title: updating win32 window to display generated barcode
body: <p>I am creating a barcode generator application in win32. I got the library to generate the needed barcodes and if I hardcode the UPC it displays the barcode fine. However I want to be able to enter a string in the text field and have it generate the barcode and display it on the window. </p>
  
  <p>I've looked into UpdateWindow, but the HWND variable that is in </p>
  
  <p>LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)</p>
  
  <p>is out of scope in the function that I want to call it in, and I've also tried making a global HWND hWnd, but it throws a compiler error. </p>
  
  <p>If this is not enough information, please let me know so I can update it with the necessary information.</p>