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Subject Author Date
XML::Twig::XPath - strange problem Henry Law 11-10-2005
Posted by Henry Law on November 10, 2005, 9:44 am
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The following short test program is the kernel of a problem I'm having
using xpath searches with XML::Twig::XPath elements. Can someone help
me find out what I'm doing wrong? It seems that if I use my own
subroutine to do the xpath search I get an error from the Twig code, but
if I code the same statements inline the results are what I expect.

============ Test program ================
#!/usr/bin/perl

use strict;
use warnings;
use XML::Twig::XPath;
# ----------------- problem section of code -------------------
# This way fails
#my $da_xml = new XML::Twig::XPath;
#$da_xml = get_xml();

# This way works
my $da_doc = new XML::Twig::XPath;
$da_doc->safe_parsefile("test.xml");
my $da_xml = $da_doc->root;
# -------------------------------------------------------------

my @f = $da_xml->findnodes("//bkfile[b_filename='testfile']");
$f[0]->print;
print "\n";

sub get_xml {
my $da_xml = new XML::Twig::XPath;
$da_xml->safe_parsefile("test.xml");
return $da_xml->root;
}

============= test.xml ==============
<bkdirectory>
<bkfile>
<dbid>445</dbid>
<b_path>/nfb</b_path>
<b_filename>testfile</b_filename>
</bkfile>
</bkdirectory>
=====================================

My results, using the program as above:
[nfb@neptune lib]$ ./tryerr.pl
<bkfile><dbid>445</dbid><b_path>/nfb</b_path><b_filename>testfile</b_filename></bkfile>

Switching the comments so the "failing" section is active I get:
[nfb@neptune lib]$ ./tryerr.pl
Can't use an undefined value as a HASH reference at
/usr/lib/perl5/site_perl/5.8.6/XML/Twig/XPath.pm line 114.

Environment is Fedora Core 4, Perl 5.8.6; XML::Twig::XPath is "v 1.15
2005/07/05 10:47:19"

--

Henry Law <>< Manchester, England


Posted by metaperl@gmail.com on November 11, 2005, 1:19 am
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the author of XML::Twig reads perlmonks.org regularly. he also is
responsive to direct emails, so you might shoot him one. He's a good
guy (mirod on perlmonks, MIchael Rodriguez in real life.)



Posted by Michel Rodriguez on November 11, 2005, 4:12 pm
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Henry Law wrote:
> The following short test program is the kernel of a problem I'm having
> using xpath searches with XML::Twig::XPath elements. Can someone help
> me find out what I'm doing wrong? It seems that if I use my own
> subroutine to do the xpath search I get an error from the Twig code, but
> if I code the same statements inline the results are what I expect.

Weird,

It looks like when returning from the sub, the twig field for the root
element is lost (it becomes undef), which causes problems later on (the
XML::XPath object is attached to the twig itself).

I'll look some more (you can file a bug with RT if you want)

Thanks.

--
mirod


Posted by Michel Rodriguez on November 11, 2005, 4:29 pm
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Henry Law wrote:
> It seems that if I use my own subroutine to do the xpath search I get
> an error from the Twig code, but if I code the same statements inline
> the results are what I expect.

It is a problem of scope: the XML::Twig::XPath object goes out of scope
at the end of the sub, so you can't use it any more.

You have to have get_xml return the XML::Twig::XPath object.

and then write:

my $da_doc= get_xml(); # so the document object hangs around
my $da_xml= $da_doc->root;


I can't decide whether this is a bug or a feature though (read: if it is
a bug, it's a hard one to fix ;--). The document (XML::Twig::XPath
object) is destroyed because the link from the root node
(XML::Twig::XPath::Elt) to the document is weak, to avoid memory leaks.
If I don't weaken that link, then memory leaks happen.

A better error message might be all I can do.

Does this help?

--
mirod


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