On Fri, 11 May 2018, Segher Boessenkool wrote: > > In general such address-based comparisons steps are invalid; they lack > > anti-reflexivity. So comparators are not actually allowed to do that. > > I don't know what you mean here? Every comparator is required to be > *reflexive*! Subtracting two pointers to elements of the same array gives > a perfectly fine total order as far as I see (it is the same as the > difference between the array indices of those elements).
I meant "anti-commutativity"; sorry about the mixup. Such strategy does not give a valid total order because the order changes while in-progress sorting reorders elements: suppose you have elements A and B such that A precedes B and compare A < B via address test. At some point, the sort routine may swap A with some unrelated element C, moving A past B. Now B < A. This is a contradiction. > In any case, every qsort call that is converted to one with this weaker > contract will need to be checked. Or we can wait for bug reports ;-) I think such comparators have a good chance of failing existing qsort_chk validation. Alexander
