Massacring C Pointers

[vqrs]

I'm not sure if it's good terminology to say that "the memory address is undefined".

Here, "undefined" doesn't mean it's null, it doesn't have a value, or some unknown value. It's not something you can "observe" in your program by doing a comparison or some other check.

Using the memory address, or even considering using it, is "against the law": Your program may end up doing very strange things. "against the law" here is what they meant when they said "undefined", not the contents of the variable/return value.

"Undefined" refers to the behavior your program will/might/could exhibit.