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Tie-TZ-2
River stage one • 2 direct dependents • 2 total dependents
/ Tie::TZ

NAME

Tie::TZ - tied $TZ setting %ENV and calling tzset()

SYNOPSIS

use Tie::TZ qw($TZ);
$TZ = 'GMT';

{ local $TZ = 'EST+10';
  ...
}

DESCRIPTION

This module provides a tied $TZ variable which gets and sets $ENV{'TZ'}. When it sets %ENV it calls POSIX::tzset(), ensuring the C library notices the change for subsequent localtime, etc.

$TZ = 'GMT';
# does  $ENV{'TZ'} = 'GMT';  POSIX::tzset();

For a plain set you can just as easily store and tzset yourself (or have a function to do the combo). The power of a tied variable comes when using local to have a different timezone temporarily. Any goto, return, die, etc, exiting the block will restore the old setting, including a tzset for it.

{ local $TZ = 'GMT';
  print ctime();
  # TZ restored at block exit
}

{ local $TZ = 'GMT';
  die 'Something';
  # TZ restored when the die unwinds
}

Storing undef to $TZ deletes $ENV{'TZ'}, unsetting the environment variable. This generally means the timezone goes back to the system default local time (usually per from /etc/timezone).

As an optimization, if a store to $TZ is already what $ENV{'TZ'} contains then tzset is not called. This is helpful if some of the settings you're working are all the same. If there's never any different value applied then POSIX module is not even loaded at all.

Uses

Quite often tzset is not actually needed. Decent C libraries look for a new TZ each time in the various localtime etc functions. But tzset keeps you out of trouble on older systems, or with any external libraries directly accessing the C global variables like timezone or daylight.

Perl's own calls to localtime etc do a tzset themselves where necessary to cope with old C libraries (based on a configure test, see Config). However in 5.8.8 and earlier Perl didn't do that on localtime_r, and the latter in some versions of GNU C can need an explicit tzset; the net result being that you need tzset in threaded Perl 5.8.8 (whether using threads or not). Of course even when Perl recognises C library limitations you might not be so lucky deep in external libraries.

EXPORTS

By default nothing is exported and you can use the full name $Tie::TZ::TZ,

use Tie::TZ;
$Tie::TZ::TZ = 'GMT';

Or import $TZ in the usual way (see Exporter) as a shorthand

use Tie::TZ '$TZ';
$TZ = 'GMT';

OTHER NOTES

The local trick above of course works with $ENV{'TZ'} directly too if you're happy that you don't need tzset, eg. local $ENV{'TZ'} = 'EST+10'. The Env module can let you tie a $TZ name for that too if desired. However undef ends up meaning an empty string instead of unset, which may be subtly different, and it also provokes a warning from Perl prior to 5.10.

When you get sick of the C library timezone handling have a look at DateTime::TimeZone. Its data tables make it big (though no doubt you could turf what you don't use) but it's all Perl and is much friendlier if you're working in multiple zones more or less simultaneously.

SEE ALSO

POSIX, Env, DateTime::TimeZone

HOME PAGE

http://www.geocities.com/user42_kevin/tie-tz/index.html

COPYRIGHT

Copyright 2008 Kevin Ryde

Tie-TZ is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version.

Tie-TZ is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with Tie-TZ. If not, see http://www.gnu.org/licenses.