PRB: Metafiles and Palettes

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.0 and 3.1

SYMPTOMS

The metafile list inside of GDI's heap grows by a WORD.

CAUSE

This happens each time an application selects a palette into a metafile device context (DC).

RESOLUTION

If a palette is never deleted, then this block is never freed and keeps on growing until GDI's heap fills and the system crashes.

This problem is heightened by the fact that the application typically selects the default stock palette back in so the application can free its custom palette. Because an application cannot delete the default stock palette, this memory is never freed inside of GDI's heap.

MORE INFORMATION

Below is the code necessary to correct the problem. There are no rules regarding when to call FlushGDIHeap(); however, Microsoft recommends doing this after a major operation, such as after saving a file or printing. Determining whether you need to flush the GDI heap depends on how often your application selects in a palette in a metafile. Optionally, you could periodically call TOOLHELP and see if the GDI resources are greater than X%.

Sample Code

SEG       segGDI, segNil;
HANDLE    hPalSys;
int       iPal;
BOOL      fEnableFlushHeap;

/* ---------------------------------------------
   The following code is used to initialize the
   FlushGDIHeap function.
   --------------------------------------------- */

  HANDLE hGDISegment;

  /* Called once during application initialization.  Sets up the
     state for FlushGDIHeap.  */
  /* Make sure this version of Windows has the bug. */
  if (wWinVer >= 0x400)
          return;

  /* This is from MSDN and it gets GDI's DS.
   * The module handle return by LoadLibrary is actually the module's DS.
   */
  hGDISegment = LoadLibrary("GDI.EXE");
  if (hGDISegment <= HINSTANCE_ERROR)
          return;

  FreeLibrary(hGDISegment);

  Glob(segGDI) = HIWORD(GlobalLock(hGDISegment));
  if (Glob(segGDI) != segNil)
  {
    GlobalUnlock(hGDISegment);
    Glob(hPalSys) = GetStockObject(DEFAULT_PALETTE);
    // Debug version of Windows has palette metafile list
    // heap handle in different offset than in retail Windows
    // versions greater than 3.0.
    // The offset is in WORD's.
    Glob(iPal) = (GetSystemMetrics(SM_DEBUG) != 0 &&
            FGTWinVer30()) ? 11 : 9;
    Glob(fEnableFlushHeap) = TRUE;
  }

/*=========================================================================

  HANDLE hPal;
  SEG segTmp;
  WORD *pPal, *pw, w2, iPal;

  /* If the flush heap operation is not enabled then return. */
  if (!Glob(fEnableFlushHeap))
          return;

  /* Switch to GDI's heap for Local memory functions. */
  segTmp = Glob(segGDI);
  hPal = Glob(hPalSys);
  /* Offset in Pal data structure to second metafile list handle.
  * This varies between debug and retail (see InitOLE).
  */
  iPal = Glob(iPal);
  _asm {
          push ds
          mov  ds,segTmp
       }

  /* Shrink the metafile list table down for the system palette. */
  if ((pPal = (WORD *)LocalLock(hPal)) != NULL)
  {
     /* First free memory from the standard object metafile list.
      * All GDI objects have this list associated with them if
      * they are selected into metafiles.
      */

     if (pPal[4] != hNil)
     {
        if ((pw = (WORD *)LocalLock(pPal[4])) != NULL)
        {
           w2 = *pw;
           LocalUnlock(pPal[4]);
           LocalReAlloc(pPal[4], (w2+1) * sizeof(WORD), LHND);
        }
     }

     /* Second, free memory from the secondary metafile list.
      * Only palettes have this list.
      */

     if (pPal[iPal] != hNil)
     {
       if ((pw = (WORD *)LocalLock(pPal[iPal])) != NULL)
       {
          w2 = *pw;
          LocalUnlock(pPal[iPal]);
          LocalReAlloc(pPal[iPal], (w2+1) * sizeof(WORD), LHND);
       }
     }

     LocalUnlock(hPal);
  }

  /* Restore application's data segment. */
  _asm {
         pop ds
       }