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Very Dirty But Very Useful Debugging Trick
Submitted by |
Just a short tip, that I found very useful about bug hunting.
Many times when an assert fails or a memory exception happen is just too late to
try to know what was wrong.
And when is a very strange situation is very hard to breakpoint before it
happens and try to understand why does it failed.
So, you have the debug window, in break mode, just over the line where the
exceptions happen, or where the assert failed.
If you change to the Disassembly window, you can see the memory address for each
line of assembly code, and their correspondence with c source.
The tip consist in changing the EIP register to the address of a previous line
of code of the same function, using the watch window.
That way you can go a bit back in execution and debug, step by step a few lines
before the exception or the assert, so you can get more useful info about what
was wrong.
You can go back in code outside the current function, but you have to be very
careful about the ESP and EBP registers and always take in account the
unpredictable results in execution of unsuitable use of that tip. If you are
curious about the correct values of ESP and EBP, you can look at the begin and
end of the assembly code of the current function and do some reverse engineering
of your compiler's calls protocol.
Alberto Garcia-Baquero Vega
Nebula Entertainment
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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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