Load a .so object with QLibrary from PyQt

[ilian]

Hello, I've been using QLibrary many times, however now I have a case when I want to load .so from PyQt. Following their docs it should be done the same way, but since it's python I don't follow why load() returns with False. Here is the simple .so code:

interface.h:

#ifndef INTERFACE_H
#define INTERFACE_H

#ifdef __cplusplus
extern "C" {
#endif

int  init(int argc, char** argv);

#ifdef __cplusplus
};
#endif

I have and a .cpp file :
interface.cpp

#include "interface.h"
#include "testwidget.h"

#include <QApplication>

int init(int argc, char **argv)
{
    QApplication a(argc, argv);
    MyWidget w;
    w.init();
    return  a.exec();
}

Assume that testwidget is just a 200 x 200 empty window. So here is what is happening in python:

import  os as UNIX
import  sys
from PyQt5 import QtCore

def main(*args, **kwargs):
    testlib = QtCore.QLibrary('/home/ilian/Qt/build-testpyqtlib-Desktop_Qt_5_8_0_GCC_64bit-Debug/libtestpyqtlib.so')

    res = testlib.load()
    if res:
        print("OK, loaded library!")
    else:
        print("Failed to load library!")

if __name__ == "__main__":
    main(sys.argv)

Don't fire at me why I am doing it with 'PyQt, it's something inherited and I have to work with it. Just tell me why this call fails here? Everything is in place as directories. It just does not loads the .so` file.

Regards.

#endif // INTERFACE_H

[jsulm]

@ilian

What does http://doc.qt.io/qt-5.9/qlibrary.html#errorString say after load()?

[ilian]

@jsulm That is dull now... it loaded. Maybe it was something with the python interpreter setup from the PyCharm don't know exactly.

[jsulm]

@ilian In any case you should print out the output of errorString() to get more information next time it fails.

[ilian]

@jsulm Are you aware of PyQt, it resolved my init function with some sip.voidptr thing, that I don't know how to call. Originally I'd get a C function pointer but this sip.voidptr is not callable.

[jsulm]

@ilian What init function do you mean? Is it a function from your library? If so then sip.voidptr probably means that it could not resolve the function.
See http://doc.qt.io/qt-5.9/qlibrary.html#resolve, you need to export your functions to be able to resolve them.

[ilian]

@jsulm I have an init function

extern "C" int init(int argc, char **argv)
{
    QApplication a(argc, argv);
    MyWidget w;
    w.init();
    return  a.exec();
}

resolve() returns a non null value, which is sip.voidptr, and I am expecting this to be a C function pointer but, I have no idea how to call it.

[jsulm]

@ilian Please take a look at the link I posted before: you need to export your function, else it cannot be resolved.

You are aware that a.exec() will block until you close your app? That means init() call will block.

[ilian]

@jsulm I am aware yes, removed it, and just show the widget. However, I did the example with MY_EXPORT macro, I still get this voidptr. I will test in C++ program to see if there everything is OK.

[jsulm]

@ilian This void pointer is valid, but I don't know how to use it if it points to a function. Never did this with PyQt.

[ilian]

@jsulm I've made a similar C++ program with the function pointer, and it does what I want, open a widget and enters it's event loop. In short, as C++ program, loading the library everything is fine, as PyQt - it's not, or the reslove form PyQt and that sip.voidptr are some weird stuff, we know not of.

[ilian]

libc = CDLL('/home/ilian/git-projects/build-testpyqtlib-Desktop_Qt_5_7_1_GCC_64bit-Debug/libtestpyqtlib.so') That line of code solved it all, just called libc.init(0, 0) and the widget from the lib appeared...

[jsulm]

@ilian said in Load a .so object with QLibrary from PyQt:

sip.voidptr

this is not weird stuff :-)
It is documented, I just don't know how to use it if it contains a function pointer - I could not find anything in SIP documentation for this use case.

[ilian]

@jsulm Yes, same here, I don't know how to solve that problem, I was expecting a better examples how to make it the C way here, alas none was present.