD); falls back to LOG_DAEMON

  • LOG_RAS - Remote Access Service (VPN / PPP) (Mac OS X); falls back to LOG_AUTH

  • LOG_REMOTEAUTH - remote authentication/authorization (Mac OS X); falls back to LOG_AUTH

  • LOG_SECURITY - security subsystems (firewalling, etc.) (FreeBSD); falls back to LOG_AUTH

  • LOG_SYSLOG - messages generated internally by syslogd

  • LOG_USER (default) - generic user-level messages

  • LOG_UUCP - UUCP subsystem


  • LOG_EMERG - system is unusable

  • LOG_ALERT - action must be taken immediately

  • LOG_CRIT - critical conditions

  • LOG_ERR - error conditions

  • LOG_WARNING - warning conditions

  • LOG_NOTICE - normal, but significant, condition

  • LOG_INFO - informational message

  • LOG_DEBUG - debug-level message


  • Invalid argument passed to setlogsock

    (F) You gave setlogsock() an invalid value for $sock_type .

  • eventlog passed to setlogsock, but no Win32 API available

    (W) You asked setlogsock() to use the Win32 event logger but the operating system running the program isn't Win32 or does not provides Win32 compatible facilities.

  • no connection to syslog available

    (F) syslog() failed to connect to the specified socket.

  • stream passed to setlogsock, but %s is not writable

    (W) You asked setlogsock() to use a stream socket, but the given path is not writable.

  • stream passed to setlogsock, but could not find any device

    (W) You asked setlogsock() to use a stream socket, but didn't provide a path, and Sys::Syslog was unable to find an appropriate one.

  • tcp passed to setlogsock, but tcp service unavailable

    (W) You asked setlogsock() to use a TCP socket, but the service is not available on the system.

  • syslog: expecting argument %s

    (F) You forgot to give syslog() the indicated argument.

  • syslog: invalid level/facility: %s

    (F) You specified an invalid level or facility.

  • syslog: too many levels given: %s

    (F) You specified too many levels.

  • syslog: too many facilities given: %s

    (F) You specified too many facilities.

  • syslog: level must be given

    (F) You forgot to specify a level.

  • udp passed to setlogsock, but udp service unavailable

    (W) You asked setlogsock() to use a UDP socket, but the service is not available on the system.

  • unix passed to setlogsock, but path not available

    (W) You asked setlogsock() to use a UNIX socket, but Sys::Syslog was unable to find an appropriate an appropriate device.


Manual Pages


SUSv3 issue 6, IEEE Std 1003.1, 2004 edition,

GNU C Library documentation on syslog,

Solaris 10 documentation on syslog,

Mac OS X documentation on syslog,

IRIX 6.5 documentation on syslog,

AIX 5L 5.3 documentation on syslog,

HP-UX 11i documentation on syslog,

Tru64 5.1 documentation on syslog,

Stratus VOS 15.1,


RFC 3164 - The BSD syslog Protocol, -- Please note that this is an informational RFC, and therefore does not specify a standard of any kind.

RFC 3195 - Reliable Delivery for syslog,


Syslogging with Perl,

Event Log

Windows Event Log,


Tom Christiansen <tchrist (at)> and Larry Wall <larry (at)>.

UNIX domain sockets added by Sean Robinson <robinson_s (at)> with support from Tim Bunce <Tim.Bunce (at)> and the perl5-porters mailing list.

Dependency on replaced with XS code by Tom Hughes <tom (at)>.

Code for constant() s regenerated by Nicholas Clark <nick (at)>.

Failover to different communication modes by Nick Williams <Nick.Williams (at)>.

Extracted from core distribution for publishing on the CPAN by Sébastien Aperghis-Tramoni <sebastien (at)>.

XS code for using native C functions borrowed from Unix::Syslog, written by Marcus Harnisch <marcus.harnisch (at)>.

Yves Orton suggested and helped for making Sys::Syslog use the native event logger under Win32 systems.

Jerry D. Hedden and Reini Urban provided greatly appreciated help to debug and polish Sys::Syslog under Cygwin.


Please report any bugs or feature requests to bug-sys-syslog (at), or through the web interface at I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.


You can find documentation for this module with the perldoc command.

  1. perldoc Sys::Syslog

You can also look for information at:


Copyright (C) 1990-2008 by Larry Wall and others.


This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.