__declspec(dllexport) and __declspec(dllimport) in C++

I often see __declspec(dllexport) / __declspec(dllimport) instructions on Windows, and __attribute__((visibility("default"))) on Linux with functions, but I don't know why.

Could you explain to me, why do I need to use theses instructions for shared libraries?

The Windows-exclusive __declspec(dllexport) is used when you need to call a function from a Dll (by exporting it) , that can be accessed from an application.

Example This is a dll called "fun.dll" :

// Dll.h :

#include <windows.h>

extern "C" {

 __declspec(dllexport) int fun(int a);   // Function "fun" is the function that will be exported

}

// Dll.cpp :

#include "Dll.h"

int fun(int a){
return a + 1;
}

You can now access the "fun" from "fun.dll" from any application :

#include <windows.h>

typedef int (fun)(int a);  // Defining function pointer type

int call_fun(int a){
  int result = 0;

  HMODULE fundll = LoadLibrary("fun.dll");   // Calling into the dll
  
  if(fundll){
    fun* call_fun = (fun*) GetProcAddress(fundll, "fun"); // Getting exported function
     if(call_fun){
       result = call_fun(a);    // Calling the exported fun with function pointer
     }       
  }
return result;
}