GetLastError on WinDBG

Today, after a series of design posts, we'll be looking at something quite different.

If you happen to be debugging a native application, you may find yourself stepping through code that calls a Win32 API and then never checks the last error, maybe because it's only looking at the result for pass/fail information. The error is there somewhere, as of course GetLastError can get it, but how to get to it?

If you're using WinDBG, part of the Debugging Tools for Windows, there are two debugger extensions you can combine to get this information. The first is !teb. This extension will provide you with information on the current thread, with an output similar to the following. Don't worry if some of the fields don't match your output - the format for this command has changed over time.

0:001> !teb
TEB at 7FFDD000
   ExceptionList:    76ffdc
   Stack Base:       770000
   Stack Limit:      76f000
   SubSystemTib:     0
FiberData:        1e00
   ArbitraryUser:    0
   Self:             7ffdd000
   EnvironmentPtr:   0
ClientId:         324.48c
   Real ClientId:    324.48c
   RpcHandle:        0
   Tls Storage:      0
   PEB Address:      7ffdf000
   LastErrorValue:   2
   LastStatusValue:  0
   Count Owned Locks:0
   HardErrorsMode:   0

Note the LastErrorValue field. This is the value that GetLastError reports; it lives at the thread level, which is how you can call into Win32 APIs from different threads and make sure each one gets its own value - it's a "thread global", if you will.

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