From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm1-x32c.google.com (mail-wm1-x32c.google.com [IPv6:2a00:1450:4864:20::32c]) by sourceware.org (Postfix) with ESMTPS id 749D939874BB for ; Tue, 22 Sep 2020 09:10:11 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 749D939874BB Received: by mail-wm1-x32c.google.com with SMTP id q9so2522087wmj.2 for ; Tue, 22 Sep 2020 02:10:11 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=WTdxb0zAPu2kxNDHiiVz1yQGtJI5ctg2vuStvZG9n+c=; b=PKtbH5gO6SvnPmfGvepYvEyTZVliSb/cO/kIl7sNrsg/oZ+hXo0o+M5VxbGbZKqlgC im6a1tCNqxLqz+T3SkjL8PvEq/mhkTQpkGYa2LM9lZQsjjczLG7dJ/ajARAMBBMhdRJZ efFly1uNDX7GVbRhsmrVfBEDkeYwIleNbrIHl10FVmSw7S5T9czzGLwLJsIJ+8+Yvvps Jp97Y+1NgRpyjwKLfBkKY8kHVSZ6eaejvazRTBZbAnWkAAVGMQIICTJrFx+FB+qUHvgK jiauCzOc6//QfqrKrTXWZ45+GRWGYaek59PYWg0jc7skprs0ErxlgTDRHrvkdrB+ICKg dctw== X-Gm-Message-State: AOAM532FKOEQ7T3eiQuXCDsSP3AsdbUyy9xVDUZUAwK9x2CRYR8x6sGd KEC7hk8wFJ0jKBZOISKq9Hg= X-Google-Smtp-Source: ABdhPJwHlrSXW/ckAl7IDPO3bePA4/ajKFF3Ux+50Cp/0vdT8b497QF7Kk3cJihG3d27RYwI1kiDtA== X-Received: by 2002:a7b:cb97:: with SMTP id m23mr3347723wmi.173.1600765810537; Tue, 22 Sep 2020 02:10:10 -0700 (PDT) Received: from [192.168.1.143] ([170.253.60.68]) by smtp.gmail.com with ESMTPSA id t4sm25506608wrr.26.2020.09.22.02.10.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 22 Sep 2020 02:10:09 -0700 (PDT) Subject: Re: Expose 'array_length()' macro in To: Ville Voutilainen , Jonathan Wakely , Florian Weimer Cc: gcc@gcc.gnu.org, libstdc++ , Libc-alpha , libc-coord@lists.openwall.com, LKML 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> From: Alejandro Colomar Message-ID: <34bae433-f67e-877d-f871-928cb55d2a87@gmail.com> Date: Tue, 22 Sep 2020 11:10:08 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-4.5 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_ENVFROM_END_DIGIT, FREEMAIL_FROM, NICE_REPLY_A, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 22 Sep 2020 09:10:13 -0000 [[ 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 # 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 # endif { # if __cplusplus >= 201703L return std::size(arr); # else return N; # endif } template # if __cplusplus >= 201103L constexpr # endif inline std::ptrdiff_t # if __cplusplus >= 202002L __array_slength(const T(&arr)[N]) # else __array_slength(const T(&)[N]) # endif # if __cplusplus >= 201103L noexcept # endif { # if __cplusplus >= 202002L return std::ssize(arr); # else return N; # endif } #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) ; }