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  MS Visual C++ Conditional Breakpoints
  Submitted by

There haven't been any tips for a while, so I thought I'd share this feature I found the other day.

I have a single class that implements a byte stream used all over the place which transfers data in a separate thread. This thread moves all the data streams along their merry way (i.e. there's one static thread, not one thread for each object). Wouldn't you know it, I had a bug I had to track down even though I thought the code was solid. The problem was that I had to have at least 10 instances of this class to even run the program, and everything was happening in a different thread at a 10ms interval, so I couldn't just set a breakpoint in a method because it'd be impossible to track. Enter conditional breakpoints:

[Note: this only applies to Microsoft Visual C++, sorry]
What you do is set a breakpoint where you normally would (F9). Now, edit breakpoints (Alt-F9). Select the breakpoint you just created, and hit the "Condition" tab to bring up the "Breakpoint Condition" dialog. There are a couple things you can do from this dialog. The most useful is that you can set a condition for the breakpoint.

For my problem, I set a breakpoint where I created the byte stream that I thought was causing the problem. I noted its address in memory. Then I went into the class code for my worker thread, and set a breakpoint with the condition "pByteStream == 0xAEF38201" (or whatever the address was of the offending stream) . Then I continue execution. Like magic, the worker thread breaks when it gets to my offending stream, ignoring all the others. I also used this to put breaks in the byte stream code itself with the condition "this == 0xAEF38201".

(Another nice thing is that, at least in debug builds, your program always allocates variables in the same place in memory if you follow the same execution path; you only have to manually check the address once).

The other options in this dialog allow you to declare the size of a dereferenced void pointer to use with a the compare logic, and allow you to set a number of times to skip the breakpoint before actually breaking (so you can look at the execution every Nth loop).

Granted, this is all documented and not a "secret", but it's something I'd never discovered until my back was against the wall. Hope this helps with your debugging.

The zip file viewer built into the Developer Toolbox made use of the zlib library, as well as the zlibdll source additions.


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