C++ Shared Library (Dynamically Linked Library) Example

1. shared.h

#pragma once

// extern: https://docs.microsoft.com/en-us/cpp/cpp/extern-cpp?view=vs-2019
// why extern "C" : https://stackoverflow.com/a/42183390/2790983
extern "C" {
    // function declaration
    int add(int, int);
}

2. shared.cpp

#include <iostream>
#include "shared.h"

// function definition
int add(int a, int b) {
    return a + b;
}

3. main.cpp

#include <iostream>
#include <dlfcn.h> // https://pubs.opengroup.org/onlinepubs/7908799/xsh/dlfcn.h.html
#include "shared.h"

int main() {
    
    // declaration (template) of add function
    // function typedef: http://www.iso-9899.info/wiki/Typedef_Function_Type
    typedef int add_t(int, int);
    
    // load shared library
    // https://pubs.opengroup.org/onlinepubs/7908799/xsh/dlopen.html
    void *handler = dlopen("./shared.so", RTLD_LAZY);
  
    // extract `add` symbol (pointer) and convert it to function pointer
    // https://pubs.opengroup.org/onlinepubs/7908799/xsh/dlsym.html
    add_t *add = (add_t*) dlsym( handler, "add" );
    
    // execute function
    std::cout << (*add)(5, 2) << std::endl;
}

4. commands.sh

# compilation process: https://en.wikibooks.org/wiki/C%2B%2B_Programming/Programming_Languages/C%2B%2B/Code/Compiler/Linker
# Shared library concept: http://ehuss.com/shared/

# 1. Creating the shared library
# Command reference: https://www.oreilly.com/library/view/c-cookbook/0596007612/ch01s05.html
g++ -dynamiclib -fPIC shared.cpp -o shared.so # MacOS

# 2. Creating the main executable file
g++ main.cpp -o main.exe

# 3. Running main executable
./main.exe