shared object stuff

README.md

$ g++ -o libcode.so code.cpp -ldl -rdynamic -shared -fPIC -std=c++20
$ g++ -o main main.cpp -rdynamic -std=c++20 -Os

The -rdynamic flag when compiling the executable adds the unused symbols to the table so we can use the definition from the main/executable file in the shared object's foo() call.

code.cpp

#include <string>

extern "C" void print(std::string);

extern "C" void foo() { print("foo called print but without definition"); }

main.cpp

#include <dlfcn.h>
#include <format>
#include <functional>
#include <iostream>
#include <string>

extern "C" void print(std::string s) { std::cout << s << '\n'; }

int main() {
  void *shared_object = dlopen("./libcode.so", RTLD_LAZY | RTLD_GLOBAL);
  if (shared_object == nullptr) {
    std::cout << std::format("Error loading shared object: {}\n", dlerror());
    return 1;
  }

  std::function<void()> so_foo =
      reinterpret_cast<void (*)()>(dlsym(shared_object, "foo"));
  if (so_foo == nullptr) {
    std::cout << std::format("Error finding symbol \"print\": {}\n", dlerror());
    return 2;
  }

  so_foo();
}