DOCUMENT:Q77653  27-JUL-2001  [visualc]
TITLE   :PRB: Bitfield Breakpoint Halts Execution on Neighboring Fields
PRODUCT :Microsoft C Compiler
PROD/VER:winnt:
OPER/SYS:
KEYWORDS:kbCodeView kbDebug kbide kbVC

======================================================================
-------------------------------------------------------------------------------
The information in this article applies to:

 - The Integrated Debugger, included with:
    - *EDITOR Please do not choose this product*Microsoft Visual C++ for Windows* use 1699 
    - *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225 
    - Microsoft CodeView for MS-DOS 
-------------------------------------------------------------------------------

SYMPTOMS
========

When a break expression specifies a bit field, the debugger does not halt
execution at that breakpoint.

CAUSE
=====

The byte is the smallest addressable unit in the debugger; it does not support
specifying an individual bitfield in a breakpoint. The larger unit that contains
the bitfield determines when execution halts.

MORE INFORMATION
================

Consider the sample code below. In Win16 (Windows), x0 through x7 are stored in
the same memory unit (unsigned int), but x8 is stored in the next; in Win32
(Windows NT), x0 through x8 are all stored in the same memory unit. If you set a
breakpoint to halt execution when x0 changes, execution in Win16 halts on each
of the first three statements in main() because each one modifies the same unit;
execution in Win32 halts on all four statements of main() for the same reason.

Sample Code
-----------

      /*
       * Compile options needed: /Zi /Od
       */ 

      typedef struct
      {
         unsigned int x0:2;
         unsigned int x1:2;
         unsigned int x2:2;
         unsigned int x3:2;
         unsigned int x4:2;
         unsigned int x5:2;
         unsigned int x6:2;
         unsigned int x7:2;
         unsigned int x8:2;
      } node;

      node NodeVar;

      void main(void)
      {
         NodeVar.x0++;
         NodeVar.x1++;
         NodeVar.x3++;
         NodeVar.x8++;
      }

Additional query words:

======================================================================
Keywords          : kbCodeView kbDebug kbide kbVC 
Technology        : kbVCsearch kbAudDeveloper kbIntegratedDebugger
Version           : winnt:
Issue type        : kbprb

=============================================================================

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.

Copyright Microsoft Corporation 2001.