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From: Ben Bacarisse <ben@bsb.me.uk>
Newsgroups: comp.lang.c
Subject: Re: Baby X is bor nagain
Date: Thu, 20 Jun 2024 22:39:57 +0100
Organization: A noiseless patient Spider
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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.

-- 
Ben.