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Posted 23 Jul 2008

Toggle hardware data/read/execute breakpoints programmatically

, 23 Jul 2008
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Simple code to introduce a hardware breakpoint mechanism.


I decided to write this article about hardware breakpoints for the following reasons:

  • Visual C++ only supports write-only data breakpoints. You might want to trigger a break when data is read as well.
  • You might not be using Visual C++, so chances are that your debugger uses some slow software-based mechanism.
  • You might want to set/remove a breakpoint programmatically.
  • You may be interested in low level CPU stuff!


  • Works for x86 and x64.
  • Supports upto 4 hardware breakpoints per thread.

Debug Registers

x86/x64 contains a set of debug registers, named DR0, DR1, DR2, DR3, DR6, and DR7. These registers are 32-bit when in 32-bit mode, and 64-bit when in long mode. DR0, DR1, DR2, and DR3 contain the linear addresses of the breakpoint, and DR7 contains the bits explained here:

0-7Flags for each of the 4 debug registers (2 for each). The first flag is set to specify a local breakpoint (so the CPU resets the flag when switching tasks), and the second flag is set to specify a global breakpoint. In Windows, obviously, you can only use the first flag (although I haven't tried the second).

2 bits for each register, defining when the breakpoint will be triggered:

  • 00b - Triggers when code is executed
  • 01b - Triggers when data is written
  • 10b - Reserved
  • 11b - Triggers when data is read or written

2 bits for each register, defining the size of the breakpoint:

  • 00b - 1 byte
  • 01b - 2 bytes
  • 10b - 8 bytes
  • 11b - 4 bytes

We use SetThreadContext to set the necessary flags for the thread. After that, when the breakpoint is triggered, an exception of the value EXCEPTION_SINGLE_STEP is raised.

Setting the Breakpoint

HANDLE SetHardwareBreakpoint(HANDLE hThread,HWBRK_TYPE Type,HWBRK_SIZE Size,void* s);
  • hThread - Handle to the thread for which the breakpoint is to be set.
  • Type - Type of the breakpoint:
  • Size - Size of the breakpoint:
    • HWBRK_SIZE_1
    • HWBRK_SIZE_2
    • HWBRK_SIZE_4
    • HWBRK_SIZE_8
  • addr - The address of the breakpoint.

The function returns a handle to the breakpoint, to be used later in RemoveHardwareBreakpoint. It can return 0 if:

  • You do not have access to the thread.
  • You have set the maximum number of breakpoints for that thread (4).

Removing the Breakpoint

bool RemoveHardwareBreakpoint(HANDLE hBrk);

Removes the breakpoint, returning true on success.


int __stdcall WinMain(HINSTANCE,HINSTANCE,LPSTR,int)
 char c1[100] = {0};
 lstrcpyA(c1,"Hello 1");
 HANDLE hX1 = 0;

 hX1 = SetHardwareBreakpoint(GetCurrentThread(), 
   volatile char a1 = c1[2];
   // To ensure that it won't be optimized out.
 __except(GetExceptionCode() == STATUS_SINGLE_STEP)
   MessageBoxA(0,"Breakpoint hit!",0,MB_OK);
 return 0; 

I wait for your comments and questions!


  • July 24, 2008 - First post.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Michael Chourdakis
Greece Greece
I'm working in C++, PHP , Java, Windows, iOS and Android.

I 've a PhD in Digital Signal Processing and I specialize in Pro Audio applications.

My home page:

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Comments and Discussions

QuestionBreakpoints don't trigger in the debugger? Pin
Member 86976873-Apr-12 16:02
memberMember 86976873-Apr-12 16:02 
AnswerRe: Breakpoints don't trigger in the debugger? Pin
Michael Chourdakis3-Apr-12 21:02
memberMichael Chourdakis3-Apr-12 21:02 
Data breakpoints are triggered in any context, no matter if you are debugging or not. The try/catch simply allows the application to catch the exception and continue. When debugging, the exception goes into the debugger which (in case of VS) does NOT break because it knows there's a hardware breakpoint.

Of course the try/except is not needed. It's there only to demonstrate that the code actually throws. In real life, you set the breakpoint using SHB() and you wait for the debugger to break when the breakpoint is hit.
Michael Chourdakis - Music & DSP Engineer - Software & Research

GeneralRe: Breakpoints don't trigger in the debugger? Pin
Member 86976873-Apr-12 22:09
memberMember 86976873-Apr-12 22:09 
GeneralRe: Breakpoints don't trigger in the debugger? Pin
Member 86976879-Apr-12 16:42
memberMember 86976879-Apr-12 16:42 
GeneralRe: Breakpoints don't trigger in the debugger? Pin
Member 869768710-Apr-12 10:47
memberMember 869768710-Apr-12 10:47 

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