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


 use Unix::Syslog qw(:macros);  # Syslog macros
 use Unix::Syslog qw(:subs);    # Syslog functions
 openlog $ident, $option, $facility;
 syslog $priority, $format, @formatargs;
 $oldmask = setlogmask $mask_priority;


This module provides an interface to the system logger syslogd(8) via Perl's XSUBs. The implementation attempts to resemble the native libc-functions of your system, so that anyone being familiar with syslog.h should be able to use this module right away.

In contrary to Sys::Syslog(3), this modules does not open a network connection to send the messages. This can help you to avoid opening security holes in your computer (see FAQ).

The subs imported by the tag macros are simply wrappers around the most important #defines in your system's C header file syslog.h. The macros return integer values that are used to specify options, facilities and priorities in a more or less portable way. They also provide general information about your local syslog mechanism. Check syslog(3) and your local syslog.h for information about the macros, options and facilities available on your system.

The following functions are provided:

openlog $ident, $option, $facility

opens a connection to the system logger. $ident is an identifier string that syslogd(8) prints into every message. It usually equals the process name. $option is an integer value that is the result of ORed options. $facility is an integer value that specifies the part of the system the message should be associated with (e.g. kernel message, mail subsystem).

syslog $priority, $format, @formatargs

Generates a log message and passes it to the system logger. If syslog() is called without calling openlog() first, probably system dependent default values will be used as arguments for an implicit call to openlog().

$priority is an integer value that specifies the priority of the message. Alternatively $priority can be the ORed value of a priority and a facility. In that case a previously selected facility will be overridden.

In the case that syslog() is called without calling openlog() first and priority does not specify both a priority and a facility, a default facility will be used. This behaviour is most likely system dependent and the user should not rely on any particular value in that case.

$format is a format string in the style of printf(3). Additionally to the usual printf directives %m can be specified in the string. It will be replaced implicitly by the contents of the Perl variable $! ($ERRNO). @formatargs is a list of values that the format directives will be replaced with subsequently.


closes the connection to the system logger.

setlogmask $mask_priority

sets the priority mask and returns the old mask. Logging is enabled for the priorities indicated by the bits in the mask that are set and is disabled where the bits are not set. Macros are provided to specify valid and portable arguments to setlogmask(). Usually the default log mask allows all messages to be logged.

priorityname $priority

returns a string containing the name of $priority as string. If this functionality has not been enabled at installation, the function returns undef.

facilityname $facility

returns a string containing the name of $facility as string. If this functionality has not been enabled at installation, the function returns undef.

NOTE: The behaviour of this module is system dependent. It is highly recommended to consult your system manual for available macros and the behaviour of the provided functions.


The functions openlog(), syslog() and closelog() return the undefined value. The function setlogmask returns the previous mask value.


Open a channel to syslogd specifying an identifier (usually the process name) some options and the facility: openlog "", LOG_PID | LOG_PERROR, LOG_LOCAL7;

Generate log message of specified priority using a printf-type formatted string: syslog LOG_INFO, "This is message number %d", 42;

Set log priority mask to block all messages but those of priority LOG_DEBUG: $oldmask = setlogmask(LOG_MASK(LOG_DEBUG))

Set log priority mask to block all messages with a higher priority than LOG_ERR: $oldmask = setlogmask(LOG_UPTO(LOG_ERR))

Close channel to syslogd: closelog;


  1. What is the benefit of using this module instead of Sys::Syslog?

    Sys::Syslog always opens a network connection to the syslog service. At least on Linux systems this may lead to some trouble, because

    Peter Stamfest <> pointed out some other advantages of Unix::Syslog, I didn't came across my self.

  2. Well, is there any reason to use Sys::Syslog any longer?

    Yes! In contrary to Unix::Syslog, Sys::Syslog works even if you don't have a syslog daemon running on your system as long as you are connected to a log host via a network and have access to the syslog.h header file of your log host to generate the initial files for Sys::Syslog (see Sys::Syslog(3) for details). Unix::Syslog only logs to your local syslog daemon which in turn may be configured to distribute the message over the network.

  3. Are calls to the functions provided by Unix::Syslog compatible to those of Sys::Syslog?

    Currently not. Sys::Syslog requires strings to specify many of the arguments to the functions, while Unix::Syslog uses numeric constants accessed via macros as defined in syslog.h. Although the strings used by Sys::Syslog are also defined in syslog.h, it seems that most people got used to the numeric arguments. I will implement the string based calls if there are enough people ($min_people > 10**40) complaining about the lack of compatibility.


syslog(3), Sys::Syslog(3), syslogd(8), perl(1)


Marcus Harnisch <>