Bus error -- Re: [mapserver-users] Re: Perl/mapscript error

[Puneet Kishor]

Hi Lowell,

My problem is specifically in mapObj... the DESTROY sub in the mapObj. The
error message you refer to is the only error message.

You asked for my map file... I can send you the file, but it is really a
very simple map file. The problem is not with that... the problem is with
some error trapping... I have got my perl code/map file/data combo working
now. However, if something goes wrong in my code, this error crops up.

here is what I have gathered...

if I make some mistake in my coding, mapscript.pm is unable to recover from
it... in other words, while the actual error is not in the mapscript.pm
code, it is the mapscript.pm code that croaks because the mapObj that it is
trying to DESTROY doesn't exist anymore.

Btw, this is not necessarily a mapscript.pm specific thing... that is, there
are others who have experienced such an error with other scripts. This is
also not an OS specific thing... I posted an inquiry about "bus error" on
perl at macosx and a few there have seen this crop up here and there. But, note
that Giorgio also experienced this error... I am assuming he is not using OS
X.

In other words, there is some assertion checking or error-trapping that can
be implemented in mapscript.pm which will cause DESTROY to not fail if the
object it is trying to destroy (in this case, mapObj) doesn't exist. This
way mapscript.pm can fail gracefully, and not cause a fatal error for Perl
altogether.

Thanks,

pk/



> -----Original Message-----
> From: Lowell Filak [mailto:lfilak at medinaco.org]
> Sent: Monday, April 29, 2002 2:44 PM
> To: Puneet Kishor; Giorgio Volpe
> Cc: mapserver-users at lists.gis.umn.edu
> Subject: Re: Bus error -- Re: [mapserver-users] Re: 
> Perl/mapscript error
> 
> 
> Any gurus feel free to step forward, in the mean-time I have 
> a couple of questions:
> Puneet - is "(in cleanup) Not a HASH reference at 
> /Library/Perl/darwin/mapscript.pm ine 1468." the ONLY error 
> message in your log? Are there ANY other messages in the log 
> around the same time (within a second or so)?
> Giorgio - which DESTROY subroutine did you insert your print 
> statement into, mapObj, layerObj, classObj, ...? It looks 
> like layerObj as opposed to mapObj where Puneet is having trouble.
> Lowell F.
> 
> The following message was sent by Puneet Kishor 
> <pkishor at GeoAnalytics.com> on Sun, 28 Apr 2002 10:29:51 -0500.
> 
> > thanks for analysis Giorgio... I will try out some of your 
> shenanigans 
> > detailed here...
> > 
> > since it has not been reported exhaustively, I guess there 
> is something 
> > about our implementation that is bringing this out. My 
> guess is so... if 
> > there is an error in the Perl script (because of our own 
> programming 
> > mistake), the script goes kablooie and the map object 
> vanishes without a 
> > warning. However, the coded garbage collector tries to clean it up, 
> > except it doesn't find anything. So, it croaks.
> > 
> > >
> > > We need a PERL guru ...!
> > 
> > I think we already have a few on this list, probably 
> starting with Steve 
> > Lime at the very top considering he wrote most of this 
> wizardry to begin 
> > with... I am sure between him and Steve W and Lowell and 
> Paul Ramsey, 
> > etc., we will be able to squash this.
> > 
> > pk/
> > 
> > 
> > 
> > 
> > On Sunday, April 28, 2002, at 10:20  AM, Giorgio Volpe wrote:
> > 
> > > Puneet Kishor wrote:
> > >>
> > >> ok... I have spent a good part of today battling this, and have
> > >> discovered the following...
> > >>
> > >> one of the reasons I am getting these errors because I 
> have -w switch 
> > >> on
> > >> my shebang line. After much finagling, I can get the 
> image to draw, but
> > >> with the -w switch I get the following error as well --
> > >>
> > >> (in cleanup) Not a HASH reference at 
> /Library/Perl/darwin/mapscript.pm
> > >> line 1468.
> > >>
> > >> that line in mapscript.pm is the one that is trying to 
> clean up... and
> > >> failing. Specifically,
> > >>
> > >> sub DESTROY {
> > >>      my $self = tied(%{$_[0]});
> > >>      delete $ITERATORS{$self};
> > >>      if (exists $OWNER{$self}) {
> > >>          mapscriptc::delete_mapObj($self);
> > >>          delete $OWNER{$self};
> > >>      }
> > >> }
> > >
> > > I've the same problem!
> > > It is not a warning but a fatal error for perl.
> > >
> > > The problem, i guess, is related to the use of tied hash 
> to support
> > > mapscript objects ...
> > >
> > > I've added a PRINT ...
> > >
> > > sub DESTROY {
> > >     print STDERR "!!!! Destroing $_[0] ...\n";
> > >     my $self = tied(%{$_[0]});
> > >     ...
> > >
> > > The DESTRUCTOR is called two times! The first one with 
> $_[0] that is a
> > > reference to an hash, the second to a scalar so function 
> "tied" raises
> > > the error!
> > >
> > > !!!! Destroing layerObj=HASH(0x83e925c) ...
> > > !!!! Destroing layerObj=SCALAR(0x83d8ef4) ...
> > >
> > > Empirically i've seen that, if I undef the variable containing the
> > > reference to the object before returning from the sub 
> where i use it,
> > > than the error disappears ... (also if the two call to 
> the DESTRUCTOR
> > > are always there, but probably the undef value is passed)
> > >
> > >
> > > We need a PERL guru ...!
> > >
> > > --
> > >
> > >     Giorgio
> > >
> > > -----------------------------------------
>