How do I load a shared object in C++?

Question

I have a shared object (a so - the Linux equivalent of a Windows dll) that I'd like to import and use with my test code.

I'm sure it's not this simple ;) but this is the sort of thing I'd like to do..

#include "headerforClassFromBlah.h"

int main()
{
    load( "blah.so" );

    ClassFromBlah a;
    a.DoSomething();
}

I assume that this is a really basic question but I can't find anything that jumps out at me searching the web.

Answer 1

There are two ways of loading shared objects in C++

For either of these methods you would always need the header file for the object you want to use. The header will contain the definitions of the classes or objects you want to use in your code.

Statically:

#include "blah.h"
int main()
{
  ClassFromBlah a;
  a.DoSomething();
}

gcc yourfile.cpp -lblah

Dynamically (In Linux):

#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(1);
    }
    dlerror();    /* Clear any existing error */
    cosine = dlsym(handle, "cos");
    if ((error = dlerror()) != NULL)  {
        fprintf (stderr, "%s\n", error);
        exit(1);
    }
    printf ("%f\n", (*cosine)(2.0));
    dlclose(handle);
    return 0;
}

*Stolen from dlopen Linux man page The process under windows or any other platform is the same, just replace dlopen with the platforms version of dynamic symbol searching.

For the dynamic method to work, all symbols you want to import/export must have extern'd C linkage.

There are some words Here about when to use static and when to use dynamic linking.