Question

I am loading a DLL from an application and I want the DLL to be able to use a function implemented in the application. The definition of the function is put in a header and is included in the DLL. Can anyone tell me what I am doing wrong here or whether this can’t be done? Thanks for any help.

App:

include <API.h>

extern "C" int Mult(int x, int y)
{
    return x*y;
}

int main(int argc, char **argv)
{
if(argc <= 1)
    return -1;

void * pHandle = dlopen(argv[1], RTLD_NOW);
if(pHandle == NULL)
{
    cout << "Can't find DLL : " << argv[1] << endl;
    cout << dlerror() << endl;
    return -2;
}
else
    cout << "DLL Loaded" << endl;

The API.h file:

#ifndef __APP_API__
#define __APP_API__

extern "C" int Mult(int x, int y);

#endif

And the DLL:

#include <API.h>

int Imp1::GetFunctionID()
{
   return Mult(42, 42);
}

When run, this gives me the error:

Can’t find DLL : ./ll.d.so
./ll.d.so: undefined symbol: Mult

Please help. Thanks

Answer 1

You should instruct your compiler to put all the symbols of your executable (App) in it’s dynamic symbol table. Otherwise, as Marcus Lindblom said, the dependencies would be one way only. With g++, the option is -rdynamic.