IO::Default - replace select() with $DEFOUT, $DEFERR, $DEFIN


   use IO::Default;

   open LOG, ">/var/log/my.log";
   $DEFOUT = LOG;               # instead of select(LOG);

   open $DEFERR, ">/var/log/my.err";
   warn "Badness!";             # sends to $DEFERR

   $DEFIN = \*STDIN;            # barewords or globs work
   @data = <>;                  # reads from $DEFIN now

   use FileHandle;              # provide OO file methods

   $DEFOUT = \*MYFILE;          # need to use globs if want OO
   $DEFOUT->autoflush(1);       # set $| on whatever $DEFOUT is
   $DEFERR->autoflush(1);       # ditto

   $DEFIN->untaint;             # untaint default input stream


Currently, Perl provides a somewhat clumsy way of manipulating the default filehandle, and no easy way of manipulating default error and input handles. This module serves the dual purpose of providing a means around this, as well as serving as a prototype for a proposed Perl 6 feature.

This module replaces the use of select() and the default filehandle with three variables, $DEFOUT, $DEFERR, and $DEFIN, that are the default output, input, and error filehandles. By default, they point to STDOUT, STDERR, and nothing, respectively. The reason $DEFIN doesn't do anything until you assign to it is because assigning to it wipes out @ARGV. See the BUGS below.

To change what the default filehandle is for output, simply assign a filehandle or filehandle glob to $DEFOUT:

   open LOG, ">/var/log/my.log" or die;
   $DEFOUT = LOG;                       # bare filehandles ok
   print "Here's some data";            # goes to LOG

The same can be easily done with $DEFERR for default errors:

   open ERR, ">/var/log/my.err" or die;
   $DEFERR = \*ERR unless $have_a_tty;  # glob refs ok too
   warn "Danger, Will Robinson!";       # goes to ERR

Finally, this module changes the semantics of <> if you assign to $DEFIN. Normally, the <> ARGV filehandle will iterate through command line arguments. This is still the default. However, if you explicitly assign a filehandle to $DEFIN, then this changes the semantics and input is instead read from the handle:

   open MOTD, "</etc/motd" or die;
   print while (<>);                    # just reads /etc/motd

Why do this? Well, passing filehandles in and out of functions is a pain in Perl 5, requiring you to use globs. If you simply want to change the default input for a sub function somewhere, have it read from <>:

   sub get_data {
       my @data = <>;
       # do more stuff
       return @data;

Then from the top level do something like this:

   # Figure out our input stream
   $DEFIN = get_handle || \*STDIN;
   @data = get_data;

And now you don't have to pass filehandles in and out of functions anymore just for dealing with default input and output. Note that here <> and <$DEFIN> are synonymous.


This module should NOT be used in production code because it is considered unstable and subject to change.

Unfortunately, getting $DEFIN to work in Perl 5 is hairy, since ARGV is so special. As such, assigning to $DEFIN will wipe out whatever you have in @ARGV at the time. It also can't set $ARGV, since the file that was opened is unknown.

Luckily, if you never assign or access $DEFIN, then <> retains its magic powers, so if you don't like this simply don't use $DEFIN.

This module really just does some trickery to reopen the STD filehandles and point them to different places. As such, mixing print calls to $DEFERR and STDERR will send the output to the same place (contrary to the Perl 6 proposal).


For complete details on the Perl 6 proposal, please visit Comments are welcome.


Copyright (c) 2000, Nathan Wiger <>. All Rights Reserved.

This module is free software; you may copy this under the terms of the GNU General Public License, or the Artistic License, copies of which should have accompanied your Perl kit.