3 ms·

They're from different objects, so it's not meaningful (according to the standard) to compare them. That's what makes it UB.

• jjnoakes 8y ago
  "Not meaningful" and "undefined behavior" have very different meanings. I agree it's not meaningful, but the standard does define the result of comparing pointers to two unrelated objects with "==", so it's not UB.
  • Rusky 8y ago
    Where? The post cites C11 section 6.5.8 paragraph 5, which seems to disagree with you. Of course == comparing unrelated pointers is something you need to be able to do, but relational comparisons (including those based on pointer arithmetic) don't seem to be defined anywhere...
    • jjnoakes 8y ago
      Right, comparisons with < and > are UB, but I'm talking about ==, which is well-defined. It's hard to tell which operator "tzahola" was originally talking about, but I took "comparing the address" in this context (especially given his example code) to mean "==", or at least, include it, which makes the statement "comparing the addresses of stack variables is undefined behavior" incorrect in this context.
      • tzahola 8y ago
        You're right. There's no undefined behaviour for ==: Two pointers compare equal if and only if both are null pointers, both are pointers to the same object (including a pointer to an object and a subobject at its beginning) or function, both are pointers to one past the last element of the same array object, or one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space.