[Distutils] Undefined symbol when loading a library using dlopen

[Erik Bray]

On Tue, Aug 26, 2014 at 5:54 PM, Jerome Fuselier
<jerome.fuselier at free.fr> wrote:
> 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.

I don't think this question really has to do with distutils' role in
this.  But have you tried declaring

extern "C" {
    int simple(void);
}

in func.cpp?  I think that should do it (to get around C++ name mangling).

Best of luck,

Erik