WinDBG Tutorial – Part 1

Items covered


          Attaching to process

          Inspecting local variables (dv, ??)

          Inspecting last error (.lastevent)

          Evaluating parameters (x)

          Setting value (e)


Code used


For this introductory test, we will work with the following test program (Win32 C++ console project):

#include “stdafx.h”



int _tmain(int argc, _TCHAR* argv[])


      printf(“Enter two numbers: “);

      int a, b;

      scanf(“%d%d”, &a,&b);

      int rez;

      rez = a / b;

      printf(“The division is: %d”,  rez);


      return 0;


What the program does is obvious: it reads two integer variables from the command line and calculates the result of their division. The potential error is again obvious: if the latter variable is 0, we’ll get a division by zero exception.


Let’s get to work:

Compile and run the program from console:


Set the symbol path in WinDBG to the Debug folder of the C++ program (where EmptyApp.exe and PDB files are located).

Open WinDBG and attach to the process named “EmptyApp”

For the moment, we will choose Go, as no exception as occurred yet:

0:001> g

(The message in WinDBG will be “Debugee is running”)

Let’s now input two values: a = 6, b = 0

When you press ENTER, winDBG will break because it receives a First-hand exception:

(1e38.1b00): Integer divide-by-zero – code c0000094 (first chance)

First chance exceptions are reported before any exception handling.

This exception may be expected and handled.

This exception may be expected and handled (had we inserted a try/catch block in the C++ program), but it is not. Let’s go further and receive the second chance exception:

0:000> g

(1e38.1b00): Integer divide-by-zero – code c0000094 (!!! second chance !!!)

Assuming we didn’t know what happened, let’s find out the call stack:

0:000> k

ChildEBP RetAddr 

0022f948 00fa1a28 Emptyapp!wmain+0x79 [d:\home\allinoner\emptyapp\emptyapp\emptyapp.cpp @ 14]

0022f998 00fa186f Emptyapp!__tmainCRTStartup+0x1a8 [f:\dd\vctools\crt_bld\self_x86\crt\src\crtexe.c @ 579]

0022f9a0 763d10dc Emptyapp!wmainCRTStartup+0xf [f:\dd\vctools\crt_bld\self_x86\crt\src\crtexe.c @ 399]

0022f9ac 77301e9e kernel32!BaseThreadInitThunk+0xe

0022f9ec 77301e71 ntdll!__RtlUserThreadStart+0x70

0022fa04 00000000 ntdll!_RtlUserThreadStart+0x1b


The error, therefore, originates at line 14 in the source emptyapp.cpp

Since we do have the source for this file, let’s see what line 14 reads:

                rez = a / b;

Hmm, we get a division by zero and the line of code contains a division…

Let’s dump the local variables:

0:000> dv

           argc = 1

           argv = 0x00511310

              b = 0

            rez = -858993460

              a = 6

OK, question was answered, we want to divide a = 6 by b = 0.

Just out of curiosity: what is argv?

0:000> ?? argv

wchar_t ** 0x00511310


Since argc is 1, let’s see the value of the first argument:

0:000> ?? argv[0]

wchar_t * 0x00511318



OK, let’s go further: let’s modify variable b so that no division by zero is thrown:

Firstly, get its address:

0:000> x b

0022f934 b = 0

Secondly, set value of b to 3, for instance:

0:000> e 22f934 3

Inspecting all local variables again:

0:000> dv

           argc = 1

           argv = 0x00511310

              b = 3

            rez = -858993460

              a = 6

This is it. Let’s let the program finish:

0:000> g

eax=5f24dff0 ebx=00000000 ecx=5f1812bc edx=5f1812bc esi=77377380 edi=77377340

eip=772e8244 esp=0022f8b0 ebp=0022f8cc iopl=0         nv up ei pl zr na pe nc

cs=001b  ss=0023  ds=0023  es=0023  fs=003b  gs=0000             efl=00000246


772e8244 c3              ret

The command prompt reads:

Enter two numbers: 6 0

The division is: 2


And finally, to make sure program has exited:

0:000> .lastevent

Last event: 1e38.1b00: Exit process 0:1e38, code 0

  debugger time: Sun Jul 12 21:08:08.080 2009 (GMT+3)

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