Setting Dynamic Breakpoints in WinDbg

• Microsoft Win32 Software Development Kit (SDK), versions 3.1, 3.5, 3.51, and 4.0

SUMMARY

The WinDbg breakpoint command contains a metacircular interpreter; that is, you can execute commands dynamically once a breakpoint is hit. This allows you to perform complex operations, including breaking when an automatic variable has changed, as described below.

The command interpreter of WinDbg allows any valid C expression to serve as a break condition. For example, to break whenever a static variable has changed, use the following expression in the Expression field of the breakpoint dialog box:

   &<variablename>

This technique does not work for automatic variables because the address of an automatic variable may change depending on the value that the stack pointer has upon entering the function that defines the automatic variable. This is one case where the breakpoint needs to be redefined dynamically.

For this purpose, a breakpoint can be enabled at function start and disabled at function exit, so that the address of the variable is recomputed.

MORE INFORMATION

Suppose that the name of the function is "subroutine" and the local variable name is "i". The following steps will be used:

1. Start the program and step into the function that defines the automatic variable with the commands:

         g subroutine
         p
         bp500 ={subroutine}&i /r4 /C"?i"
   

   The breakpoint number is chosen to be large so that the breakpoint will be well out of range of other breakpoints. Note that /r4 indicates a length of 4 because i is an integer. Make this number larger for other data types. The command "?i" prints out the value of i.

2. Next, disable this first breakpoint with the command

         bd500
   

   because the address of i may change. The breakpoint will be enabled when in the scope of function subroutine.

3. The second breakpoint definition is set at the entry point of the function:

         bp .<FirstLine> /C"be 500;g"
   

   This is where the breakpoint is enabled. Note that <FirstLine> is the line number of the first statement in the function subroutine.

4. The last breakpoint is set at the end of the function

         bp .<LastLine> /C"bd 500;g"
   

   and will disable the breakpoint again. Note that <LastLine> is the line number of the last statement in the function subroutine.

   Note that if the function has more than one exit point, multiple breakpoints may have to be defined.