Windows-API-examples/firststeps/centering.c

#include <windows.h>

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
void CenterWindow(HWND);

int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
                    PWSTR pCmdLine, int nCmdShow) {
                    
    MSG  msg;    
    WNDCLASSW wc = {0};
    wc.lpszClassName = L"Center";
    wc.hInstance     = hInstance;
    wc.hbrBackground = GetSysColorBrush(COLOR_3DFACE);
    wc.lpfnWndProc   = WndProc;
    wc.hCursor       = LoadCursor(0, IDC_ARROW);
  
    RegisterClassW(&wc);
    CreateWindowW(wc.lpszClassName, L"Center",
                  WS_OVERLAPPEDWINDOW | WS_VISIBLE,
                  100, 100, 250, 150, 0, 0, hInstance, 0);  

    while (GetMessage(&msg, NULL, 0, 0)) {
  
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    return (int) msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, 
    WPARAM wParam, LPARAM lParam) {
    
  switch(msg) {
  
      case WM_CREATE: {
      
          CenterWindow(hwnd);
          return 0;
      }

      case WM_DESTROY: {
      
          PostQuitMessage(0);
          return 0;
      }
  }

  return DefWindowProcW(hwnd, msg, wParam, lParam);
}

void CenterWindow(HWND hwnd) {

    RECT rc;
    
    GetWindowRect(hwnd, &rc) ;
    
    SetWindowPos(hwnd, 0, 
        (GetSystemMetrics(SM_CXSCREEN) - rc.right)/2,
        (GetSystemMetrics(SM_CYSCREEN) - rc.bottom)/2,
         0, 0, SWP_NOZORDER|SWP_NOSIZE);
}