From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [216.205.24.124]) by sourceware.org (Postfix) with ESMTP id 9A6C5386F802 for ; Tue, 22 Sep 2020 09:40:27 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 9A6C5386F802 Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-341-SuCItxG2MgebvJ5vQjuOtA-1; Tue, 22 Sep 2020 05:40:23 -0400 X-MC-Unique: SuCItxG2MgebvJ5vQjuOtA-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id BF4761017DC2; Tue, 22 Sep 2020 09:40:21 +0000 (UTC) Received: from localhost (unknown [10.33.36.46]) by smtp.corp.redhat.com (Postfix) with ESMTP id 6971810013BD; Tue, 22 Sep 2020 09:40:21 +0000 (UTC) Date: Tue, 22 Sep 2020 10:40:20 +0100 From: Jonathan Wakely To: Alejandro Colomar Cc: Ville Voutilainen , Florian Weimer , gcc@gcc.gnu.org, libstdc++ , Libc-alpha , LKML , libc-coord@lists.openwall.com Subject: Re: Expose 'array_length()' macro in Message-ID: <20200922094020.GS6061@redhat.com> References: <946e9377-0558-3adf-3eb9-38c507afe2d0@gmail.com> <874knr8qyl.fsf@oldenburg2.str.redhat.com> <875z875si2.fsf@oldenburg2.str.redhat.com> <20200921140100.GA449323@redhat.com> <20200921220443.GP6061@redhat.com> <34bae433-f67e-877d-f871-928cb55d2a87@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <34bae433-f67e-877d-f871-928cb55d2a87@gmail.com> X-Clacks-Overhead: GNU Terry Pratchett X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Spam-Status: No, score=-9.2 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H5, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=unavailable autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libstdc++@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libstdc++ mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 22 Sep 2020 09:40:29 -0000 On 22/09/20 11:10 +0200, Alejandro Colomar via Libstdc++ wrote: >[[ CC += LKML ]] > >Thanks for all your input. I learned some C++ :) > >The following code works for all C and C++ standards: >g++ --std={c++98, c++03, c++11, c++14, c++17, c++20} >gcc --std={c89, c99, c11, c18, c2x} >With `-Wall -Wextra -Werror -pedantic -Wno-vla -Wno-sizeof-pointer-div`. >It doesn't compile when '+ __array_length(p)' is uncommented. >It compiles, and returns the correct value (18). > With some exceptions: >c++ doesn't accept the VLA (w[]): > >array_length.c: In function 'int main()': >array_length.c:101:22: error: no matching function for call to >'__array_slength(int [y])' > 101 | + __array_slength(w) > | ^ >array_length.c:38:1: note: candidate: 'template >std::ptrdiff_t __array_slength(const T (&)[N])' > 38 | __array_slength(const T(&)[N]) > | ^~~~~~~~~~~~~~~ >array_length.c:38:1: note: template argument deduction/substitution >failed: >array_length.c:101:22: note: variable-sized array type 'long int' is >not a valid template argument > 101 | + __array_slength(w) > | ^ > >But we can live with limited support for VLAs in C++. >So I needed to comment '+ __array_slength(w)', >and then the program correctly returns 11. > >As Ville suggested, I renamed the function/macro to __array_[s]length(). >However, (some) BSDs already provide nitems() in , >so it probably wouldn't be very drastic to >provide this function/macro with the name '[s]nitems()' there. > >Would you like to add anything else before I write the patch? > > >BTW, I should note one more thing: > >Linux has a macro named '__must_be_array()' with the same API, >but slightly different internal implementation, >so they should be aware of this change. >However, I don't think they include a lot, >so maybe it doesn't break anything; but they should be aware anyway. >I CC'd the LKML so they are aware and can give any suggestions. > >Thanks, > >Alex > > >------------------------------------------------------------------------ > >#if defined(__cplusplus) > ># include ># if __cplusplus >= 201703L ># include ># endif > >template You need to use reserved names here. ># if __cplusplus >= 201103L >constexpr ># endif >inline std::size_t ># if __cplusplus >= 201703L >__array_length(const T(&arr)[N]) ># else >__array_length(const T(&)[N]) ># endif ># if __cplusplus >= 201103L >noexcept If this is going to be part of Glibc then you can use its __THROW macro instead. ># endif >{ ># if __cplusplus >= 201703L > return std::size(arr); ># else > return N; ># endif >} I don't see much point in using std::size here. If you're going to provide the alternative implementation for when std::size isn't defined, why not just use it always? template #if __cplusplus >= 201103L constexpr #endif inline std::size_t __array_length(const _Tp(&)[_Num]) __THROW { return _Num; } This only requires , not . > >template This declaration is wrong, the array extent has type std::size_t. The type you return from the function doesn't change that. The __array_slength definition should be identical to __array_length except for its name and return type. template #if __cplusplus >= 201103L constexpr #endif inline std::ptrdiff_t __array_slength(const _Tp(&)[_Num]) __THROW { return _Num; } > > >#else /* !defined(__cplusplus) */ >#include > ># define __is_same_type(a, b) \ > __builtin_types_compatible_p(__typeof__(a), __typeof__(b)) ># define __is_array(arr) (!__is_same_type((arr), &(arr)[0])) > ># if __STDC_VERSION__ >= 201112L ># define __must_be(e, msg) ( \ > 0 * (int)sizeof( \ > struct { \ > _Static_assert((e), msg); \ > char ISO_C_forbids_a_struct_with_no_members__; \ > } \ > ) \ >) ># else ># define __must_be(e, msg) ( \ > 0 * (int)sizeof( \ > struct { \ > int : (-!(e)); \ > char ISO_C_forbids_a_struct_with_no_members__; \ > } \ > ) \ >) ># endif > ># define __must_be_array(arr) __must_be(__is_array(arr), "Must be an >array!") > ># define __array_len(arr) (sizeof(arr) / sizeof((arr)[0])) ># define __array_length(arr) (__array_len(arr) + __must_be_array(arr)) ># define __array_slength(arr) ((ptrdiff_t)__array_length(arr)) >#endif > > >int main(void) >{ > int a[5]; > const int x = 6; > int v[x]; > int y = 7; > int w[y]; > int *p; > (void)p; > (void)v; > (void)w; > > return __array_slength(a) > + __array_slength(v) > + __array_slength(w) >// + __array_length(p) > ; >} >