Path: utzoo!attcan!uunet!fernwood!apple!usc!elroy.jpl.nasa.gov!jpl-devvax!lwall
From: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: Operations on filehandles
Message-ID: <7020@jpl-devvax.JPL.NASA.GOV>
Date: 8 Feb 90 06:35:28 GMT
References: <1004@frankland-river.aaii.oz.au>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 57

In article <1004@frankland-river.aaii.oz.au> pem@frankland-river.aaii.oz.au (Paul E. Maisano) writes:
: I was wondering what sort of object a filehandle was and what can be done
: with one besides printing to them etc.
: 
: Specifically, can you save them in a local context in some way, similar to
: what the local() function does with variables.
:
: For example, if I open a file in a subroutine and use a file handle called
: FILE which I open and then close, how do I know I'm not stomping on
: another filehandle called FILE which has already been opened somewhere else
: in my program.
: 
: I could create a package to avoid the possibility of a naming conflict but
: this does not really solve the problem.

I believe you can do a local on a name using the type globbing syntax:

    sub foo {
	local(*FILE);

	open(FILE,'foo');
	etc.

I haven't exercised it heavily, but the code is supposedly there to create
a new name if you aren't assigning another name to *FILE.  Just as normal
variables created with a null value when you don't assign something to them.

: What if I want to write a recursive subroutine which opens a file ? How do
: I keep separate filehandles for each recursive call ?

Same way.

: Is there a simple way to do this or must I generate a unique filehandle name
: on each invocation ?

That's pretty simple, though it's easy to generate unique filehandles too:

	$genhandle = 'AAAAAA';			# increments to AAAAAB

	sub foo {
	    local($FILE) = $genhandle++;	# magical autoincrement

	    open($FILE,'foo');
	    ...
		&foo();
	    ...
	    close $FILE;
	}

But I THINK the other way is more efficient, if you reference the filehandle
much, since it has to do a symbol table lookup every time you say $FILE,
but references to FILE already point to the symbol table entry upon compilation.
But it probably takes a little longer to local(*FILE) than local($FILE).

It's 6 1/2 of one, and uh, uh, half a baker's dozen of the other.

Larry