PRB: Bitfield Breakpoint Halts Execution on Neighboring Fields

• The Integrated Debugger, included with:
  • Microsoft Visual C++ for Windows
  • Microsoft Visual C++ 32-bit Edition
  • 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
Version : winnt:
Platform : winnt
Issue type : kbprb