How to Create DLL Files

 // MathLibrary.h - Contains declarations of math functions 
 #pragma once 
 #ifdef MATHLIBRARY_EXPORTS 
 #define MATHLIBRARY_API __declspec(dllexport) 
 #else 
 #define MATHLIBRARY_API __declspec(dllimport) 
 #endif 
 // The Fibonacci recurrence relation describes a sequence F 
 // where F(n) is { n = 0, a 
 //               { n = 1, b 
 //               { n > 1, F(n-2) + F(n-1) 
 // for some initial integral values a and b. 
 // If the sequence is initialized F(0) = 1, F(1) = 1, 
 // then this relation produces the well-known Fibonacci 
 // sequence: 1, 1, 2, 3, 5, 8, 13, 21, 34, ... 
 // Initialize a Fibonacci relation sequence 
 // such that F(0) = a, F(1) = b. 
 // This function must be called before any other function. 
 extern 
 "C" 
 MATHLIBRARY_API 
 void 
 fibonacci_init 
 ( 
 const 
 unsigned 
 long 
 long 
 a 
 , 
 const 
 unsigned 
 long 
 long 
 b 
 ); 
 // Produce the next value in the sequence. 
 // Returns true on success and updates current value and index; 
 // false on overflow, leaves current value and index unchanged. 
 extern 
 "C" 
 MATHLIBRARY_API 
 bool 
 fibonacci_next 
 (); 
 // Get the current value in the sequence. 
 extern 
 "C" 
 MATHLIBRARY_API 
 unsigned 
 long 
 long 
 fibonacci_current 
 (); 
 // Get the position of the current value in the sequence. 
 extern 
 "C" 
 MATHLIBRARY_API 
 unsigned 
 fibonacci_index 
 (); 

 // MathLibrary.cpp : Defines the exported functions for the DLL. 
 #include 
 "stdafx.h" // use pch.h in Visual Studio 2019 
  
 #include 
 <utility> 
  
 #include 
 <limits.h> 
  
 #include 
 "MathLibrary.h" 
  
 // DLL internal state variables: 
 static 
 unsigned 
 long 
 long 
 previous_ 
 ; 
 // Previous value, if any 
 static 
 unsigned 
 long 
 long 
 current_ 
 ; 
 // Current sequence value 
 static 
 unsigned 
 index_ 
 ; 
 // Current seq. position 
 // Initialize a Fibonacci relation sequence 
 // such that F(0) = a, F(1) = b. 
 // This function must be called before any other function. 
 void 
 fibonacci_init 
 ( 
 const 
 unsigned 
 long 
 long 
 a 
 , 
 const 
 unsigned 
 long 
 long 
 b 
 ) 
 { 
 index_ 
 = 
 0 
 ; 
 current_ 
 = 
 a 
 ; 
 previous_ 
 = 
 b 
 ; 
 // see special case when initialized 
 } 
 // Produce the next value in the sequence. 
 // Returns true on success, false on overflow. 
 bool 
 fibonacci_next 
 () 
 { 
 // check to see if we'd overflow result or position 
 if 
 (( 
 ULLONG_MAX 
 - 
 previous_ 
 < 
 current_ 
 ) 
 || 
 ( 
 UINT_MAX 
 == 
 index_ 
 )) 
 { 
 return 
 false 
 ; 
 } 
 // Special case when index == 0, just return b value 
 if 
 ( 
 index_ 
 > 
 0 
 ) 
 { 
 // otherwise, calculate next sequence value 
 previous_ 
 += 
 current_ 
 ; 
 } 
 std 
 :: 
 swap 
 ( 
 current_ 
 , 
 previous_ 
 ); 
 ++ 
 index_ 
 ; 
 return 
 true 
 ; 
 } 
 // Get the current value in the sequence. 
 unsigned 
 long 
 long 
 fibonacci_current 
 () 
 { 
 return 
 current_ 
 ; 
 } 
 // Get the current index position in the sequence. 
 unsigned 
 fibonacci_index 
 () 
 { 
 return 
 index_ 
 ; 
 }