From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00,FREEMAIL_FROM autolearn=unavailable autolearn_force=no version=3.4.4 Path: eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!feeder.eternal-september.org!aioe.org!.POSTED!not-for-mail From: Victor Porton Newsgroups: comp.lang.ada Subject: Re: Allocators design flaw Date: Sat, 14 Oct 2017 17:24:12 +0300 Organization: Aioe.org NNTP Server Message-ID: References: NNTP-Posting-Host: 8U0x309/ia0QUzusgm/krA.user.gioia.aioe.org Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7Bit X-Complaints-To: abuse@aioe.org User-Agent: KNode/4.14.10 X-Notice: Filtered by postfilter v. 0.8.2 Xref: news.eternal-september.org comp.lang.ada:48455 Date: 2017-10-14T17:24:12+03:00 List-Id: Victor Porton wrote: > Victor Porton wrote: > >> Dmitry A. Kazakov wrote: >> >>> On 2017-10-14 04:53, Victor Porton wrote: >>>> It is impossible to properly implement an allocator through a C >>>> function (such as raptor_alloc_memory() from Raptor C library) which >>>> allocates a struct and returns the pointer to the allocated struct. >>>> >>>> It is because RM13.11(21.5/3) "The Alignment parameter is a nonzero >>>> integral multiple of D'Alignment..." >>>> >>>> (If it were "The Alignment parameter is equal to D'Alignment", then we >>>> would be able just to check (in Allocate procedure implementation) that >>>> >>>> pragma Assert(Dummy_Record'Alignment mod Alignment = 0); >>>> -- where Dummy_Record is an arbitrary C-convention record >>>> -- (as all C records have the same alignment reqs) >>>> >>>> So Alignment parameter may be arbitrarily big and the C function >>>> alignment may not conform to it. >>> >>> Usually allocators return addresses already rounded and there is nothing >>> to worry about. >>> >>>> Let us think how to work around (in Ada 202x) of this design flaw. >>> >>> If any it is _alloc_memory() flaw, not Ada's. >> >> It is an Ada flaw, because Ada must be able to interact with legacy C >> libraries. > > Moreover, the same logic would lead to say that there is malloc() standard > C flaw. > > It is not constructive to criticize an ISO standard of C. I mean, it is because we cannot change C standard for better compatibility with Ada. But we can change Ada 202x for better compatibility with C libraries. >>> Add max alignment + log max alignment - 1 to the desired size. Add log >>> max alignment to the returned address and round to the required >>> alignment. Place the offset to original address in front (log alignment >>> length). Return the rounded address. When freed use the stored offset to >>> get the original address. >> -- Victor Porton - http://portonvictor.org