Re: [Plplot-devel] Peculiar pyqt4 linking error on Linux with Qt4.6.1

[Alan W. I. <ir...@be...>]

On 2010-03-04 10:04-0500 Hazen Babcock wrote:

> Did you mean to send [your first post on this topic] to the dev mailing list?

Yes, I have now forwarded that post to the list, and this post is
on list as well.

>
> Do you have another version of Qt on your system in a place where it could 
> get picked up instead?

Yes.  In fact, during my research to answer you in more detail, I actually
stumbled on an amazingly simple solution to this complex issue.

First I collected a whole bunch of
data with nm, ldd, and readelf, and the conclusion is that PLplot
and the /home/software/qtsdk-2010.01/qt/lib/ version of Qt4 are linked
perfectly with correct DT_RPATH values.  The "undefined" symbol,
_Z9qBadAllocv is defined by /home/software/qtsdk-2010.01/qt/lib/libQtCore.so.4
but not the system version /usr/lib/libQtCore.so.4.  Presumably this
symbol reflects some API that was added somewhere between my system
version of Qt4 (4.5.2) and the downloaded version of Qt4 (4.6.1)
in /home/software/qtsdk-2010.01/qt/lib which is why it is undefined
for the system version.

Second, I looked closely at your pyqt4 example, and the command

from PyQt4 import QtCore, QtGui

loads, 
/usr/lib/python-support/python-qt4/python2.5/PyQt4/QtCore.so
/usr/lib/python-support/python-qt4/python2.5/PyQt4/QtGui.so

which from ldd data (without LD_LIBRARY_PATH set) on those plug-ins also
loads /usr/lib/libQtCore.so.4 and /usr/lib/libQtGui.so.4 which are the _old_
system versions of those libraries.

Your pyqt4 example later executes the command

import plplot_pyqt4

which dynamically loads

plplot_pyqt4.so and (according to ldd data on that extension module)
libQtSvg.so.4, libQtXml.so.4, libQtGui.so.4, and libQtCore.so.4.  Since
at this point, libQtSvg.so.4 and libQtXml.so.4 have not previously
been loaded, they are loaded from the correct
/home/software/qtsdk-2010.01/qt/lib, but libQtGui.so.4, and libQtCore.so.4
have already been loaded from the old versions so that is not done
again, and bingo! we have the undefined symbol issue.

The amazingly simple fix for this complex issue is to do

import plplot_pyqt4

_before_

from PyQt4 import QtCore, QtGui

in the example.  The first import command loads in consistent versions (from
/home/software/qtsdk-2010.01/qt/lib) for all Qt libraries used by
plplot_pyqt4 and all Qt libraries recursively used by those libraries.  Then
the second "from PyQt4 ..." command also uses those consistent versions
since they are already loaded.

This fix (with comments in the code as to why) has been done for revision
10844.

What a relief to finally arrive at such a simple solution to this complex
dynamic loading issue!  It strongly reminds me (probably for
exactly the same reasons) of all the library ordering shenanigans we
had to go through in the static library era.

Alan
__________________________
Alan W. Irwin

Astronomical research affiliation with Department of Physics and Astronomy,
University of Victoria (astrowww.phys.uvic.ca).

Programming affiliations with the FreeEOS equation-of-state implementation
for stellar interiors (freeeos.sf.net); PLplot scientific plotting software
package (plplot.org); the libLASi project (unifont.org/lasi); the Loads of
Linux Links project (loll.sf.net); and the Linux Brochure Project
(lbproject.sf.net).
__________________________

Linux-powered Science
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