Perl Cookbook

Storing Filehandles in Variables

Problem

You want to use a filehandle like a normal variable so you can pass it to or return it from a function, store it in a data structure, and so on.

Solution

If you already have a regular symbolic filehandle like STDIN or LOGFILE, use the typeglob notation, *FH. This is the most efficient approach.

$variable = *FILEHANDLE;        # save in variable
subroutine(*FILEHANDLE);        # or pass directly

sub subroutine {
    my $fh = shift;
    print $fh "Hello, filehandle!\n";
}

If you want anonymous filehandles, see the return_fh function below, or use the new method from the IO::File or IO::Handle module, store that in a scalar variable, and use it as though it were a normal filehandle:

use FileHandle;                   # make anon filehandle
$fh = FileHandle->new();

use IO::File;                     # 5.004 or higher
$fh = IO::File->new();

Discussion

You have many choices for passing filehandles into a subroutine or storing them in a data structure. The simplest and fastest way is through the typeglob notation, *FH. It may help you to conceptualize the asterisk as the type symbol for a filehandle. Like the little colored balls from high school chemistry that stood for atomic particles, it’s not really true, but it is a convenient mental shorthand. By the time you understand where this model breaks down, you won’t need it anymore.

That works cleanly for simple things, but what if you wanted to make an array of filehandles whose names you didn’t know? As shown in Chapter 11, generating anonymous arrays, hashes, ...