220 30246 article Path: news.gmane.org!.POSTED!not-for-mail From: "'Neil MacIntosh' via ISO C++ Standard - Future Proposals" Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: RE: P0298: A byte type definition: with undefined pointer arithmetic? 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[2a01:111:f400:fe40::706]) by mx.google.com with ESMTPS id 74si28313886iou.249.2017.01.04.11.21.44 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Wed, 04 Jan 2017 11:21:44 -0800 (PST) Received-SPF: pass (google.com: domain of neil.macintosh@microsoft.com designates 2a01:111:f400:fe40::706 as permitted sender) client-ip=2a01:111:f400:fe40::706; Original-Received: from DM5PR03MB2825.namprd03.prod.outlook.com (10.175.105.11) by DM5PR03MB2828.namprd03.prod.outlook.com (10.175.105.14) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384_P384) id 15.1.817.10; Wed, 4 Jan 2017 19:21:42 +0000 Original-Received: from DM5PR03MB2825.namprd03.prod.outlook.com ([10.175.105.11]) by DM5PR03MB2825.namprd03.prod.outlook.com ([10.175.105.11]) with mapi id 15.01.0817.009; Wed, 4 Jan 2017 19:21:42 +0000 Thread-Topic: P0298: A byte type definition: with undefined pointer arithmetic? Thread-Index: AQHSZndgl96bd9O4NEuh6V6iYPQ+OaEosBhg In-Reply-To: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [50.46.251.73] x-ms-office365-filtering-correlation-id: ce0b471a-9ea0-481c-242b-08d434d6ea8c x-microsoft-antispam: UriScan:;BCL:0;PCL:0;RULEID:(22001);SRVR:DM5PR03MB2828; x-microsoft-exchange-diagnostics: 1;DM5PR03MB2828;7:81rfTwSl5GLj/LW1EoFGps/rXSOHwx5a5MUOTHtEDkqYXAKB+Ixr0Laln47rpDbAOmrWZf7JsyPm/0cRM5t7o+IEbv0QvMnRfvDblxKc5Q3D2SJp5Ue3wWVFnV8Khwh7hGPUal1Drg96T5fN1KRA3pff9T+nVorcrhWORRxEouUs7ZmICrKWAQmquy4xWfFdAIJdeJUjQtoJZOv47+4AooG/2fKXt3TKdDGJ2Nxx2cr44bMbgcmt2T+BibkvqVr6fssEE9GGKMZtc4gQtl2i9Pjb1/nOueRnWV7GvJR8Y/MqK9b5DNZrZJK7vMTPJstPfaSIeEoAcTGBjB4XNfF7mWQEBWR+t41ZMprny8c2cbLfmvGeWR4TWBcVTDQShZ2DTFiggNyXuuaC+1649VuP8gRt0IDnSZi6E6o8nRv/I6NoOOMG4BiVVUTkEKc86KHI6CCGI3BVI5/Cgjx3ONEILEsxl83Q3DVsfq92Iaj3oUg= x-microsoft-antispam-prvs: x-exchange-antispam-report-test: UriScan:(190756311086443)(189930954265078)(219752817060721); x-exchange-antispam-report-cfa-test: BCL:0;PCL:0;RULEID:(61425038)(6040375)(601004)(2401047)(8121501046)(5005006)(10201501046)(3002001)(6055026)(61426038)(61427038)(6041248)(20161123560025)(20161123555025)(20161123562025)(20161123564025)(6072148)(6047074);SRVR:DM5PR03MB2828;BCL:0;PCL:0;RULEID:;SRVR:DM5PR03MB2828; x-forefront-prvs: 0177904E6B x-forefront-antispam-report: SFV:NSPM;SFS:(10019020)(6009001)(7916002)(39860400002)(39850400002)(39450400003)(39410400002)(39840400002)(377454003)(199003)(13464003)(189002)(10090500001)(2501003)(66066001)(68736007)(2900100001)(76176999)(101416001)(92566002)(2906002)(189998001)(107886002)(122556002)(5001770100001)(105586002)(106116001)(97736004)(38730400001)(6436002)(54356999)(106356001)(74316002)(7736002)(99286003)(50986999)(305945005)(9686002)(561944003)(2950100002)(25786008)(77096006)(6506006)(33656002)(10290500002)(5660300001)(3660700001)(5005710100001)(6116002)(86612001)(3280700002)(102836003)(3846002)(229853002)(86362001)(81166006)(8936002)(55016002)(7696004)(8990500004)(8676002)(81156014);DIR:OUT;SFP:1102;SCL:1;SRVR:DM5PR03MB2828;H:DM5PR03MB2825.namprd03.prod.outlook.com;FPR:;SPF:Non e;PTR:InfoNoRecords;A:1;MX:1;LANG:en; received-spf: None (protection.outlook.com: microsoft.com does not designate permitted sender hosts) spamdiagnosticoutput: 1:99 spamdiagnosticmetadata: NSPM X-OriginatorOrg: microsoft.com X-MS-Exchange-CrossTenant-originalarrivaltime: 04 Jan 2017 19:21:42.7810 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 72f988bf-86f1-41af-91ab-2d7cd011db47 X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM5PR03MB2828 X-Original-Sender: neil.macintosh@microsoft.