• About
• Features
• Recent changes
• Download
• Installation
• Usage
• Command Line Options
• The EXCHNDL.DLL
• Introduction
• Using ExcHndl for you own purposes
• Example
• Appendix
• The IMAGEHLP.DLL Saga
• Suggested Reading

About

Dr. Mingw is a Just-in-Time (JIT) debugger. When the application throws an unhandled exception, Dr. Mingw attaches itself to the application and collects information about the exception, using the available debugging information.

Features

Dr. Mingw can read debugging information in Stabs formats — generated by the Gnu C/C++ Compiler, and in a PDB file — generated by the Microsoft Visual C++ Compiler.

Windows NT/2000 platform is supported, as well as Windows 95/98. On older Windows versions though, the IMAGEHLP.DLL isn't included or it's a rather old version. Dr. Mingw doesn't require it, but relies upon it to resolve symbols in modules compiled by the Microsoft tools. See below for more information.

Recent changes

Download

Dr. Mingw can be downloaded here.

Git repository available here

Installation

To install enter

drmingw -i

Dr. Mingw will register itself as the JIT debugger by writting into the system registry. On Windows NT/2000 make sure you have Administrator rights. See Enabling Postmortem Debugging for more information.

If the installation is sucessful, the following message box should appear:

To enable other options they must be set them allong with the -i option. For example,

drmingw -i -v

If you still have trouble installing, edit the included drmingw.reg file to reflect the drmingw.exe executable path and load it on the system registry.

Usage

You can test Dr. Mingw by running the sample test.exe. Depending of your Windows version, you'll see a familiar dialog:

If you request to debug the program, Dr. Mingw will attach to the faulting application, collect information about the exception, and display the dialog

To resolve the addresses it's necessary to compile the application with debugging information. In case of address is in a DLL with no debugging information, it will resolve to the precedent exported symbol.

Command Line Options

The Dr. Mingw command line uses the following syntax:

*drmingw *[*-h | --help*] [*-V | --version*] [*-i | --install*] [*-a | --auto*] [*-u | --uninstall*]
[*-p* _pid_ | *--process-id=*_pid_]  [*-e* _event_ | *--event=*_event_]
[*-v | --verbose*]

The following table describes the Dr. Mingw command-line options. All comand-line options are case-sensitive.

The EXCHNDL.DLL

Introduction

Although internally Dr. Mingw behaves much like a debugger, from the outside it is like a standalone exception handler. But for its own debugging purposes, Dr. Mingw has a internal exception handler that is completly seperate from the main code.

This exception handler resides in exchndl.c. When drmingw.exe is loaded, the code in exchndl.c is automatically executed (by the gcc static constructor/destructor mechanism) and registers itself as a exception handler.

This exception handler is much lighter than Dr. Mingw debugging system because it doesn't have to deal with interprocess communication. The exception handling routine runs in the same process context of the faulting application (drmingw, in this case).

Using ExcHndl for you own purposes

If you incorporate ExcHndl in you own programs, especially those that you release to others, you can have almost the same exception information that you would get with Dr. Mingw, but with no need of the end user to have Dr. Mingw installed.

You can use ExcHndl in two ways:

• linking exchndl.o and libbfd.a with your program objects
• linking exchndl.o and libfd.a in the EXCHNDL.DLL and dinamically loading it at run-time. This can be done by linking just exchndl2.o or explicitly calling

LoadLibrary("exchndl.dll")i

The latter method is preferred because you have smaller executables and don't need to link the BFD library in all builds. The application wont fail if the EXCHNDL.DLL is missing.

Example

The sample test.exe application uses the second method above. Copy EXCHNDL.DLL to executable directory. When you run it, even before general protection fault dialog box appears, it's written to the test.RPT file a report of the fault.

Here is how test.RPT should look like:

-------------------

Error occured on Sunday, May 7, 2000 at 20:22:03.

C:\home\jrfonseca\drmingw\src\test.exe caused an Access Violation in module C:\WINDOWS\system32\msvcrt.dll Writing to location 00000008.

Registers:
eax=00003039 ebx=00000064 ecx=00000008 edx=0244fec0 esi=00401211 edi=0244fec0
eip=78027470 esp=0244fcd8 ebp=0244fea8 iopl=0         nv up ei pl nz na pe nc
cs=001b  ss=0023  ds=0023  es=0023  fs=0038  gs=0000             efl=00010202

Call stack:
78027470  C:\WINDOWS\system32\msvcrt.dll:78027470  wscanf
7802544B  C:\WINDOWS\system32\msvcrt.dll:7802544B  sscanf
00401241  C:\home\jrfonseca\drmingw\src\test.exe:00401241  YetAnotherFunction  //C/home/jrfonseca/drmingw/src/test.cxx:14
00401272  C:\home\jrfonseca\drmingw\src\test.exe:00401272  MyWonderfulFunction  //C/home/jrfonseca/drmingw/src/test.cxx:19
004012A9  C:\home\jrfonseca\drmingw\src\test.exe:004012A9  main  //C/home/jrfonseca/drmingw/src/test.cxx:24
004011C1  C:\home\jrfonseca\drmingw\src\test.exe:004011C1
004011EB  C:\home\jrfonseca\drmingw\src\test.exe:004011EB
77E87903  C:\WINDOWS\system32\KERNEL32.dll:77E87903  SetUnhandledExceptionFilter

Appendix

The IMAGEHLP.DLL Saga

Taken from several Bugslayer articles of MSJ.

The IMAGEHLP.DLL symbol engine first appeared in Windows NT 4.0. The beta Windows NT 5.0 SDK had new parts of IMAGEHLP.H that dealt with source and line information. In the meantime, the November 1998 Platform SDK showed and the IMAGEHLP.DLL that shipped with it supported the new source and line handling. There are several different versions of IMAGEHLP.DLL. The only one that does not support the new source and line information is the original Windows NT 4.0 version.

The IMAGEHLP.DLL version 5.00.1678.1 dynamically links to MSPDB50.DLL. It first tries to load MSDBI.DLL, and if that is not found it will load MSPDB50.DLL, so it works with both Visual C++ 5.0 and 6.0. To use it with Visual C++ 6.0, copy MSPDB50.DLL to MSPDB60.DLL. If you want to get symbols from the field, you will have to compile with CodeView symbols and use .DBG files to get them. Keep in mind that MSPDB50.DLL, like MSDBI.DLL, is not redistributable.

The IMAGEHLP.DLL version 5.00.1878.1, which comes with the Windows 2000 Beta 2 Platform SDK, hard links against MSDBI.DLL instead of dynamically loading MSPDB50.DLL as in earlier versions to read PDB files. The problem is that MSDBI.DLL is not redistributable.

IMAGEHLP.DLL now uses DEBUGHLP.DLL.

If you want IMAGEHLP.DLL, it's available in:

• Platform SDK
• WinDBG Debugger
• Windows 2000

Suggested Reading

• A Crash Course on the Depths of Win32 Structured Exception Handling, MSJ January 1997
• MSJEXHND - Part 1, Under the Hood, MSJ April 1997
• MSJEXHND - Part 2, Under the Hood, MSJ May 1997
• Bugslayer, MSJ, August 1998
• The Win32 Debugging Application Programming Interface