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From: | Keith Thompson <keithsthompson AT gmail DOT com> |
Date: | Sun, 6 Jan 2019 17:19:25 -0800 |
Message-ID: | <CAAHpriOFzks092zT+chJw4Dd91rPsO0mR3wO1gue8btARmBHKw@mail.gmail.com> |
Subject: | clang++: M_PI is visible in conforming mode |
To: | cygwin AT cygwin DOT com |
Cc: | Keith Thompson <Keith DOT S DOT Thompson AT gmail DOT com> |
The macro M_PI, which expands to a constant approximating the value of pi, is defined by POSIX, but not by ISO C or ISO C++. It is not a reserved identifier, so it should be available for use as a user-defined identifier. The problem: Including <cmath> causes M_PI to be defined, even when the C++ compiler is invoked in what should be a conforming mode. This problem occurs with clang++. It does not occur with g++. EXPECTED: Program compiles without error and prints "3". OBSERVED: Program is rejected at compile time. (The program is rejected when g++ or clang++ is invoked without options. That's not a bug.) Using <math.h> rather than <cmath> doesn't change the symptom. A similar program in C does not exhibit the problem. I *think* the problem is in the "math.h" header, which should arrange for M_PI and similar macros not to be defined in conforming mode. It's also possible that a fix might involve updates to clang++. I haven't fully investigated the twisty maze of macro definitions and nested #includes in math.h. (I acknowledge that writing your own definition of M_PI is not a good idea, and that the definition in this program is particularly poor style.) I'm using 64-bit Cygwin on Windows 10, with all the latest updates. See also https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1810695 a report for a similar bug on Ubuntu (affecting both g++ and clang++). Output illustrating the problem follows: $ uname -a CYGWIN_NT-10.0 eddie 2.11.2(0.329/5/3) 2018-11-08 14:34 x86_64 Cygwin $ clang++ --version clang version 5.0.1 (tags/RELEASE_501/final) Target: x86_64-unknown-windows-cygnus Thread model: posix InstalledDir: /usr/bin $ cat M_PI_bug.cpp #include <iostream> #include <cmath> int main() { const int M_PI = 22/7; std::cout << M_PI << '\n'; } // This is not reasonable code. // The issue is that a conforming C++ compiler must accept it. // Expected output: 3 $ g++ -std=c++17 -pedantic-errors M_PI_bug.cpp && ./M_PI_BUG 3 $ clang++ -std=c++17 -pedantic-errors M_PI_bug.cpp && ./M_PI_BUG M_PI_bug.cpp:4:15: error: expected unqualified-id const int M_PI = 22/7; ^ /usr/include/math.h:617:15: note: expanded from macro 'M_PI' #define M_PI 3.14159265358979323846 ^ 1 error generated. -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple
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