现在本菜鸟再看windows程序设计的第四章，关于滚动条的程序截取部分如下：
   switch (message)
     {
     case WM_CREATE:
          hdc = GetDC (hwnd) ;
         
          GetTextMetrics (hdc, &tm) ;
          cxChar = tm.tmAveCharWidth ;
          cxCaps = (tm.tmPitchAndFamily & 1 ? 3 : 2) * cxChar / 2 ;
          cyChar = tm.tmHeight + tm.tmExternalLeading ;
         
          ReleaseDC (hwnd, hdc) ;

               // Save the width of the three columns
         
          iMaxWidth = 40 * cxChar + 22 * cxCaps ;
          return 0 ;
         
     case WM_SIZE:
          cxClient = LOWORD (lParam) ;
          cyClient = HIWORD (lParam) ;

               // Set vertical scroll bar range and page size

          si.cbSize = sizeof (si) ;
          si.fMask  = SIF_RANGE | SIF_PAGE ;
          si.nMin   = 0 ;
          si.nMax   = NUMLINES - 1 ;
          si.nPage  = cyClient / cyChar ;
          SetScrollInfo (hwnd, SB_VERT, &si, TRUE) ;

               // Set horizontal scroll bar range and page size

          si.cbSize = sizeof (si) ;
          si.fMask  = SIF_RANGE | SIF_PAGE ;
          si.nMin   = 0 ;
          si.nMax   = 2 + iMaxWidth / cxChar ;
          si.nPage  = cxClient / cxChar ;
          SetScrollInfo (hwnd, SB_HORZ, &si, TRUE) ;
          return 0 ;
         
     case WM_VSCROLL:
               // Get all the vertial scroll bar information

          si.cbSize = sizeof (si) ;
          si.fMask  = SIF_ALL ;
          GetScrollInfo (hwnd, SB_VERT, &si) ;

               // Save the position for comparison later on

          iVertPos = si.nPos ;

          switch (LOWORD (wParam))
          {
          case SB_TOP:
               si.nPos = si.nMin ;
               break ;
               
          case SB_BOTTOM:
               si.nPos = si.nMax ;
               break ;
               
          case SB_LINEUP:
               si.nPos -= 1 ;
               break ;
               
          case SB_LINEDOWN:
               si.nPos += 1 ;
               break ;
               
          case SB_PAGEUP:
               si.nPos -= si.nPage ;
               break ;
               
          case SB_PAGEDOWN:
               si.nPos += si.nPage ;
               break ;
               
          case SB_THUMBTRACK:
               si.nPos = si.nTrackPos ;
               break ;
               
          default:
               break ;         
          }
               // Set the position and then retrieve it.  Due to adjustments
               //   by Windows it may not be the same as the value set.

          si.fMask = SIF_POS ;
          SetScrollInfo (hwnd, SB_VERT, &si, TRUE) ;
          GetScrollInfo (hwnd, SB_VERT, &si) ;

               // If the position has changed, scroll the window and update it

          if (si.nPos != iVertPos)
          {                    
               ScrollWindow (hwnd, 0, cyChar * (iVertPos - si.nPos),
                                   NULL, NULL) ;
               UpdateWindow (hwnd) ;
          }
          return 0 ;
我不明白的是si.nPos位置已经设定了，为什么还要重新获取，注释说有可能被windows更改，但是为什么会被windows更改，难道windows可以返回实时的滚动条位置？但是如果windows可以返回实时的滚动条位置，那就没有必要计算si.nPos的值了。。
求各位大神解答。

我是业余编程爱好者，手中正好有这本书，就拿来随便看看。。

好吧，我好像懂了，是防止滚动条越界。