Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Tim Rentsch Newsgroups: comp.lang.c Subject: Re: Baby X is bor nagain Date: Fri, 21 Jun 2024 21:10:07 -0700 Organization: A noiseless patient Spider Lines: 62 Message-ID: <86h6dlhb34.fsf@linuxsc.com> References: <20240613174354.00005498@yahoo.com> <20240617002924.597@kylheku.com> <20240618115650.00006e3f@yahoo.com> <20240618184026.000046e1@yahoo.com> <877celzx14.fsf@nosuchdomain.example.com> <87iky3svqh.fsf@bsb.me.uk> <874j9nxsdy.fsf@nosuchdomain.example.com> <874j9ns382.fsf@bsb.me.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Injection-Date: Sat, 22 Jun 2024 06:10:09 +0200 (CEST) Injection-Info: dont-email.me; posting-host="ec2cba2faddfc9625d8883c2f98b569e"; logging-data="3783390"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19pFHIH9fw9HxZFX/57eeVhg3O0MAu7R50=" User-Agent: Gnus/5.11 (Gnus v5.11) Emacs/22.4 (gnu/linux) Cancel-Lock: sha1:vcYedKIp+uY3rbBLLPEwfZxSepQ= sha1:UgkprlaWXNwDEpXlef6l8Uzws/Q= Bytes: 4592 Ben Bacarisse writes: > Keith Thompson writes: > >> Ben Bacarisse 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. I think you're misreading the first sentence. Suppose we are in court listening to an ongoing murder trial. Witness one comes in and testifies that Alice left the house before Bob. Witness two comes in (after witness one has gone) and testifies that Bob left the house before Cathy. Witness three comes in (after the first two have gone) and testifies that Cathy left the house before Alice. None of the witnesses have contradicted either of the other witnesses, but the testimonies of the three witnesses are not consistent with one another. Try a web search "consistent with" definition for more explanation. Also, for "one another", if we say the children in the Jones family get along with one another, we don't mean that each child gets along with at least one of the others, but instead mean that every child gets along with every other child, that is, that they all get along with each other. Whether or not some other reading (of that problem sentence in the C standard) is sensible, surely the reading I have suggested is a plausible one. Do you agree? It seems clear, given how the second sentence is phrased, that this suggested reading is what was intended. I don't mean to defend the quality of writing in this passage. Certainly it would be nice if the meaning could have been stated more plainly. But I think it's an overstatement to say that the first sentence in no way implies a total order.