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The Basics of Debugger Extensions: Short Term Effort, Long Term Gain

In order to talk about debugger extensions, we need to first talk about the Debugger Engine library, (DbgEng). DbgEng is a generic interface that can be used to manipulate various debuggingtargets, which can be things such as crash dumps, a live kernel, an application, etc. The library provides access to all of the types of actions that you might want to perform on a debugging target, such as setting or clearing breakpoints, reading or writing memory, translating addresses into symbolic debugging information, receiving events from the target, and so on.

The DbgEng library is quite flexible and can actually be used in standalone applications. If you wrote an application that exported all of the features of the DbgEng library, you’d end up with a debugger exactly like WinDBG or KD because, well, that’s what they are. In addition, the DbgEng library is what we’re going to talk to when we write our debugger extensions. Thus, in effect we have the same API set but two different uses. This can lead to a fair amount of confusion if you start looking through the WinDBG SDK samples because there’s a mix of applications and extensions supplied. If you’re not aware of this fact it can be difficult to find a sample to get started with.


In either case, DbgEng relies on the concept of sessions. In order to examine or manipulate the target, a debugging session must be active. When a debug event is raised from the target, the target may become acquiredat which point the debug session is active. The application or extension is now allowed to perform whatever options it wants against the target. Once finished with the target, it must be released, at which point the session is no longer active and operations are no longer allowed against the target.

Aside from sessions, the other major concept to learn about DbgEng is the set of objects that are available to the application or extension. The objects supplied are broken down into two basic categories: client objects and callback objects.

Client Objects

Client objects are used to interact with the debugger engine and provide access to all of the available DbgEng APIs used to manipulate the target via COM interfaces. The way this works is that each client object provides a QueryInterface method that is used to create instances of the various COM interfaces. So, for example, if you wanted to set a breakpoint on the target, you’d call the QueryInterface method on a client object to retrieve an instance of the COM interface that exports the set breakpoint API. Sounds a little scary, but it’s going to turn out to be straightforward so don’t worry. The list of available client COM interfaces and their uses are as follows:

  • · IDebugAdvanced – Thread context, source server, and some symbol information
  • · IDebugClient – Connect to targets, register input/output callbacks, register debugger event callbacks, etc.
  • · IDebugControl – Evaluate expressions, disassemble, execute commands, add/remove breakpoints, get input, send output, etc.
  • · IDebugDataSpaces – Read/write virtual memory, physical memory, I/O ports, MSRs, etc.
  • · IDebugRegisters – Read/write pseudo and hardware registers
  • · IDebugSymbols – Type information, module information, source file information, etc.
  • · IDebugSystemObjects – Thread, process, and processor information

Callback Objects

As their name implies, callback objects export the COM interfaces necessary to receive notifications of events happening on the target. There are three types of callback objects: input objects, output object, and debugger event objects. These allow you to scan the input supplied to the debugger engine from the user as well as the output of any debugger commands or the debugging target. In addition to this, callback objects provide a way to receive notification of process creation, thread creation, breakpoints, etc. The list of available callback COM interfaces and their uses are as follows:

  • · IDebugInputCallbacks – Receive notification of the engine waiting for user input
  • · IDebugOutputCallbacks – Receive notification of output being sent to debugger engine
  • · IDebugEventCallbacks – Receive notification of debug events


Creating Your Own Extension

Now that we have some of the basics down, we can see how to actually create a debugger extension. Debugger extensions are nothing more than DLLs that provide commands via exports and rely on the DbgEng library and any other Win32 API of their choosing. They can be built just like any other DLL, though we’ll use the WDK seeing as how that’s what the available samples use. The only other thing that you’ll need is the SDK subdirectory of the WinDBG installation, as that’s where the necessary header and library files are located.

Extension DLL Entry Points

There is a single required entry point for extension DLLs, DebugExtensionInitialize. This is where you will do all of your one time initialization for the extension DLL. A description of the entry point and the function prototype can be found in dbgeng.h:

// Initialization routine.  Called once when the 

// extension DLL is loaded.  Returns a version and
// returns flags detailing overall qualities of the 
// extension DLL. A session may or may not be active
// at the time the DLL is loaded so initialization 
// routines should not expect to be able to query
// session information.
    (__out PULONG Version, __out PULONG Flags);

A simple example of a complete DebugExtensionInitialize is as follows:

extern "C"
DebugExtensionInitialize(PULONG Version, PULONG Flags)
    // We're version 1.0 of our extension DLL
    *Version = DEBUG_EXTENSION_VERSION(1, 0);
    // Flags must be zero
    *Flags = 0;
    // Done!
    return S_OK;


There are two other entirely optional exports that you driver can provide, DebugExtensionUninitialize  andDebugExtensionNotify. DebugExtensionUninitialize can be used to undo anything that you might have done in DebugExtensionInitialize:


// Exit routine.  Called once just before the 
// extension DLL is unloaded.  As with 
// initialization, a session may or may not be 
// active at the time of the call.

