Comment by schemescape

One similar story: I was maintaining some C++ code that had a few #ifdefs in it. Someone reported a problem.

I put a breakpoint on the calling code and traced into my code. It went into the #ifdef code I expected, but the problem persisted.

Just to double-check, I let the program run until it hit the breakpoint again and traced in, but this time, it went into the #else code! That code should have been removed by the preprocessor, yet here I was, currenting stepping through it.

After questioning my understanding of the C preprocessor (and my own sanity), I luckily noticed that the module name in the debugger was very slightly different in the two cases mentioned above.

The world finally made sense again. My code was in a header file that was compiled (with different symbols defined) into two different modules and both of those modules were loaded into the same process. When I set the breakpoint in the debugger, it silently set breakpoints in both modules.