Sys::Syslog(3perl) Perl Programmers Reference Guide Sys::Syslog(3perl)

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

This is the documentation of version 0.36

use Sys::Syslog;                        # all except setlogsock()
use Sys::Syslog qw(:standard :macros);  # standard functions & macros
openlog($ident, $logopt, $facility);    # don't forget this
syslog($priority, $format, @args);
$oldmask = setlogmask($mask_priority);

"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).

"Sys::Syslog" exports the following "Exporter" tags:

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

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).
  • "noeol" - When set to true, no end of line character ("\n") will be appended to the message. This can be useful for some syslog daemons. Added in "Sys::Syslog" 0.29.
  • "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. Added in "Sys::Syslog" 0.15.
  • "nonul" - When set to true, no "NUL" character ("\0") will be appended to the message. This can be useful for some syslog daemons. Added in "Sys::Syslog" 0.29.
  • "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. Added in "Sys::Syslog" 0.22.
  • "pid" - Include PID with each message.


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

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

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

openlog($name, "ndelay,pid", LOG_LOCAL0);
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 "WARNING" 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 ":".


# informational level
syslog("info", $message);
syslog(LOG_INFO, $message);
# information level, Local0 facility
syslog("info|local0", $message);
syslog(LOG_INFO|LOG_LOCAL0, $message);
"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.
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:

setlogmask( LOG_MASK(LOG_ERR) );

Log everything except informational messages:

setlogmask( ~(LOG_MASK(LOG_INFO)) );

Log critical messages, errors and warnings:

setlogmask( LOG_MASK(LOG_CRIT)
          | LOG_MASK(LOG_ERR)
          | LOG_MASK(LOG_WARNING) );

Log all messages up to debug:

setlogmask( LOG_UPTO(LOG_DEBUG) );
Sets the socket type and options to be used for the next call to openlog() or syslog(). Returns true on success, "undef" on failure.

Being Perl-specific, this function has evolved along time. It can currently be called as follow:

  • setlogsock($sock_type)
  • "setlogsock($sock_type, $stream_location)" (added in Perl 5.004_02)
  • "setlogsock($sock_type, $stream_location, $sock_timeout)" (added in "Sys::Syslog" 0.25)
  • setlogsock(\%options) (added in "Sys::Syslog" 0.28)

The available options are:

  • "type" - equivalent to $sock_type, selects the socket type (or "mechanism"). An array reference can be passed to specify several mechanisms to try, in the given order.
  • "path" - equivalent to $stream_location, sets the stream location. Defaults to standard Unix location, or "_PATH_LOG".
  • "timeout" - equivalent to $sock_timeout, sets the socket timeout in seconds. Defaults to 0 on all systems except Mac OS X where it is set to 0.25 sec.
  • "host" - sets the hostname to send the messages to. Defaults to the local host.
  • "port" - sets the TCP or UDP port to connect to. Defaults to the first standard syslog port available on the system.

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. See also the "host", "port" and "timeout" options.
  • "udp" - connect to a UDP socket, on the "syslog/udp" service. See also the "host", "port" and "timeout" options.
  • "inet" - connect to an INET socket, either TCP or UDP, tried in that order. See also the "host", "port" and "timeout" options.
  • "unix" - connect to a UNIX domain socket (in some systems a character special device). The name of that socket is given by the "path" option or, if omitted, the value returned by the "_PATH_LOG" macro (if your system defines it), /dev/log or /dev/conslog, whichever is writable.
  • "stream" - connect to the stream indicated by the "path" option, or, if omitted, the value returned by the "_PATH_LOG" macro (if your system defines it), /dev/log or /dev/conslog, whichever is writable. For example Solaris and IRIX system may prefer "stream" instead of "unix".
  • "pipe" - connect to the named pipe indicated by the "path" option, 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). HP-UX is a system which uses such a named pipe.
  • "console" - send messages directly to the console, as for the "cons" option of openlog().

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:


Send messages using the TCP socket mechanism on a custom port:

setlogsock({ type => "tcp", port => 2486 });

Send messages to a remote host using the TCP socket mechanism:

setlogsock({ type => "tcp", host => $loghost });

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

setlogsock(["native", "udp", "unix"]);
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 occurrence of it unless they specifically want to use a given mechanism (like TCP or UDP to connect to a remote host).
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, "die"s, 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:

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

Another example:

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

Example of use of %m:

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

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

setlogsock("udp", $remotehost);
openlog($program, 'ndelay', 'user');
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.

"Sys::Syslog" is a core module, part of the standard Perl distribution since 1990. At this time, modules as we know them didn't exist, the Perl library was a collection of .pl files, and the one for sending syslog messages with was simply lib/, included with Perl 3.0. It was converted as a module with Perl 5.0, but had a version number only starting with Perl 5.6. Here is a small table with the matching Perl and "Sys::Syslog" versions.

Sys::Syslog     Perl
-----------     ----
   undef        5.0.0 ~ 5.5.4
   0.01         5.6.*
   0.03         5.8.0
   0.04         5.8.1, 5.8.2, 5.8.3
   0.05         5.8.4, 5.8.5, 5.8.6
   0.06         5.8.7
   0.13         5.8.8
   0.22         5.10.0
   0.27         5.8.9, 5.10.1 ~ 5.14.*
   0.29         5.16.*
   0.32         5.18.*
   0.33         5.20.*
   0.33         5.22.*

Log::Log4perl - Perl implementation of the Log4j API

Log::Dispatch - Dispatches messages to one or more outputs

Log::Report - Report a problem, with exceptions and language support


SUSv3 issue 6, IEEE Std 1003.1, 2004 edition,

GNU C Library documentation on syslog,

FreeBSD documentation on syslog,

Solaris 11 documentation on syslog,

Mac OS X documentation on syslog,

IRIX documentation on syslog,

AIX 5L 5.3 documentation on syslog,

HP-UX 11i documentation on syslog,

Tru64 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,

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.

perldoc Sys::Syslog

You can also look for information at:

The source code is available on Git Hub:

Copyright (C) 1990-2012 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.

2024-02-11 perl v5.38.2