使用M_PI与C89标准 [英] Using M_PI with C89 standard

问题描述

我使用C和试图访问的常量M_PI（3.14159 ...）。我已经导入文件math.h头文件，但M_PI常量仍然不确定。通过在计算器上一番搜索，我发现我需要的#define _USE_MATH_DEFINES 添加到我的code（见例如code以下）。通常编译时，此工作正常，但我需要能够与 STD = C89 标志，我做的工作进行编译。

I'm using C and trying to get access to the constant M_PI (3.14159...). I have imported the math.h header file, but the M_PI constant was still undefined. Through some searching on StackOverflow I have found that I need to add #define _USE_MATH_DEFINES to my code (see example code below). This works fine when compiling normally, but I need to be able to compile with the std=c89 flag for the work that I'm doing.

我应该如何从一些C89 code访问M_PI？

How should I access M_PI from some C89 code?

推荐答案

一个符合标准的库文件文件math.h 不仅没有要求，但实际上的千万不要确定 M_PI 默认情况下。在此背景下'默认'的意思是 M_PI 只能获得通过具体的编译器技巧定义，通过使用保留的标识符的最常见未定义的行为。

A conforming standard library file math.h is not only not required to, but actually must not define M_PI by default. In this context 'by default' means that M_PI must only get defined through compiler-specific tricks, most often undefined behavior through the use of reserved identifiers.

只要定义常量自己（你可以使用名称 M_PI 自由，但你应该希望能够编译code有不符合要求的编译器，您必须首先检查 M_PI 尚未定义）。对于约定的缘故，不要定义 M_PI 作为比PI（的近似值）的任何其他。

Just define the constant yourself (you can use the name M_PI freely, but should you want to be able to compile the code with a non-conforming compiler, you must first check that M_PI is not already defined). For convention's sake, do not define M_PI as anything other than (the approximation of) pi.

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