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

问题描述

我正在使用 C 并试图访问常量 M_PI (3.14159...).我已导入 math.h 头文件，但 M_PI 常量仍未定义.通过在 StackOverflow 上的一些搜索，我发现我需要将 #define _USE_MATH_DEFINES 添加到我的代码中(参见下面的示例代码).这在正常编译时工作正常，但我需要能够使用 std=c89 标志来编译我正在做的工作.

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 代码访问 M_PI?

How should I access M_PI from some C89 code?

推荐答案

符合标准的标准库文件 math.h 不仅不需要，而且实际上不能默认定义 M_PI.在这种情况下，默认"意味着 M_PI 只能通过特定于编译器的技巧来定义，最常见的是通过使用保留标识符来定义未定义的行为.

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.

只需自己定义常量(您可以随意使用名称M_PI，但如果您希望能够使用不符合标准的编译器编译代码，则必须首先检查M_PI 尚未定义).为了约定俗成，不要将 M_PI 定义为 pi(的近似值)以外的任何东西.

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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