Runtime error stacktrace or location in VB6

Question

I maintain an old application written in VB6. In client's environment it raises runtime errors which I can't reproduce under debugger. Is there any way to get the stacktrace or location of error?

I mean, without putting trace statements all over the code like here or adding error handlers for logging to every procedure like here.

It seems to be a simple question. Sorry. I just don't know VB6 very well. And it is surprisingly hard to google out any information, considering how widely it is (or used to be) used.

Answer 1

It's been a while, but I don't think there is a way to get a stack trace in a VB6 application without adding an error handler and outputting the appropriate message. There were some third party tools that would add error handling to an entire application but I believe it just added "On Error Goto" error handlers throughout the code.

Just as an aside, one of the more insidious runtime errors I ever encountered in a VB6 app was when I used a font that didn't exist on the client's PC in the property of a control. This generates a runtime error that cannot be trapped in code, so no amount of error handling that I added ever uncovered the error. I finally came across it by chance. Hope this helps.

Answer 2

Try compiling to pcode and see if you still get the error. This is one common difference between the debug mode of VB6 and runtime. I used to compile to native and ran into errors that only occurred in runtime. When I switched to pcode I found either the error went away or more likely a new error that reflected the real problem cropped up and was more easily reproduced in debug mode.

If despite that you still getting the error then I really recommend starting at the top of your procedure stack and working you way down using Maero's suggestion of

On Error Goto Handler
<code>
Exit <routine>
Handler:
Err.Raise Err.Number, "(function_name)->" & Err.source, Err.Description

It is a pain but there is no real way around it.

Answer 3

The VB6 debugger is flaky sometimes. There are alternatives.

• You could try Windbg, a free standalone debugger from Microsoft. Compile your VB6 with no optimisation and "create symbolic debug info" (i.e. create PDB files), and you will be able to debug. Here's a 2006 blog post by a Microsoft guy about using Windbg with VB6, and 2004 blog post by another Microsoft guy with a brief introduction to Windbg.
• You could also use the Visual Studio 2008 debugger with VB6 and PDB files, e.g. with Visual C++ Express Edition (which is free). See this for more details.
• Both Windbg and Visual Studio expect the source code to be in exactly the same path on the debug machine as it was on the build machine when the VB6 was built. The easiest way is to build and debug on the same machine. Otherwise you might need to fiddle with SUBST to create virtual drives - or I'm told the serious way is to use a Symbol Server.

Answer 4

If you check the "Create Symbolic Debug Info" checkbox on the Project Properties/Compile tab, then you can debug in Visual Studio just like you would a native C++ application.