Shared library runtime loading and C++ classes

[Francois Goudal]

Hi,

I'm currently trying to write a plugin system for my C++ project.

I have been reading the C++-dlopen-mini-HOWTO and now I'm trying to 
implement it in my own project.
But there are some particularities in my project which makes it more 
difficult and I'm a bit blocked right now with dynamic linking issues :


For the plugins, I have defined a virtual class called CConfigPlugin, 
and right now I have a test plugin class called CTestConfigPlugin which 
inherits from the CConfigPlugin and implements all the virtual methods.

Also, together with the CTestConfigPlugin, I have a static "C" function 
called get() which creates an instance of CTestConfigPlugin and returns 
a pointer to it.

This CTestConfigPlugin class together with the static "C" function get() 
are compiled using g++ and linked to a shared library, so I have a 
test.so file


The goal is that my main program will dlopen this test.so file and run 
dlsym to find the get() symbol and call it in order to get an instance 
of CTestConfigPlugin which it will be able to call methods through the 
CConfigPlugin interface class.


All this is exactly like in the howto, now I have a difference though :

My main program contains a singleton class called CConfigController 
which I can get through the method Instance().

I need my plugins to be able to get the instance of this class and run 
some methods of it.

As a consequence, the code of the Test plugin is calling 
CConfigController::Instance().
The configcontroller.cpp file is linked with my main program, and as a 
matter of fact :
$ objdump -t a.out  | grep ConfigController
0804a881 l     F .text	0000001c 
_GLOBAL__I__ZN7mytest17CConfigController8InstanceEv
0804e6a8 g     F .text	000000d7 
_ZN7mytest17CConfigController8InstanceEv


I would have hoped that when dlopen would load my test.so, test.so which 
requires the symbol _ZN7mytest17CConfigController8InstanceEv would find 
it automatically in the main program, but obviously it doesn't work that 
way :

In the main program, dlopen fails, and dlerror returns that string :
/u/fg/test.so: undefined symbol: _ZN7avioipd17CConfigController8InstanceEv

I also have tried to link configcontroller.cpp together with the 
testconfigplugin.cpp into the .so, but it doesn't help (and anyway, that 
would potentially conflicts since loading the .so would lead to 2 
symbols with the same name being there since the symbol already exists 
in the main program).


Am I trying to do something impossible, or could somebody give me a hint 
on how I can make this work ?

Thank's in advance

--
Francois Goudal
EPITA