BOOL APIENTRY DllMain( HANDLE hModule, 
                       DWORD  ul_reason_for_call, 
                       LPVOID lpReserved)
{
    return TRUE;
}

#ifndef MYLIB_H
#define MYLIB_H

#ifdef BUILDING_EXAMPLE_DLL
#define EXAMPLE_DLL __declspec(dllexport)
#else
#define EXAMPLE_DLL __declspec(dllimport)
#endif

int __stdcall EXAMPLE_DLL fun();

#endif MYLIB_H

#include "mylib.h"

__stdcall int fun()
{
    return 1;
}

g++ -c -DBUILDING_EXAMPLE_DLL mylib.cpp
g++ -shared -o mylib.dll mylib.o -Wl,--out-implib,mylib.a

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

int main(void){
    
    std::cout << fun();

    return 0;
}

g++ -c example_dll.cpp
g++ -o example_dll.exe example_dll.o -L. -lmylib

example_dll.exe
1