Learning Perl on Win32 Systems

Changing Defaults for Formats

We have often referred to the default for this or that. Well, Perl provides a way to override the defaults for just about every step. Let’s talk about these.

Using select to Change the Filehandle

Back when we talked about print, in Chapter 6, we mentioned that print and print STDOUT were identical, because STDOUT was the default for print. Not quite. The real default for print (and write, and a few other operations we’ll get to in a moment) is an odd notion called the currently selected filehandle.

The currently selected filehandle starts out as STDOUT—which makes it easy to print things on the standard output. However, you can change the currently selected filehandle with the select function. This function takes a single filehandle (or a scalar variable containing the name of a filehandle) as an argument. After the currently selected filehandle is changed, it affects all future operations that depend on the currently selected filehandle. For example:

print "hello world\n";      # like print STDOUT "hello world\n";
select (LOGFILE);           # select a new filehandle
print "howdy, world\n";     # like print LOGFILE "howdy, world\n";
print "more for the log\n"; # more for LOGFILE
select (STDOUT);            # re-select STDOUT
print "back to stdout\n";   # this goes to standard output

Note that the select operation is sticky—after you’ve selected a new handle, it stays in effect until the next select.

So, a better definition for STDOUT with respect to print and write is that STDOUT is ...