[Distutils] Undefined symbol when loading a library using dlopen

[Jerome Fuselier]

Hello,

I'm trying to wrap a C++ library which uses dlopen to load a dynamic 
library and I don't manage to get rid of an "undefined symbol" error.

I tried to simulate my problem with a simpler example so hopefully 
someone may be able to tell me what I'm doing wrong.

I have a file func.cpp which provides 2 functions :

func.cpp:

int simple(void) {
     return 12;
}

int call_lib(void) {
     int res;
     void *handle = dlopen("/home/jerome/Test/libDyn.so", RTLD_NOW);

     if (!handle) {
         return -1;
     }
     dlerror();
     char* err = 0;
     int (*fun_dyn)() = (int (*)())dlsym(handle, "fun_dyn");
     if ( (err = dlerror()) != 0 ) {
         dlclose(handle);
         return -1;
     }
     res = (*fun_dyn)();
     dlclose(handle);
     return res;
}

It is created like that:
$ g++ -Wall -g -fPIC -c -o func.o func.cpp
$ ar rcs libFun.a func.o

I have a dynamic library which is called from call_lib() which depends 
on libFun.a:

libDyn.cpp:

#include "func.hpp"

int fun_dyn(void) {
     return simple();
}

It is created like that:
$ g++ -Wall -g -fPIC -c -o libDyn.o libDyn.cpp
$ g++ -Wall -g -fPIC -shared -o libDyn.so libDyn.o

I'm using swig to wrap the libFun.a library with this mylib.i interface 
file:

%module mylib

int simple(void);
int call_lib(void);

Then I'm using this setup.py file to create the Python module:

from distutils.core import setup, Extension

mylib = Extension("_mylib",
                   sources=['mylib.i'],
                   language="c++",
                   extra_link_args=['-rdynamic'],
                   libraries=["Fun", "dl"],
                  )

setup(name="MyLib",
       platforms=["Linux"],
       ext_modules=[mylib],
       py_modules = ["mylib"])


It's a simplified example but when I compile everything I get this error 
in Python:
 >>> import _mylib
 >>> _mylib.call_lib()
failed to open shared object file
/home/jerome/Test/libDyn.so: undefined symbol: simple

If I do a nm _mylib.so I see that the symbol simple is present in the 
library so I'm wondering why dlopen doesn't see it when it tries to load 
the library.

I know I can recompile libDyn.so to link libFun.a but this would means I 
would need to recompile all the plugins provided for the library I'm 
trying to wrap and that's not really a solution for my project.

Sorry for the long email. I can provide a tar.gz of the source code if 
needed.

Thanks for your help
Jerome