Re: [Pyfltk-user] Mystery swig error when exiting pyfltk program

[Andreas H. <and...@bl...>]

Hi Dima

I now got a better understanding of this error, but still no real 
solution. The problem as I can see it is that there are two Swig modules 
present. Each Swig module has a pointer to a global data structure 
containing all the swig type info: swig_module->types

When cleaning up, the function SWIG_Python_DestroyModule is called which 
deallocates the type info. If this is called before the destructor of 
the Fl_Window is called, then the error you are seeing is shown, because 
part of the information needed to cleanly deallocate the Fl_Window 
structure is contained in the type info and is now missing. As I see it, 
this is basically a race condition between the two modules. Besides the 
factors you have mentioned, also exchanging the import order of fltk and 
Fl_Gl_Image_Widget seems to resolve the problem. In that case, 
deallocation of the type info is done by the fltk module, properly after 
deallocating the Fl_Window.

Long story short, at the moment I see no possibility to really fix this.

Best regards

Andreas

On 6/27/25 00:18, Dima Kogan wrote:
> Hello!
>
> Can one of you help debugging a mystery error please? I have this
> script, that does more-or-less nothing:
>
>    import Fl_Gl_Image_Widget
>    from fltk import *
>
>    def func():
>        5
>
>    window = Fl_Window(100,200,"xxx")
>    window.end()
>    window.show()
>
>    Fl.run()
>
> I run it, the window pops up, I press Escape, the application exits.
> This is right. But as it exits it prints to the console:
>
>    swig/python detected a memory leak of type 'Fl_Window *', no destructor found.
>
> Does this mean something to any of you? I can make this message go away
> by doing unrelated-looking things. I can remove the definition of func()
> above (which doesn't do anything anyway). Or I can remove the "import
> Fl_Gl_Image_Widget" (which also doesn't do anything). Doing any of these
> silences the error.
>
> This is on a very recent Debian, running everything from packages:
>
>    dima@shorty:~/projects/mrcam$ dpkg -l python3-fltk python3-gl-image-display libfltk1.4
>    ii  python3-fltk:amd64       1.4.2.0-2    amd64        Python wrapper for the Fast Light Toolkit
>    ii  python3-gl-image-display 0.23-1       amd64        OpenGL-based image visualizer
>    ii  libfltk1.4:amd64 1.4.3-1      amd64        Fast Light Toolkit - main shared library
>
> I just upgraded libfltk1.4 from 1.4.2-7; it made no difference.
>
> python3-gl-image-display is mine:
>
>    https://github.com/dkogan/GL_image_display
>
> It also uses swig. Note that here I'm just doing the import; I'm NOT
> actually using it.
>
> Suggestions? Since unrelated-looking things make the problem disappear
> for me, I wouldn't be surprised if yall have trouble reproducing.
>
> Suggestions?
>
> Thanks!
>
>
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