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From: Michael S <already5chosen@yahoo.com>
Newsgroups: comp.lang.c
Subject: Re: Baby X is bor nagain
Date: Fri, 21 Jun 2024 00:56:05 +0300
Organization: A noiseless patient Spider
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On Thu, 20 Jun 2024 22:39:57 +0100
Ben Bacarisse <ben@bsb.me.uk> wrote:

> Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
> 
> > Ben Bacarisse <ben@bsb.me.uk> writes:
> > [...]  
> >> On a C language point, I don't think the standard says anything
> >> about sorting with non-order functions like the one above.  Is an
> >> implementation of qsort permitted to misbehave (for example by not
> >> terminating) when the comparison function does not implement a
> >> proper order relation?  
> >
> > N1570 7.22.5p4 (applies to bsearch and qsort):
> > """
> > When the same objects (consisting of size bytes, irrespective of
> > their current positions in the array) are passed more than once to
> > the comparison function, the results shall be consistent with one
> > another. That is, for qsort they shall define a total ordering on
> > the array, and for bsearch the same object shall always compare
> > the same way with the key.
> > """
> >
> > That's a "shall" outside a constraint, so violating it results in
> > undefined behavior.  
> 
> I think it should be clearer.  What the "that is" phrase seems to
> clarify in no way implies a total order, merely that the repeated
> comparisons or the same elements are consistent with one another.
> That the comparison function defines a total order on the elements
> is, to me, a major extra constraint that should not be written as an
> apparent clarification to something the does not imply it: repeated
> calls should be consistent with one another and, in addition, a total
> order should be imposed on the elements present.
> 

Malcolm's comparison function does not establish total order on
arbitrary strings, but it is both possible and likely that it
establishes total order on all sets of inputs that he cares about.