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Q77653: PRB: Bitfield Breakpoint Halts Execution on Neighboring Fields

Article: Q77653
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s): 
Keyword(s): kbCodeView kbDebug kbide kbVC
Last Modified: 27-JUL-2001

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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 
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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

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