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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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5.8 Typeglobs and Filehandles

Perl uses an internal type called a typeglob to hold an entire symbol table entry. The type prefix of a typeglob is a *, because it represents all types. This used to be the preferred way to pass arrays and hashes by reference into a function, but now that we have real references, this is seldom needed.

The main use of typeglobs in modern Perl is create symbol table aliases. This assignment:

*this = *that;

makes $this an alias for $that, @this an alias for @that, %this an alias for %that, &this an alias for &that, etc. Much safer is to use a reference. This:

local *Here::blue = \$There::green;

temporarily makes $Here::blue an alias for $There::green, but doesn't make @Here::blue an alias for @There::green, or %Here::blue an alias for %There::green, etc. See "Symbol Tables" (perlmod) in the Perl Library Reference Manual (Volume 2) for more examples of this. Strange though this may seem, this is the basis for the whole module import/export system.

Another use for typeglobs is to pass filehandles into a function or to create new filehandles. If you need to use a typeglob to save away a filehandle, do it this way:

$fh = *STDOUT;

or perhaps as a real reference, like this:

$fh = \*STDOUT;

See 8 for examples of using these as indirect filehandles in functions.

Typeglobs are also a way to create a local filehandle using the local() operator. These last until their block is exited, but may be passed back. For example:

sub newopen {
    my $path = shift;
    local  *FH;  # not my!
    open   (FH, $path)          or  return undef;
    return *FH;
}
$fh = newopen('/etc/passwd');

Now that we have the *foo{THING} notation, typeglobs aren't used as much for filehandle manipulations, although they're still needed to pass brand new file and directory handles into or out of functions. That's because *HANDLE{IO} only works if HANDLE has already been used as a handle. In other words, *FH must be used to create new symbol table entries; *foo{THING} cannot. When in doubt, use *FH.

All functions that are capable of creating filehandles (open(), opendir(), pipe(), socketpair(), sysopen(), socket(), and accept()) automatically create an anonymous filehandle if the handle passed to them is an uninitialized scalar variable. This allows the constructs such as open(my $fh, ...) and open(local $fh,...) to be used to create filehandles that will conveniently be closed automatically when the scope ends, provided there are no other references to them. This largely eliminates the need for typeglobs when opening filehandles that must be passed around, as in the following example:

sub myopen {
    open my $fh, "@_"
         or die "Can't open '@_': $!";
    return $fh;
}
{
    my $f = myopen("</etc/motd");
    print <$f>;
    # $f implicitly closed here
}

Note that if an initialized scalar variable is used instead the result is different: my $fh='zzz'; open($fh, ...) is equivalent to open( *{'zzz'}, ...). use strict 'refs' forbids such practice.

Another way to create anonymous filehandles is with the Symbol module or with the IO::Handle module and its ilk. These modules have the advantage of not hiding different types of the same name during the local(). See the bottom of for an example.

ISBN 9781906966027Perl Language Reference ManualSee the print edition