Path: utzoo!attcan!uunet!samsung!usc!elroy.jpl.nasa.gov!jpl-devvax!lwall
From: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: Bug -- variable corruption inside sub/foreach
Message-ID: <7018@jpl-devvax.JPL.NASA.GOV>
Date: 8 Feb 90 05:57:00 GMT
References: <4080010@hpausla.aso.hp.com>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 32

In article <4080010@hpausla.aso.hp.com> brian@hpausla.aso.hp.com (Brian Coogan) writes:
: I seem to have managed to find another bug in perl.  A variable $file is
: corrupted by reading from a filehandle (variable name doesn't seem
: to matter, nor does it matter whether $file is local or not, or the
: first/most recent variable use beforehand.)  I've worked around this
: bug, but it lost me a lot of time.  The variable gets corrupted to
: the value of $_.

Oddly enough, it's not really a perl bug in this case.  What you've got is

	do checkfiles($_);

	sub checkfiles {
	    foreach $file (@_) {
		while (<file>) {
		    ...
		}
	    }
	}

What you have to remember is that @_ is an array of references to the
actual parameters (not copies, as in 2.0), and that foreach iterates over
an array by making the variable ($file, in this case) to be a reference
to the actual array elements.  Hence, when checkfiles is called with $_,
it ends up aliased to $file.  So reading into $_ then clobbers $file too.

That's the fun of passing parameters by reference.  I still think it's
worth it for the efficiency gain.  If it worries you, just be consistent
about copying your parameters out of @_ into something local.  Or tell
yourself to worry about aliasing whenever you don't.  Or something like that.

Larry