Thread: Unable to get an out of bounds reference on my virtual machine.

The relative position of variables in memory do not necessarily correspond to the relative position of their declarations. (Which is a reason why accessing an array out of bounds results in undefined behaviour.)

That said, you could try swapping the order of the two declarations to see if it makes any difference.

Originally Posted by KCfromNC

But yeah, you're hoping that undefined behavior works in the way you'd expect it to, and you see that it's not always the case.

By definition, undefined behaviour won't work in any way you expect it to, at least not consistently, because it is undefined!

There are several reasons why what you expect doesn't happen, up to and including that C compilers are written by different people with different ideas but only have to conform to the C specification to be called C compilers. And the C specifications say NOTHING about where things should be stored in memory at all. For all you know, a compiler could be storing things at 1Mb gaps apart and then going back and filling in the gaps, or not even putting things into memory AT ALL (instead using processor registers, etc.). Hell, it could be putting them into completely random parts of memory, so long as it keeps track of them somehow (and you NEVER get to know how it keeps track from inside the C code itself).

You're trying to trigger a particular result of undefined behaviour that, presumably, your professor demonstrated. If you're not using exactly the same machine, the exact same OS, the exact same compiler, the exact same compilation step and the exact same source code, it won't happen. And even if you DO, there's no guarantee that what you witnessed will happen. Your professor almost certainly knows this, and has to keep that example setup working because it DOESN'T always trigger as nicely as you would hope.

Hell, depending on what you're doing, on some OS's you could (in theory) crash the program like that by triggering DEP, segfaults or other similar protections and never see any result at all.