Maybe inf or nan?

https://stackoverflow.com/a/42723797/2077386

No, but you need a nan check in the above code. And a negative zero check if you are picky for sorting maybe.

Maybe theyre talking about NaN?Comparing against NaN return false. That doesn't violate transitivity, but it can confuse many sorting algorithms.

Trivially, NaN != NaN, so this function is not even properly reflexive (i.e. for a = b = NaN we find cmp(&a,&b) = cmp(&b,&a) = 1).

I mean, yeah. It's not like there is a lot of meaning to NaN. For most applications, trying to sort NaNs is probably a bug in itself. So I think this is likely a perfectly valid comparison function.

To make a point, here is a sorting function I'm using.

    static int cmp_seqnos(const void *a, const void *b)
    {
        uint64_t x = (const uint64_t *) a;
        uint64_t y = (const uint64_t *) b;
        if (y - x < ((uint64_t) 1 << 63))
            return -1;
        return x != y;
    }

I think this should work if there are uint64_t a, b: b - a < ((uint64_t) 1 << 63) and all values x from the set to be sorted are in the range a..b. Note that a < b isn't a requirement here.

Sorting such numbers can be useful when dealing with sliding windows.