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@microsoft.com; spf=pass (google.com: domain of neil.macintosh@microsoft.com designates 2a01:111:f400:fe40::706 as permitted sender) smtp.mailfrom=Neil.MacIntosh@microsoft.com; dmarc=pass (p=QUARANTINE dis=NONE) header.from=microsoft.com X-Original-From: Neil MacIntosh 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:30246 Archived-At: Hello Kazutoshi, There are plenty of examples of programs that perform byte-oriented access = in the domain of operating systems and communications protocols. Consider r= eading or writing a wire-protocol, for example. In fact, you seem to go on = to give a brief sketch of exactly the sort of byte oriented access people w= ant to perform. Rather than speculate on core wording myself, and whether or not the patter= n you described is undefined behavior, I'll just observe that the Core Work= ing Group reviewed the proposal (twice - once in Oulu, once in Issaquah) an= d approved it for adoption. If you have specific questions about array-of-byte access to object represe= ntations, then these questions would be completely orthogonal to the std::b= yte proposal. They would exist today with unsigned char, and they would con= tinue to exist whether or not std::byte is adopted. So modifying the propos= al to address that issue is unnecessary. This proposal is about creating a = distinct type for accessing bytes of memory - something that can also be do= ne today using unsigned char or char. The semantics of such access should n= ot be changed by the proposal...it's a different issue. Hope that helps, Neil. -----Original Message----- From: Kazutoshi Satoda [mailto:k_satoda@f2.dion.ne.jp]=20 Sent: Wednesday, January 4, 2017 2:43 AM To: Neil MacIntosh ; std-proposals@isocpp.org Subject: P0298: A byte type definition: with undefined pointer arithmetic? from P0298R2: https://na01.safelinks.protection.outlook.com/?url=3Dhttp%3A%= 2F%2Fwww.open-std.org%2Fjtc1%2Fsc22%2Fwg21%2Fdocs%2Fpapers%2F2016%2Fp0298r2= ..pdf&data=3D02%7C01%7CNeil.MacIntosh%40microsoft.com%7C931c7d44cdb941e437b2= 08d4348e8184%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C63619123404167591= 7&sdata=3DCl8GsBUSn8IgERF6qnnr7G3uCHZBeOmF1cFvwaqajVQ%3D&reserved=3D0 > Motivation and Scope > Many programs require byte-oriented access to memory. ... Could you please show examples of such programs? As I read N4623 (WG21 2016-11 Issaquah Minutes) and saw that the proposal w= as about to pass through, I want to understand more concretely what is goin= g. I suspect that such programs do something like obtaining unsigned char* whi= ch points to an int object via reinterpret_cast and iterating through it as= if the pointer points to an element of unsigned char[sizeof(int)], as I pe= rsonally have seen many such programs. However, AFAIK, the current standard doesn't (or fails to) give well-define= d behavior to such byte-oriented access on objects of arbitrary types. N4618 5.7 [expr.add] p6 says: > For addition or subtraction, if the expressions P or Q have type=20 > "pointer to cv T", where T and the array element type are not similar=20 > (4.5), the behavior is undefined. Then, if "Many programs require ..." is saying what I suspect, I think defi= ning such byte-oriented accesses should be done before (or within) the prop= osal. It will also resolve CWG issue 1701: "Array vs sequence in object representation" https://na01.safelinks.protection.outlook.com/?url=3Dhttp%3A%2F%2Fwg21.cmee= rw.net%2Fcwg%2Fissue1701&data=3D02%7C01%7CNeil.MacIntosh%40microsoft.com%7C= 931c7d44cdb941e437b208d4348e8184%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0= %7C636191234041685924&sdata=3DRP%2BMlYq7xWlz5BrM13jeaG9fUL6U13iUEpApP5Plgk8= %3D&reserved=3D0 -- k_satoda --=20 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 e= mail 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/DM5PR03MB282513F1EFF95A2C111963B189610%40DM5PR03= MB2825.namprd03.prod.outlook.com. .