#define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <dlfcn.h>

       int dladdr(void *addr, Dl_info *info);

       void *dlvsym(void *handle, char *symbol, char *version);

       The function dladdr() takes a function pointer and tries to resolve name and file where it is located.  Information is stored in the  Dl_info
       structure:

           typedef struct {
               const char *dli_fname;  /* Pathname of shared object that
                                          contains address */
               void       *dli_fbase;  /* Address at which shared object
                                          is loaded */
               const char *dli_sname;  /* Name of nearest symbol with address
                                          lower than addr */
               void       *dli_saddr;  /* Exact address of symbol named
                                          in dli_sname */
           } Dl_info;

 Load the math library, and print the cosine of 2.0:

       #include <stdio.h>
       #include <stdlib.h>
       #include <dlfcn.h>

       int
       main(int argc, char **argv)
       {
           void *handle;
           double (*cosine)(double);
           char *error;

           handle = dlopen("libm.so", RTLD_LAZY);
           if (!handle) {
               fprintf(stderr, "%s\n", dlerror());
               exit(EXIT_FAILURE);
           }

           dlerror();    /* Clear any existing error */

           <span style="color:#ff0000;">/* Writing: cosine = (double (*)(double)) dlsym(handle, "cos");
              would seem more natural, but the C99 standard leaves
              casting from "void *" to a function pointer undefined.
              The assignment used below is the POSIX.1-2003 (Technical
              Corrigendum 1) workaround; see the Rationale for the
              POSIX specification of dlsym(). */

           *(void **) (&cosine) = dlsym(handle, "cos");</span>

           if ((error = dlerror()) != NULL)  {
               fprintf(stderr, "%s\n", error);
               exit(EXIT_FAILURE);
           }

           printf("%f\n", (*cosine)(2.0));
           dlclose(handle);
           exit(EXIT_SUCCESS);
       }

//----------
//main.cpp:
//----------
#include "polygon.hpp"
#include <iostream>
#include <dlfcn.h>

int main() {
    using std::cout;
    using std::cerr;

    // load the triangle library
    void* triangle = dlopen("./triangle.so", RTLD_LAZY);
    if (!triangle) {
        cerr << "Cannot load library: " << dlerror() << '\n';
        return 1;
    }

    // reset errors
    dlerror();
    
    // load the symbols
    create_t* create_triangle = (create_t*) dlsym(triangle, "create");
    const char* dlsym_error = dlerror();
    if (dlsym_error) {
        cerr << "Cannot load symbol create: " << dlsym_error << '\n';
        return 1;
    }
    
    destroy_t* destroy_triangle = (destroy_t*) dlsym(triangle, "destroy");
    dlsym_error = dlerror();
    if (dlsym_error) {
        cerr << "Cannot load symbol destroy: " << dlsym_error << '\n';
        return 1;
    }

    // create an instance of the class
    polygon* poly = create_triangle();

    // use the class
    poly->set_side_length(7);
        cout << "The area is: " << poly->area() << '\n';

    // destroy the class
    destroy_triangle(poly);

    // unload the triangle library
    dlclose(triangle);
}


//----------
//polygon.hpp:
//----------
#ifndef POLYGON_HPP
#define POLYGON_HPP

class polygon {
protected:
    double side_length_;

public:
    polygon()
        : side_length_(0) {}

    virtual ~polygon() {}

    void set_side_length(double side_length) {
        side_length_ = side_length;
    }

    virtual double area() const = 0;
};

// the types of the class factories
typedef polygon* create_t();
typedef void destroy_t(polygon*);

#endif

//----------
//triangle.cpp:
//----------
#include "polygon.hpp"
#include <cmath>

class triangle : public polygon {
public:
    virtual double area() const {
        return side_length_ * side_length_ * sqrt(3) / 2;
    }
};


// the class factories
extern "C" polygon* create() {
    return new triangle;
}

extern "C" void destroy(polygon* p) {
    delete p;
}