DOCERR: Incorrect Hard-Coded Values in SDK BITMAP.C Sample

 Last reviewed: January 9, 1996
Article ID: Q108316
The information in this article applies to:
  • Microsoft Windows Software Development Kit (SDK) for Windows version 3.1

SYMPTOMS

When running the BITMAP.C sample, no problems occur if the code is compiled and run as-is, without modifications. However, many possible symptoms can occur if the user modifies or adds to the code.

CAUSE

BITMAP.C contains several calls to the function GetObject(), all of which pass a constant for the cbBuffer parameter, rather than a sizeof(BITMAP); that is 

   GetObject(hBitmap, 16, &Bitmap);
rather than: 

   GetObject(hBitmap, sizeof(BITMAP), &Bitmap);
Unfortunately, the size of the BITMAP structure under Windows 3.1 is 14 bytes, so each such call to GetObject overwrites the variable after Bitmap. Depending on what that variable is, this code could produce a wide range of erratic behavior.

In the BITMAP.C sample code, the variable following Bitmap is 

   BOOL bTrack = FALSE;        // TRUE if user is selecting a region.
(Note that both Bitmap and bTrack are global.) Because bTrack is set to TRUE only while the mouse button is down, and the GetObject calls are done only during initialization and when a menu item is selected (which can't be done with the mouse button down), this problem becomes apparent only when the user modifies the code (for example by adding some other variable after Bitmap).

RESOLUTION

Check any code that is based on the old BITMAP.C sample, and replace the constant 16 with sizeof(BITMAP): 

   GetObject(hBitmap, sizeof(BITMAP), &Bitmap).

Additional reference words: 3.10 docerr
KBCategory: kbprg kbfile
KBSubcategory: GdiBmp

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Last reviewed: January 9, 1996
© 1998 Microsoft Corporation. All rights reserved. Terms of Use.