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 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: "Dmitry A. Kazakov" Newsgroups: comp.lang.ada Subject: Re: Allocators design flaw Date: Sat, 14 Oct 2017 16:26:37 +0200 Organization: Aioe.org NNTP Server Message-ID: References: NNTP-Posting-Host: MajGvm9MbNtGBKE7r8NgYA.user.gioia.aioe.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-Complaints-To: abuse@aioe.org User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 X-Notice: Filtered by postfilter v. 0.8.2 Content-Language: en-US Xref: news.eternal-september.org comp.lang.ada:48457 Date: 2017-10-14T16:26:37+02:00 List-Id: On 2017-10-14 16:03, 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. >> >> 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. > > As far as I understand, it will not work, because the C library I am writing > bindings for may try to free an object allocated by me (or I my need to free > an object allocated by the library). The last sentence describes freeing memory, i.e. for Deallocate. -- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.de