220 30402 article Path: news.gmane.org!.POSTED!not-for-mail From: "D. B." Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: P0298: A byte type definition: with undefined pointer arithmetic? 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[2a00:1450:400c:c09::22a]) by mx.google.com with ESMTPS id dk9si68239794wjb.198.2017.01.07.12.03.54 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 07 Jan 2017 12:03:54 -0800 (PST) Received-SPF: pass (google.com: domain of db0451@gmail.com designates 2a00:1450:400c:c09::22a as permitted sender) client-ip=2a00:1450:400c:c09::22a; Original-Received: by mail-wm0-x22a.google.com with SMTP id t79so71644717wmt.0 for ; Sat, 07 Jan 2017 12:03:54 -0800 (PST) X-Received: by 10.223.178.9 with SMTP id u9mr6170328wra.110.1483819434242; Sat, 07 Jan 2017 12:03:54 -0800 (PST) Original-Received: by 10.28.144.66 with HTTP; Sat, 7 Jan 2017 12:03:53 -0800 (PST) In-Reply-To: X-Original-Sender: db0451@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of db0451@gmail.com designates 2a00:1450:400c:c09::22a as permitted sender) smtp.mailfrom=db0451@gmail.com; dmarc=pass (p=NONE dis=NONE) header.from=gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:30402 Archived-At: --f403045eaca849f7c4054586a22d Content-Type: text/plain; charset=UTF-8 On Sat, Jan 7, 2017 at 7:52 PM, Chris Hallock wrote: > I, for one, would be shocked if the Committee (or anyone, really) actually > wants byte-oriented pointer arithmetic within objects to be undefined. I > strongly anticipate that CWG will accept issue 1701 as a defect and make it > well-defined. > I hope you're right. > Remember, we're not really talking about the byte accesses themselves; > such aliasing is already well-defined for char and unsigned char. It is > only the charPtr + i (or charPtr[i]) expression that is currently > undefined, and that expression can be awkwardly worked around anyway on > some platforms (e.g., cast to integer, add, cast back to pointer). > Oh, sure, on most platforms/compilers, even the non-workaround syntax just works, because the compilers don't go to lengths to maliciously subvert people's intuition. That doesn't make it well-defined per the IS. In fact, I'm pretty sure that what you said still isn't valid. It's not the arithmetic that's the problem: it's the fact that it's performed in such a way as to generate a final pointer that doesn't point into the same contiguous array as the original source pointer (because a "sequence" of unsigned char is not an array, formally). By the same token, subtracting or comparing pointers within such a "sequence" is not valid. See: https://www.securecoding.cert.org/confluence/display/c/ARR36-C.+Do+not+subtract+or+compare+two+pointers+that+do+not+refer+to+the+same+array These all seem like adjacent facets of the same issue to me. -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CACGiwhG2gei5%3Dk1tO%2BZVkMjoFSMyEVXwetwBKtKXQRqAXV8mgg%40mail.gmail.com. --f403045eaca849f7c4054586a22d Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On S= at, Jan 7, 2017 at 7:52 PM, Chris Hallock <christopherhallock= @gmail.com> wrote:
I, for one, would be shocked if the Committee (= or anyone, really) actually wants byte-oriented pointer arithmetic within o= bjects to be undefined. I strongly anticipate that CWG will accept issue 17= 01 as a defect and make it well-defined.

<= div>I hope you're right.

=C2=A0
Remember, we're not = really talking about the byte accesses themselves; such aliasing is already= well-defined for char and unsigned char. It is only the char= Ptr + i (or charPt= r[i]) expression that is currently undefined, and that expression ca= n be awkwardly worked around anyway on some platforms (e.g., cast to intege= r, add, cast back to pointer).

Oh= , sure, on most platforms/compilers, even the non-workaround syntax just wo= rks, because the compilers don't go to lengths to maliciously subvert p= eople's intuition. That doesn't make it well-defined per the IS.
In fact, I'm pretty sure that what you said still isn&#= 39;t valid. It's not the arithmetic that's the problem: it's th= e fact that it's performed in such a way as to generate a final pointer= that doesn't point into the same contiguous array as the original sour= ce pointer (because a "sequence" of unsigned char is not an array= , formally). By the same token, subtracting or comparing pointers within su= ch a "sequence" is not valid. See:=C2=A0 https://www.securecodi= ng.cert.org/confluence/display/c/ARR36-C.+Do+not+subtract+or+compare+two+po= inters+that+do+not+refer+to+the+same+array These all seem like adjacent= facets of the same issue to me.

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