What is your favourite Windbg tip/trick? [closed]

Question

My favorite is the command .cmdtree <file> (undocumented, but referenced in previous release notes). This can assist in bringing up another window (that can be docked) to display helpful or commonly used commands. This can help make the user much more productive using the tool.

Initially talked about here, with an example for the <file> parameter: http://blogs.msdn.com/debuggingtoolbox/archive/2008/09/17/special-command-execute-commands-from-a-customized-user-interface-with-cmdtree.aspx

Example: alt text http://blogs.msdn.com/photos/debuggingtoolbox/images/8954736/original.aspx

To investigate a memory leak in a crash dump (since I prefer by far UMDH for live processes). The strategy is that objects of the same type are all allocated with the same size.

• Feed the !heap -h 0 command to WinDbg's command line version cdb.exe (for greater speed) to get all heap allocations:

"C:\Program Files\Debugging Tools for Windows\cdb.exe" -c "!heap -h 0;q" -z [DumpPath] > DumpHeapEntries.log

• Use Cygwin to grep the list of allocations, grouping them by size:

grep "busy ([[:alnum:]]\+)" DumpHeapEntries.log \
| gawk '{ str = $8; gsub(/\(|\)/, "", str); print "0x" str " 0x" $4 }' \
| sort \
| uniq -c \
| gawk '{ printf "%10.2f %10d %10d ( %s = %d )\n", $1*strtonum($3)/1024, $1, strtonum($3), $2, strtonum($2) }' \
| sort > DumpHeapEntriesStats.log

• You get a table that looks like this, for example, telling us that 25529270 allocations of 0x24 bytes take nearly 1.2 GB of memory.

   8489.52        707      12296 ( 0x3000 = 12288 )
  11894.28       5924       2056 ( 0x800 = 2048 )
  13222.66     846250         16 ( 0x2 = 2 )
  14120.41     602471         24 ( 0x2 = 2 )
  31539.30    2018515         16 ( 0x1 = 1 )
  38902.01    1659819         24 ( 0x1 = 1 )
  40856.38        817      51208 ( 0xc800 = 51200 )
1196684.53   25529270         48 ( 0x24 = 36 )

• Then if your objects have vtables, just use the dps command to seek some of the 0x24 bytes heap allocations in DumpHeapEntries.log to know the type of the objects that are taking all the memory.

0:075> dps 3be7f7e8
3be7f7e8  00020006
3be7f7ec  090c01e7
3be7f7f0  0b40fe94 SomeDll!SomeType::`vftable'
3be7f7f4  00000000
3be7f7f8  00000000

It's cheesy but it works :)

The following command comes very handy when looking on the stack for C++ objects with vtables, especially when working with release builds when quite a few things get optimized away.

dpp esp Range

Being able to load an arbitrary PE file as dump is neat:

windbg -z mylib.dll

Query GetLastError() with:

!gle

This helps to decode common error codes:

!error error_number

Almost 60% of the commands I use everyday..

dv /i /t
?? this
kM (kinda undocumented) generates links to frames
.frame x
!analyze -v
!lmi
~

Explanation

1. dv /i /t [doc]
   1. dv - display names and values of local variables in the current scope
   2. /i - specify the kind of variable: local, global, parameter, function, or unknown
   3. /t - display data type of variables
2. ?? this [doc]
   1. ?? - evaluate C++ expression
   2. this - C++ this pointer
3. kM [doc]
   1. k - display stack back trace
   2. M - DML mode. Frame numbers are hyperlinks to the particular frame. For more info about kM refer to http://windbg.info/doc/1-common-cmds.html
4. .frame x [doc]
   1. Switch to frame number x. 0 being the frame at top of stack, 1 being frame 1 below the 0th frame, and so on.
   2. To display local variables from another frame on the stack, first switch to that frame - .frame x, then use dv /i /t. By default d will show info from top frame.
5. !analyze -v [doc1] [doc2 - Using the !analyze Extension]
   1. !analyze - analyze extension. Display information about the current exception or bug check. Note that to run an extension we prefix !.
   2. -v - verbose output
6. !lmi [doc]
   1. !lmi - lmi extension. Display detailed information about a module.
7. ~ [doc]
   1. ~ - Displays status for the specified thread or for all threads in the current process.

The "tip" I use most often is one that will save you from having to touch that pesky mouse so often: Alt + 1

Alt + 1 will place focus into the command window so that you can actually type a command and so that up-arrow actually scrolls through command history. However, it doesn't work if your focus is already in the scrollable command history.

Peeve: why the heck are key presses ignored while the focus is in a source window? It's not like you can edit the source code from inside WinDbg. Alt + 1 to the rescue.

One word (well, OK, three) : DML, i.e. Debugger Markup Language.

This is a fairly recent addition to WinDbg, and it's not documented in the help file. There is however some documentation in "dml.doc" in the installation directory for the Debugging Tools for Windows.

Basically, this is an HTML-like syntax you can add to your debugger scripts for formatting and, more importantly, linking. You can use links to call other scripts, or even the same script.

My day-to-day work involves maintenance on a meta-modeler that provides generic objects and relationship between objects for a large piece of C++ software. At first, to ease debugging, I had written a simple dump script that extracts relevant information from these objects.

Now, with DML, I've been able to add links to the output, allowing the same script to be called again on related objects. This allows for much faster exploration of a model.

Here's a simplified example. Assume the object under introspection has a relationship called "reference" to another object. r @$t0 = $arg1 $$ arg1 is the address of an object to examine

$$ dump some information from $t0

$$ allow the user to examine our reference
aS /x myref @@(&((<C++ type of the reference>*)@$t0)->reference )
.block { .printf /D "<link cmd=\"$$>a< <full path to this script> ${myref}\">dump Ref</link> " }

Obviously, this a pretty canned example, but this stuff is really invaluable for me. Instead of hunting around in very complex objects for the right data members (which usually took up to a minute and various casting and dereferencing trickery), everything is automated in one click!