It is undefined behavior in C. In many languages it is defined behavior; for instance in Go, dereferencing a nil pointer explicitly panics, which is a well-defined operation. It may, of course, crash your program, and the whole topic of 'should pointers even be able to be nil?' is a valid separate other question, but given that they exist, the operation of dereferencing a nil pointer is not undefined behavior in Go.

To many people reading this this may be a "duh" but I find it is worth pointing out, because there are still some programmers who believe that C is somehow the "default" or "real" language of a computer and that everything about C is true of other languages, but that is not the case. Undefined behavior in C is undefined in C, specifically. Try to avoid taking ideas about UB out of C, and to the extent that they are related (which slowly but surely decreases over time), C++. It's the language, not the hardware that is defining UB.