Niels Vanspauwen schrieb:
Hi,
I'm trying out PyQt and have run into a weird problem which I can't
seem to resolve. I've searched the internet and the PyQt wiki for the
answer but alas. Therefore, I turn to you in the hope you can help me.
I'm running on RedHat8, using gcc 3.2.3, QT 3.3.5 (static
multithreaded), PyQt 3.15 and SIP 4.3.1.
The problem is the following: following the instructions to the
letter, I can build the PyQt libraries without any problems. I can run
example applications without problems too, _except_ for any example
that uses QCanvas. For example, if I run examples3/canvas/canvas, I
get the following error message:
QPaintDevice: Must construct a QApplication before a QPaintDevice
Looking at the QT source code, this is because the qApp pointer is 0.
Looking at the Python code of the example though, it's clear that the
QApplication constructor is called _before_ any constructor of a
QPaintDevice descendant, so something is very wrong.
After some debugging, it appears to me that there are _two_ copies of
the qApp pointer in the program! One is defined in qt.so and the other
in qtcanvas.so. For example, if I do: "nm qt.so | grep qApp | c++filt"
I get:
00987b0c t _GLOBAL__D_qApp
00987af0 t _GLOBAL__I_qApp
00a52670 t qAppendItems(QTextEngine*, int&, int&, BidiControl&,
QChar::Direction)
004a6466 t func_qAppName(_object*, _object*)
004a6506 t func_qAppClass(_object*, _object*)
00927600 T qAppName()
0092760a T qAppClass()
00f5e638 D qApp
00f1d59c D sipNm_qt_qApp
00f1e0af D sipNm_qt_qAppClass
00f1e090 D sipNm_qt_qAppName
The interesting part is the "D qApp", clearly indicating that qApp is
defined in the data section.
Doing the same command on qtcanvas.so yields:
002a80ec t _GLOBAL__D_qApp
002a80d0 t _GLOBAL__I_qApp
0030f7d4 t qAppendItems(QTextEngine*, int&, int&, BidiControl&,
QChar::Direction)
0041077c T qAppName()
00410786 T qAppClass()
006d8a24 D qApp
Notice the last line: qApp also appears to be defined here!
Therefore I conclude that Qt's qapplication.o file must be included in
both qt.so and qtcanvas.so, which seems odd to me.
What I _think_ happens when I run the examples3/canvas/canvas.py
application is the following:
1. qt.qApp get's initialized to 0 by importing the module qt and
loading the qt.so file
2. qtcanvas.qApp get's initialized to 0 by importing the module
qtcanvas and loading the qtcanvas.so file. I don't know how the loader
resolves the conflict of the double definition of qApp, but I'm
guessing it just creates two copies.
3. qt.qApp get's initialized by calling QApplication constructor
4. When the QCanvas is constructed, it refers to qtcanvas.qApp (since
that reference should have been resolved already when building the .so
file), which is still 0, so we get the error.
So far for my research. Now my questions: given that I find so little
help on the web for this, I'm guessing that there's nothing wrong with
the way PyQt works, but there's something wrong with my setup or with
the way I built PyQt. I'm assuming that examples3/canvas/canvas works
fine on your system.
Do you have any idea what I could be doing wrong?
You can't build SIP/PyQt as dynamic loadable modules linked with a
static Qt library. You either have to link SIP/PyQt with a shared Qt
library or you have to build SIP/PyQt as static libraries and link
everything into a single Python interpreter.
Ulli
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