SPUG: Something to loose sleep over

[David Waring]

Sorry about my mistake. The program was not really printing ABC but of course
ARRAY(0x8102b84)BC. But I am afraid you are all missing the point. 
The point is that the subroutine is behaving as if the same 
parameters are being passed each time, when they are not. This 
behaviour is dependent on using shift to get the values of @_ and 
shifting all values off. Interestingly the misbehaviour can be seen 
even if @_ is printed before the shifts. I am not talking about a bug 
in my code. The code is good it works fine on many machines. It just 
appears to be a bug in one installation of Perl. I am just wondering 
if anyone can tell what might be going on in this bad version of Perl.





>Think that this line will give you what you want in terms of printing:
>	print "array = '@{shift()}' @_\n";
>
>As for the weirdness you're seeing -- are you using strict? :)
>
>This works on both 5.6 and 5.005_03
>
>Here's the code.  It works with and without the $x = shift lines.
>
>#!/usr/bin/perl
>
>@A = (1..3);
>@B = (4..6);
>@C = (7..9);
>
>asubroutine(\@A,"B","C");
>asubroutine(\@B,"E","F");
>asubroutine(\@C,"Y","Z");
>
>sub asubroutine {
>  print "array = '@{shift()}' @_\n";
>}
>
>Results:
>array = '1 2 3' B C
>array = '4 5 6' E F
>array = '7 8 9' Y Z
>
>
>On Fri, 28 Apr 2000, David Waring wrote:
>
> > Warning this is a demonstration of a bug that only appears in certain
> > installations of Perl. It is a weird bug and I am really pointing it
> > out as a topic of conversation. Ignore this if you are not interested
> > in really yucky stuff.
> >
> >
> > We have recently had a problem with passing parameters to a
> > subroutine. It only appears on one of our many boxes that are all
> > running the same version of Perl (can't remember the version but it
> > is pre 5.6) All boxes are running AIX.
> >
> >
> > To paraphrase the code as closely as possible
> >
> >
> > .....
> > # call a subroutine several times with different parameters
> >
> > asubroutine(\@A,"B","C");
> > asubroutine(\@D,"E","F");
> > asubroutine(\@X,"Y","Z");
> >
> >
> > sub asubroutine{
> > 	print @_,\n";
> > 	$x= shift @_;
> > 	$y= shift @_;
> > 	$z= shift @_;
> > }
> >
> > RESULTS
> > ABC
> > ABC
> > ABC
> >
> > whoa thats weird the subroutine is acting like it was called three
> > times with the same parameters
> >
> > Now we can fix this in several ways. All of the following changes
> > behaved properly an had the expected results of
> > ABC
> > DEF
> > XYZ
> >
> > 1) get rid of the multiple shifts and instead grab all the incoming
> > params at once as any normal person would do
> >
> > ($x,$y,$z) = @_;
> >
> > 2) forget about $z and only shift in the x y values leaving @_ with a
> > single value
> >
> > 3) stop passing the reference in the params just pass the 'normal' scalars;
> >
> > 4) removing all the shifts and just printing @_
> > I mean how on earth can what is done later with @_ affect the values
> > in it?????????
> >
> > Doing #1 but then setting @_ = (); -- which should be pretty damn
> > close to shifting out all the values from @_ worked normally.
> >
> > The Print statement before the shifts was not important. The wrong
> > values were being set for x y z anyway.
> >
> >
> >
> >
> > We will be reinstalling Perl on this box but does anyone have any
> > idea what in the deepest bowels of Perl could be going on here????
> >
> > David Waring
> >
> >
> >
> >
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> >
> >


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