NAME
IO::Default - replace select() with $DEFOUT, $DEFERR, $DEFIN
SYNOPSIS
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
DESCRIPTION
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;
$DEFIN = MOTD;
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.
BUGS
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).
REFERENCES
For complete details on the Perl 6 proposal, please visit
http://dev.perl.org/rfc/129.html. Comments are welcome.
AUTHOR
Copyright (c) 2000, Nathan Wiger <
[email protected]>. 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.
