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.
Dr. Mingw can read debugging information in DWARF and Stabs formats — generated by the Gnu C/C++ Compiler, and in a PDB file — generated by the Microsoft Visual C++ Compiler. It relies upon the DbgHelp library to resolve symbols in modules compiled by the Microsoft tools.
The functionality to resolve symbols and dump stack backtraces is provided as DLLs so it can be embedded on your applications/tools.
See here for summary of changes between releases.
Detailed changelog available here.
Dr. Mingw can be downloaded here.
Git repository available here.
Currently you will need to download and install the 32 bits version in order to debug 32 bits applications, and also the 64 bits version if you want to debug 64 bits applications.
To install enter
drmingw -i
Dr. Mingw will register itself as the JIT debugger by writting into the system registry. Make sure you have Administrator rights. See this page for more information on how this works.
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
You can easily try Dr. Mingw by building and running the included sample. 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.
The following table describes the Dr. Mingw command-line options. All comand-line options are case-sensitive.
| Short | Long | Action |
|---|---|---|
| -h | --help | Print help and exit |
| -V | --version | Print version and exit |
| -i | --install | Install as the default JIT debugger |
| -a | --auto | Automatically start (used with -i or --install) |
| -u | --uninstall | Uninstall |
| -p pid | --process-id=pid | Attach to the process with the given identifier |
| -e event | --event=event | Signal an event after process is attached |
| -b | --breakpoints | Treat breakpoints as exceptions |
| -v | --verbose | Verbose output |
The MgwHelp library aims to be a drop-in replacement for the DbgHelp library, that understand MinGW symbols. It provides the same interface as DbgHelp library, but it is able to read the debug information produced by MinGW compilers/linkers.
MgwHelp is used by Dr.MinGW and ExcHndl below to lookup symbols.
But the hope is that it will eventually be used by third-party Windows development tools (like debuggers, profilers, etc.) to easily resolve symbol on binaries produced by the MinGW toolchain.
MgwHelp relies on libdwarf to read DWARF debugging information. It can optionally also use BFD to read stabs debugging information.
NOTE: It's still work in progress, and only exports a limited number of symbols. So it's not a complete solution yet
The exchndl.dll is a embeddable exception handler. It produces the similar output to Dr. Mingw, but it can be bundled into your applications. The exception handling routine runs in the same process context of the faulting application.
If you deploy ExcHndl together your own programs you can have almost the same exception information that you would get with Dr. Mingw, but with no need for the end user to install Dr. Mingw installed.
You can use ExcHndl by:
-
including
exchndl.dll,mgwhelp.dll,dbghelp.dll,symsrv.dll, andsymsrv.yeswith your application binaries -
and loading
exchndl.dllwhen your application starts, either by-
compiling and linking
exchndl2.cxxsource file -
or by explicitly calling
LoadLibrary("exchndl.dll")
-
The sample sample.exe application uses the second method above. Copy all DLLs mentioned above to the executable directory. When you run it, even before general protection fault dialog box appears, it's written to the sample.RPT file a report of the fault.
Here is how sample.RPT should look like:
------------------- Error occured on Tuesday, June 25, 2013 at 08:18:51. z:\projects\drmingw\sample\sample.exe caused an Access Violation at location 74D2ECC0 in module C:\Windows\syswow64\msvcrt.dll Writing to location 00000001. Registers: eax=00003039 ebx=00000064 ecx=00000001 edx=0028fe50 esi=00003039 edi=0000006f eip=74d2ecc0 esp=0028fc5c ebp=0028fe30 iopl=0 nv up ei pl nz na pe nc cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010202 AddrPC Params 74D2ECC0 0028FE50 00403064 00000000 msvcrt.dll!_strxfrm_l 74D2EDC8 74D2E991 00403064 00000000 msvcrt.dll!sscanf 74D2ED67 00403067 00403064 00000001 msvcrt.dll!sscanf 004013DE 00000008 60000000 40166666 sample.exe!Function [z:\projects\drmingw\sample/sample.cxx @ 12] 00401D3E 00000004 40B33333 0028FF08 sample.exe!Class::StaticMethod(int, float) [z:\projects\drmingw\sample/sample.cxx @ 17] 00401D5E 00401970 00714458 0000000C sample.exe!Class::Method() [z:\projects\drmingw\sample/sample.cxx @ 21] 004013F9 00000001 00572F38 00571B38 sample.exe!main [z:\projects\drmingw\sample/sample.cxx @ 27] 004010FD 7EFDE000 F769D382 00000000 sample.exe 77269F42 00401280 7EFDE000 00000000 ntdll.dll!RtlInitializeExceptionChain 77269F15 00401280 7EFDE000 00000000 ntdll.dll!RtlInitializeExceptionChain
Make sure you don't use Dr.Mingw and exchndl at the same time -- the latter seems to interfere with the former by some obscure reason.
This options are essential to produce suitable results are:
-
-g: produce debugging information -
-fno-omit-frame-pointer: use the frame pointer (frame pointer usage is disabled by default in some architectures likex86_64and for some optimization levels; and it may be impossible to walk the call stack without it)
You can choose more detailed debug info, e.g., -g3, -ggdb. But so far I have seen no evidence this will lead to better results, at least as far as Dr.MinGW is concerned.
Callers put the return IP address on the stack. Therefore the source lines we get when looking the address is not the line of the call, but instead the line of the instruction immediately succeeding the call.
Dr. Mingw uses DbgHelp to handle .PDB files so it has the same behavior.
If you test on the machine you built, you typically need to do nothing. Otherwise you'll need to tell where your .PDBs are trhough the _NT_SYMBOL_PATH environment variable, using the syntax described here.
- binutil's addr2line (included in MinGW)
- cv2pdb - DWARF to PDB converter
- google-breakpad