Josef Biehler

Posted on Apr 20, 2020 • Updated on Apr 22, 2020

How to debug an unmanaged application with WinDbg

#clr #debug #internals #windbg

Why use WinDbg if I have Visual Studio with a more user friendly debugger?

There are a few situations where this is necessary. At work we are developing a ASP.NET WebAPI. In the production environment you can not attach your Visual Studio to debug or analyze the memory. The latter is also a reason, you should make your familiar with WinDbg. It allows (of course as other tools, too) to analyze a memory dump, taken from any process.

During my spare time the last week I started to write a .NET profiler with C++. This is a small DLL that is loaded along with a normal .NET application and allows you to monitor your .NET application. For example you get notifications if a class loads and so on. To debug that DLL I had to use WinDbg.

Get WinDbg

You can get WinDbg from here.

Our sample app

I created a very stupid C++ application. Follow this link to get the full runnable example. Here you get the compiled example. It consists of a class Program:

// ./project/ConsoleApplication1/Program.cpp

#include "Program.h";

Program::Program(int id) {
    this->id = id;
}
int Program::GetId() {
    return id;
}

And a file that contains the entry point:

// ./project/ConsoleApplication1/ConsoleApplication1.cpp

#include <iostream>
#include "Program.h";

int main()
{
    Program* pProgram = new Program(10);
    int dummy = 0;
    std::cout << "Press Enter to continue\n";
    std::cin >> dummy;
    std::cout << "Hello World!\n";
    std::cout << pProgram->GetId();
    return 0;
}

Use WinDbg

First you should start the programm. Then open the task manager, find the process and copy the PID (in this example: 24508):

Provide symbols

You should tell WinDbg where it can find the symbols of the system libraries. Go to File > Symbol File Path and insert:

srv*C:\MySymbolCache*http://msdl.microsoft.com/download/symbols

This will download all microsoft symbols to c:/MySymbolCache.

The symbols of your app
Sometimes WinDbg finds the symbols of the attached process automatically but not always. In the latter case, just copy the ConsoleApplication1.pdb file to c:/MySymbolCache.

Attach to the process

Go to File > Attach to a process, insert the PID at the bottom of the dialog and press OK.

If you get this view, everything works as expected:

WinDbg has now paused the program.

Set a breakpoint

WinDbg provides three commands for placing breakpoints:

bp: Set a breakpoint
bu: Set a unresolved breakpoint
bm: Set breakpoints to symbols that match a pattern

bp: Set a breakpoint

This is the "normal" way of placing a breakpoint. You specify the name of the application, followed by a exclamation mark and the path to the function name.

bp ConsoleApplication1!Program::GetId

If you get no error message (the invalid checksum can be ignored), you successfully placed a breakpoint:

Deleting a breakpoint

Go to Edit > Breakpoints... to get a list of all placed breakpoints. In this window you also can delete them.

bu: Deferred breakpoint

Try this command:

bp NonExistingApp!Program::NonExistingFunction

Obviously this function does not exist. Also the application name is wrong. What WinDbg now creates is a so called deferred breakpoint:

A deferred or unresolved breakpoint is one that gets active as soon as the module NonExistingApp is loaded. If you use bp and specify a location that does not exist currently, then bp does the same as bu.

bm: Set breakpoints to symbols that match a pattern

You also can specify a set of breakpoints by passing a pattern:

bm ConsoleApplication1!Program::*

This theoretically will place a breakpoint in all entry points that match that pattern. But during testing this command does nothing. So I am not sure how to use this properly :-)

Setting our breakpoint

For testing purposes we use bp as mentioned above:

bp ConsoleApplication1!Program::GetId

Let the app run

As now WinDbg is attached and the breakpoint is set, we can continue. insert g into the WinDbg commandline and press enter. g continues the execution of ConsoleApplication1. Press any key + enter in the ConsoleApplication1.exe cmd. Now the application is paused and you should see this:

Also notice the red underlined buttons at the top. With those buttons you can jump through the source code. You are also able to set the cursor into any line and press F9 to set a new breakpoint in this line.

Read local variables and manipulate them

Press F10 or one of the first two buttons to jump into the next line. The instruction pointer now is in line return id;. Go to View > Locals to open a new window with all local variables:

Let's say you want to return 5000 here. That is easy. Execute the command ?? id = 5000:

Notice how the variable has changed:

Summary

I showed you how you can easily debug your unmanaged application with WinDbg. I recommend you to make yourself familiar with WinDbg. It is a very powerful tool. Please keep in mind that for debugging a .NET application, you need some extra steps.

More information

Breakpoint Commands
WinDbg Cheatsheet

Found a typo?

As I am not a native English speaker, it is very likely that you will find an error. In this case, feel free to create a pull request here: https://github.com/gabbersepp/dev.to-posts . Also please open a PR for all other kind of errors.

Do not worry about merge conflicts. I will resolve them on my own.

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