If present, DbgEng calls the DebugExtensionNotify callback for session state changes:


// A debuggee has been discovered for the session.  
// It is not necessarily halted.
#define DEBUG_NOTIFY_SESSION_ACTIVE       0x00000000
// The session no longer has a debuggee.
#define DEBUG_NOTIFY_SESSION_INACTIVE     0x00000001
// The debuggee is halted and accessible.
// The debuggee is running or inaccessible.
    (__in ULONG Notify, __in ULONG64 Argument);

The entire point of writing this DLL is to create your own debugger commands, and those will also be exports of your DLL. These commands or, extensions must appear in the .DEF file associated with your DLL and the exports must contain only lower case letters. The function prototype of an extension command is as follows:


// Every routine in an extension DLL has the 
// following prototype. The extension may be called
// from multiple clients so it should not cache the
// client value between calls.
    (__in PDEBUG_CLIENT Client, __in_opt PCSTR Args);

You’ll note that the function prototype indicates that a pointer to a DEBUG_CLIENT structure is passed as the first parameter to the extension command.  This is actually the client object whose QueryInterface method you will use to gain access to the various client COM interfaces for target manipulation.

Let’s see a simple example command that uses the built in expression evaluator to add two and two together and then displays the result. This would be the equivalent of typing ? 2 + 2 in the WinDBG command prompt.


mycommand(PDEBUG_CLIENT4 Client, PCSTR args)
    PDEBUG_CONTROL debugControl;
    HRESULT        hr;
    DEBUG_VALUE    result;
    // Let's do a couple of simple things. First
    // thing to do is use the passed in client to
    // access the debugger engine APIs.
    // First, we'll get an IDebugControl so that we
    // can print messages.
    hr = Client->QueryInterface
                       (void **)&debugControl);
    if (hr != S_OK) {
        return hr;
    // Now we can print.
                       "mycommand running...\n");
    // Use the evaluator to evaluate an expression
    hr = debugControl->Evaluate("2 + 2",
    if (hr != S_OK) {
        return hr;
                         "Result is %d\n", result.I32);
    // Done with this.
    return S_OK;


Hopefully the steps followed are fairly straightforward at this point. We’ve taken the passed-in client object, created an instance of IDebugControl, and then executed some methods on it to perform actions.

To further drive home the pattern, we can see how we’d get the offset of a field of a data structure. For that, we need an instance of IDebugSymbols:



myothercommand(PDEBUG_CLIENT4 Client, PCSTR args)
    PDEBUG_SYMBOLS debugSymbols;
    HRESULT        hr;
    ULONG          fieldOffset;
    ULONG          typeId;
    ULONG64        module;
    // Let's find the offset of the CurrentThread
    // field of the PRCB
    hr = Client->QueryInterface
                       (void **)&debugSymbols);
    if (hr != S_OK) {
        return hr;
    // We need the "type identifier" and module 
    // containing the symbol that we're interested 
    // in.
    hr = debugSymbols->GetSymbolTypeId("nt!_KPRCB",
    if (hr != S_OK) {
        return hr;
    // Now we can get the offset.
    hr = debugSymbols->GetFieldOffset
    if (hr != S_OK) {
        return hr;
               "Offset of CurrentThread is %d\n", 
    return S_OK;

Dealing with 32-bit vs. 64-bit

Note that when you’re writing a debugger extension command, your code is always running on the host machine. Also note that the pointer size of the host machine does not necessarily match the pointer size of the target machine, due to the fact that 32-bit hosts can debug 64-bit targets and vice versa. In order to deal with this, debugger extensions treat all addresses from the target as 64-bit values. Thus you’ll note that the DbgEng APIs express pointer addresses as ULONG64 values. Any 32-bit value used in your extension command must be sign extended out to a full 64-bit value.

Alternative Debugger Extension Interfaces

To add to the confusion when it comes to writing a debugger extension, there are two alternative interfaces that you can use to write your debugger extensions. The first is the WdbgExts interface, which is the legacy debugger extension interface that existed before DbgEng came around. This interface is still available in the newest versions of the debugger, however it has two drawbacks. First, it is not the forward moving API thus it is frozen in time and will provide no new features. Second, this interface does not have any support for writing standalone applications, thus it won’t port to any kind of automated analysis tool that you might write. If you’re interested in learning more about the legacy extension model, see this article on OSR Online.

The other interface available to you is the EngExtCpp interface. This is actually just a C++ wrapper library around DbgEng to simplify common tasks such as manipulating typed data. Unlike WdbgExts, this is a fully supported interface and can be used alongside the direct DbgEng calls that we’ve discussed in this article.

Go forth!

Hopefully we’ve been able to clear up the cloud of mystery that hangs over writing debugger extensions and have set you on a journey of creating your own.

Source to a debug extension (!uniqstack) can be downloaded from: http://www.osronline.com/OsrDown.cfm/apexts.zip?name=apexts.zip&id=559

User Comments
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"RE: Reference is missing?"
Sorry about that, the article is here:



01-Apr-11, Scott Noone

"Reference is missing?"
"If you’re interested in learning more about the legacy extension model, see this article on OSR Online."

I see the above sentence at the end of this article, but how can I get the article referenced in this sentence?

01-Apr-11, Tiansheng Tan

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