Is sizeof(*ptr) undefined behavior when pointing to invalid memory?

Question

We all know that dereferencing an null pointer or a pointer to unallocated memory invokes undefined behaviour.

But what is the rule when used within an expression pa

Answer 1

Since sizeof does not evaluate its operand (except in the case of variable length arrays if you're using C99 or later), in the expression sizeof (*ptr), ptr is not evaluated, therefore it is not dereferenced. The sizeof operator only needs to determine the type of the expression *ptr to get the appropriate size.